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I E E A T I S E
METEOROLOGY.
WITH A COLLECTION OF
METEOROLOGICAL TABLES.
BY ELIAS LOOMIS, LL.D.,
PEOFESSOB OF NATURAL PHILOSOPHY AND ASTRONOMY IN YALB COLLEGE, AND AUTHOE OF A
"COURSE OF MATHEMATICS,"
NEW YORK:
HARPER & BROTHERS, PUBLISHERS,
327 TO 335 PEARL STREET,
FRANKLIN SQUARE.
1868.
Entered, according to Act of Congress, in the year 1868, by
HARPER & BROTHERS,
In the Clerk's Office of the District Court of the United States for the Southern
District of New York.
PREFACE.
WITHIN the past forty years a vast amount of meteorological
observations has been accumulated from almost every part of the
world, and particularly from the United States. Within the lim-
its of our own country we have observations, more or less exten-
sive, from more than a thousand stations, and some of these reg-
isters are very accurate and complete. So great an amount of
labor expended upon observations ought certainly to 'lead to
some valuable results. Such results have already been in part
attained, but they are generally published in very large works,
or in elaborate 'memoirs whose object is limited to the discus-
sion of special questions. Many of these memoirs are only to be
found in foreign languages, and nearly all of them are too elab-
orate to circulate freely even among the mass of tolerably intelli-
gent observers. It will probably be conceded that there has not
hitherto appeared, at least in the English language, any general
treatise on Meteorology which furnishes a comprehensive view
of the present condition of every branch of this science with a
minuteness sufficient to satisfy one who is himself engaged in the
business of observing. In the present volume an attempt has
been made to furnish a concise exposition of the principles of
Meteorology in a form adapted to use as a text-book for instruc-
tion, and at the same time to exhibit the most important results
of recent researches. That this attempt has been but partially
successful no one can be more fully aware than the author ; nev-
ertheless it is hoped that this volume will compare favorably with
any work which has hitherto appeared having the same objects
in view. This treatise has been in contemplation for many years,
during which I have been collecting materials for this purpose.
iv PKEFACE.
It would have been quite easy to have expanded the book to
double its present size, and in such a form it might have been
more satisfactory to those who are themselves engaged in original
researches ; but I have aimed to prepare a work which should
not only be useful to observers, but should also be adapted to
purposes of instruction in our colleges and scientific schools. It
is hoped that this volume may serve to stimulate observers, by
showing them the important results already deduced from their
labors, and also by calling their attention to the unsettled prob-
lems which require for their solution either more accurate or
more numerous observations.
I have again to acknowledge my obligations to Professor H.
A. Newton, who has read all the proofs of this work, and to whom
I am indebted for numerous suggestions, particularly in the last
chapter, which relates to a subject to which he has devoted spe-
cial attention.
CONTENTS.
CHAPTER I.
CONSTITUTION AND WEIGHT OF THE ATMOSPHERE.
Page
Composition of the Air Dalton's Theory of the Atmosphere 9
Construction of the Barometer Corrections for Temperature, etc 12
Self-registering Barometers Mean Height of the Barometer 15
Inequality of the Monthly Means Hourly Variations 19
Extreme Fluctuations of the Barometer Heights measured by the Barometer ... 21
CHAPTER II.
TEMPERATURE OF THE AIR AND OF THE EARTH.
Construction of the Thermometer Graduation, etc 23
Self-registering Thermometers Proper Exposure of a Thermometer 25
Hourly Variations of Temperature Mean Temperature of a Day determined 29
Mean Temperature of the Months Mean Temperature of a Place 31
Distribution of Heat over the Earth's Surface Isothermal Lines 34
Two Sides of the Atlantic compared Hottest and Coldest Months 37
Highest and Lowest observed Temperatures 39
Temperature of the Air at different Heights Limit of perpetual Snow 40
Temperature of Earth at different Depths Time of Maximum and Minimum.... 44
Information furnished by Volcanoes, Hot Springs, etc 47
Natural Ice-houses Temperature of the Sea 49
Temperature of Banks Polar Ice Anchor Ice 51
CHAPTER III.
THE MOISTURE OF THE AIR.
How Vapor is sustained in the Air Amount of Evaporation measured 54
Hygrometers Saussure's Bache's Daniell's, etc '.' 56
Weight of Vapor determined Diurnal Variation in Amount of Vapor 60
Annual Variation of Pressure of the Gaseous Atmosphere 63
CHAPTER IV.
THE MOTIONS OF THE ATMOSPHERE.
How to determine the Direction of the Wind 65
Self-registering Anemometers Whewell's Robinson's Osier's, etc 66
Average Velocity of the Wind Mean Direction of the Wind 70
The Trade Winds Winds in the Middle Latitudes Polar Winds 74
Motion of the Upper Half of the Atmosphere 76
Causes of the Winds Mode of Propagation of Winds 79
Cause of the high Barometer near the Parallel of 32 84
The Monsoons Land and Sea Breezes Hot Winds of Deserts 85
I
VI CONTENTS.
CHAPTER V.
PRECIPITATION OP THE VAPOR OF THE AIR.
SECTION L DEW.
rage
Effect of Radiation of Heat Origin of Dew 89
Circumstances favorable to Dew Where there is no Dew 91
SECTION II. HOAB-FKOST.
Formation of Hoar-frost Crystalline Structure of Hoar-frost 93
SECTION III FOG.
Fogs over Rivers in Summer Fogs in Spring and Winter 95
The Vesicular Theory Diameter of Particles of Fog Indian Summer 98
SECTION rV CLOUDS.
Classification of Clouds Cirrus Cumulus Stratus, etc 101
Height of Clouds Formation of Clouds 103
Peculiar Arrangement of Clouds Shadows of Clouds 106
SECTION V RAIN.
To measure the Amount of Rain Proper Exposure of the Gauge 108
Button's Theory of Rain Distribution of Rain over the Earth's Surface Ill
Influence of Elevation above the Sea Influence of Winds 114
Annual FaU of Rain at different Places Greatest Fall of Rain 117
Deserts Rain without Clouds 119
SECTION VI. SNOW.
How Flakes of Snow are formed Form of Snow-flakes 122
Red Snow in the Polar Regions Glaciers 126
SECTION VII. HAIL.
Size of Hailstones Form of Hailstones. 129
Track of Hail-storms Origin of the Cold which causes Hail 132
Process of the Formation of Hail Hail-rods 134
CHAPTER VI.
STORMS, TORNADOES, AND WATER-SPOUTS.
SECTION I THEORY AND LAWS OF STORMS.
Cause of Storms Why the Barometer falls under a Cloud 136
Amount of the Barometric Depression Gradual Rise and Decline of Storms ... 139
Distinction between the Direction of the Wind and that of the Storm's Progress 143
Course of Storms modified by Local Causes 145
*
SECTION II. CYCLONES.
Paths of Cyclones Gyratory- Movement of the Air in Cyclones 148
Cause of the Parabolic Course of Storms 151
SECTION III TORNADOES.
Tropical Tornadoes Effects of Tornadoes 152
SECTION IV. PILLARS OF SAND AND WATER-SPOUTS.
Whirlwinds caused by Fires Water-spouts 154
SECTION V. PREDICTIONS OF THE WEATHER.
Predictions founded on the Constancy of Climate 157
Predictions founded on the established Laws'of Storms 158
I
CONTENTS. Vii
CHAPTER VII.
ELECTRICAL PHENOMENA.
SECTION L ATMOSPHERIC ELECTRICITY, p^
Electrometers Diurnal Variation of Electricity 160
Origin of Atmospheric Electricity Electricity in dry Houses 163
SECTION II. THUNDER-STORMS.
'Lightning Different Forms of Lightning 166
Duration of Lightning Cause of Thunder 168
Rolling of Thunder Height of Thunder Clouds 170
SECTION HI. AURORA POLABIS.
Varieties of Aurora Arches Beams Corona 174
Geographical Extent of Auroras Auroral Arches 177
Structure of Auroral Arches the Corona 181
Height of the Aurora Noise of the Aurora 184
Geographical Distribution of Auroras Auroras in the Southern Hemisphere.... 186
Periodicity of Auroras Diurnal Annual and Secular 188
Disturbance of the Magnetic Needle .' 190
Theory of the Polar Light The Auroral Light is Electric Light 191
What are Auroral Beams? Circulation of Electricity about the Earth 194
Cause of the Magnetic Disturbances Cause of the Periodicity 197
Geographical Distribution explained System of Electrical Circulation 199
CHAPTER VIII.
OPTICAL METEOROLOGY.
SECTION L MIRAGE.
Mirage upon a Desert Mirage at Sea 202
Lateral Mirage Displacements 205
SECTION IL ABSORPTIONS OP LIGHT BY THE ATMOSPHERE.
Redness of the Evening Sky Blue Color of the Sky 206
Cyanometer Twilight Colors of the Morning Twilight 207
SECTION nL THE RAINBOW.
Dimensions of the Rainbow computed Conditions of Visibility 210
Supernumerary Bows Their Theory explained 211
Size of the Drops of Rain Fog-bow explained 213
SECTION IV. CORONA.
Order of Colors in Coronre Cause of Coronse 214
SECTION V. HALOS AND PARHELIA.
Halo of 22 Radius Theory of this Halo 216
Halo of 4G Radius Halo of 90 Radius 218
Parhelic Circle Parhelia : :. 220
Contact Arches Their variable Form 222
Intersecting Arcs opposite to the Sun Vertical Columns 224
CHAPTER IX.
SHOOTING-STARS, DETONATING METEORS, AND AEROLITES.
SECTION L SHOOTING-STARS.
Number seen at different Hours Number seen in the different Months 225
Altitude of Shooting-stars Length of Path and Velocity 226
Cause of the Light of Shooting-stars Meteoric Orbits 228
Vlll CONTENTS.
Pag.
Periodic Meteors of November Meteoric Showers 230
Period of the November Meteors Elements of the November Meteors 233
Periodic Meteors of August Elements of the Orbit of the August Meteors 235
SECTION II. DETONATING METEORS.
Examples of Detonating Meteors Number, Velocity, etc 238
Periodicity of Detonating Meteors 240
SECTION HI. AEBOLITES.
Examples of Aerolites Number of Aerolites 241
Composition of Aerolites Peculiarities of Aerolites 244
Widmannstaten Figures Periodicity of Aerolites , 245
Origin of Aerolites Conclusions 247
TABLES.
I. To convert Millimetres into English Inches 251
II. To convert Metres into English Feet ". 252
III. To convert Kilometres into English Miles 253
IV. To convert French Feet into English Feet 254
V. To compare the Centesimal Thermometer with Fahrenheit's 255
VI. To compare Reaumur's Thermometer with Fahrenheit's 256
VII. Column of Air corresponding to one tenth Inch in the Barometer 257
VIII. For reducing Barometric Observations to the Freezing Point 258
IX. To determine Altitudes with the Barometer 260
X. Mean Height of the Barometer in the different Months 262
XL Mean Height of the Barometer for all Hours of the Day 263
XII. Depression of Mercury in Glass Tubes 263
XIII. To compare the Weight of a Cubic Foot of Dry Air and of Saturated Air 264
XIV. Height of Barometer corresponding to Temperature of Boiling Water 265
XV. Diurnal Variation of Temperature at New Haven, Conn 266
XVI. Diurnal Variation of Temperature at Greenwich, Eng 267
XVII. Mean Temperature for each Month, Season, and the Year 268
XVIII. Places whose Mean Temperature is above 80 Fahrenheit 270
XIX. Places whose Mean Temperature is below 18 Fahrenheit 2 70
XX. Places having a Small Monthly Range of Temperature 271
* XXI. Places having a Great Monthly Range of Temperature 271
XXII. Places having a Small Absolute Range of Temperature 2 72
XXIII. Places having a Large Absolute Range of Temperature 272
XXIV. Height of the Snow Line above the Sea 273
XXV. Factors for Dry-bulb and Wet-bulb Thermometers 273
XXVI. Relative 'Humidity of the Air 274
XXVII. Elastic Force of Aqueous Vapor 276
XXVIII. For comparing the Pressure and Velocity of the Wind 277
XXIX. Average Amount of Rain for each Month, Season, and the Year 278
XXX. Places having a Small Annual Fall ofRain 280
XXXI. Places having a Great Annual Fall of Rain 280
XXXII. Comparative Radiating Power of different Substances at Night 281
XXXIII. Fall of the Barometer in Hurricanes 281
XXXIV. Auroras, Solar Spots, and Variation of the Magnetic Needle 282
XXXV. Catalogue of the largest Iron Meteors 283
XXXVI. Aerolites fallen in the United States 284
EXPLANATION OF THE TABLES 285
INDEX.., 301
METEOROLOGY.
CHAPTER I.
CONSTITUTION AND WEIGHT OF THE ATMOSPHERE.
1. THE term meteor was formerly employed to denote those
natural phenomena which occur within the limits of our atmos-
phere, as the wind, rain, thunder, the rainbow, etc. ; and Meteor-
ology might, therefore, be defined as that branch of Natural Phi-
losophy which treats of Meteors.
This branch of science treats of the constitution and weight
of the air ; of its temperature and moisture ; of the movements
of the atmosphere ; of the precipitation of vapor in the form of
dew, hoar-frost, fog, cloud, rain, snow, and hail ; of the laws of
storms, including tornadoes and water-spouts ; with various elec-
trical phenomena, including' atmospheric electricity, thunder-
storms, and the Polar Aurora ; as also various optical phenome-
na, including the rainbow, twilight, mirage, coronas, and halos ;
to which are generally added aerolites and shooting stars.
2. Composition of the Air. Atmospheric air is not a simple sub-
stance, as was once believed, but consists of nitrogen and oxy-
gen, together with more or less vapor of water, and almost always
a little carbonic acid. The nitrogen and oxygen are combined
in the ratio of 79.1 to 20.9 by volume. These proportions are
generally the same in all parts of the globe, and at all accessible
elevations above the earth's surface. During a balloon ascent,
air^ has been collected from an elevation of 21,774 feet, and its
constitution was found to be sensibly the same as that of air at
the earth's surface.
Atmospheric air contains a little carbonic acid (from 0.0004 to
0.0006 in the open country), and a variable amount of vapor of
water. The amount of moisture in the atmosphere sometimes
10 METEOKOLOGY.
forms four per cent, of its entire weight, and sometimes is less
than a tenth of one per cent.
3. Distinction between Vapors and Gases. Aeriform bodies are
naturally divided into two classes. Some are easily reduced to
the liquid state, and are called vapors, as the vapor of water.
Others always remain in the aeriform state, or can only be re-
duced to the liquid state with the greatest difficulty. These are
called gases, such as oxygen, nitrogen, hydrogen, etc.
4. Law of Mixture of Gases. When vapors and gases are super-
posed upon each other, they obey a law different from liquids.
If we pour into the same vessel several liquids which exert no
chemical action upon each other, they will arrange themselves in
the order of their specific gravities ; the heaviest will subside to
the bottom, and the lightest will float upon the surface. But if
we introduce into the same vessel different gases, they will not
arrange themselves in separate strata in the order of their specific
gravities, but will mutually penetrate each other, and after a short
time the proportion of the several gases will be the same in every
part of the vessel. This movement of gases toward each other
has received the name of diffusion.
5. Dalton's Theory of the Atmosphere. According to the theory
of Dalton, the gases which compose the atmosphere are not in a
state of chemical combination, but the particles of either gas have
neither attraction nor repulsion for those of another, and each of
them is disposed precisely as if the others were not present. He
therefore considered that the earth is surrounded in effect by four
atmospheres, which interpenetrate each other, but without inter-
ference.
The hypothesis that there is no repulsion between the particles
of the different gases which compose the atmosphere has not been
generally received. The diffusion of gases may be explained by
supposing that the molecules of gases are situated at great dis-
tances from each other; and each gas, therefore, presents vast
pores through which the particles of the other gas may penetrate.
6. Gases in the upper Regions of the Atmosphere. In the upper
and inaccessible regions of the atmosphere there are no other
CONSTITUTION AND WEIGHT OF THE ATMOSPHERE. 11
gases than those found at the surface of the earth, for such gases
would in time penetrate to the earth's surface by the force of
diffusion. The hypothesis, therefore, which explains certain fiery
meteors by the assumption of an inflammable gas in the upper
regions of the atmosphere, is inadmissible. 4.
7. Proportions of the Gases at Great Elevations. A stratum of
air near the earth sustains the weight of the entire superincum-
bent atmosphere, and its density is thereby increased. This dens-
ity diminishes as we rise above the surface of the earth ; and
since each gas is distributed as if no other gas was present, this
diminution (which depends upon the weight of the gas) ought
not to be the same for each of the constituents of the atmosphere.
At great elevations, the proportion of these gases should there-
fore be different from what it is at the earth's surface. It has
been computed that, at the height of four miles, the proportion
of nitrogen to oxygen should be one per cent, greater than at the
earth's surface. Observation has, however, shown that there is
no such difference, a result which is attributed to the constant
agitation of the atmosphere, by which the different strata are
thoroughly mingled together.
8. Limit of the Atmosphere determined by Centrifugal Force. Since
the earth's attraction, which retains the air near to its surface, va-
ries inversely as the square of the distance from the centre, while
the centrifugal force arising from the earth's rotation increases
with the distance, there must be a certain height at which these
two forces are equal, and beyond this distance the air will be
dissipated by centrifugal force. /This point is about 25,000 miles
from the earth's centre.
9. Estimate of the Actual Height of the Atmosphere. Other con-
siderations indicate a much lower limit to the atmosphere. The
atmosphere must terminate at that height where the attraction
of the earth is just equal to the repulsion between the particles of
air, and this repulsion is diminished by the low temperature of
the upper regions. At the height of 50 miles the atmosphere is
well-nigh inappreciable in its effect upon twilight. The phenom-
ena of lunar eclipses indicate that the earth's atmosphere is ap-
preciable at the height of 66 miles. The phenomena of shooting
12
METEOROLOGY.
stars and the auroral light indicate that an appreciable atmos-
phere exists at the height of 200 or 300 miles, and probably
more than 500 miles from the earth's surface.
10. Construction of the Barometer. The weight of the atmosphere
Fig. i. is measured by a barometer. If we take a glass
tube, A B, about three feet in length, hermetically
sealed at one end and open at the other, fill
it with quicksilver, and then, closing the open
end of the tube with the finger, invert the tube,
and immerse the lower end in a cup filled with
mercury, on removing the finger the liquid will
fall only a moderate distance, and will be main-
tained at an elevation of about thirty inches above
the level of the liquid in the cup. The column
of mercury in the tube C D is supported by the
pressure of the air acting on the surface of the
mercury in the cup ; and we conclude that the
weight of a column of mercury having a height
of thirty inches is equal to that of a column of
air of the same base, extending to the top of the atmosphere.
Such an instrument is called a Barometer. The barometer meas-
ures, therefore, the pressure of the air, and, in order to ascertain
its amount, we have only to attach to the glass tube a graduated
scale.
In order to allow entire freedom of motion to the column of
mercury, the diameter of the tube should not be too small. For
a stationary barometer, a tube having an internal diameter of half
an inch is not too great.
11. How Air and Moisture are Excluded. Special care should be
taken to exclude from the tube both air and moisture, the pres-
ence of which would produce pressure upon the upper extremity
of the column of mercury, and depress it below its proper height.
It is found very difficult to attain this object perfectly. The tube
should be entirely clean, and the mercury should be filtered, and
both should be heated, in order to expel moisture. A small quan-
tity of mercury is then introduced into the tube, special care be-
ing taken to prevent the admission of air-bubbles. The tube is
then held over a charcoal fire and heated until the mercury boils,
CONSTITUTION AND WEIGHT OF THE ATMOSPHEKE. 13
the tube being held in an inclined position, so that any particles
which may adhere to the sides of the tube may easily escape.
More mercury is now added, and the operation of boiling repeat-
ed as before, and thus the tube is gradually filled.
If a barometer-tube has been well freed from air and moisture,
when the tube is suddenly inclined the mercury will strike the
top of the tube with a sharp metallic sound.
12. How the Height of the Column is Measured. The height of
the mercury in the barometer varies from day to day, and the
graduation of the scale by which its height is measured should
have a sufficient range to comprehend the extreme variations in
the height of the column. With a stationary barometer, these
variations are generally comprehended between 27 and 31 inches.
This portion of the scale is divided into tenths of an inch, and
these spaces are still farther subdivided by means of a vernier.
The graduated scale may be either fixed or movable. If the scale
be fixed, a correction will be required for the oscillations of the
Fig. 2. mercury in the tube. Suppose, when the air is at its
mean pressure, the lower extremity of the graduated
scale just touches the surface ef the mercury in the
cistern. When the pressure diminishes, the mercury
which descends from the tube fills the cistern to a
greater height, and its level rises above the lower ex-
tremity of the scale. When the pressure increases,
mercury from the cistern ascends into the tube, and
its level is left below the extremity of the scale. Thus
the lower extremity of the graduated scale alternately
sinks below the level of the cistern, and rises above
it, in neither of which cases is the true pressure of
the atmosphere directly indicated. As, however, when
we know the relative diameters of the tube and cis-
tern, the variations of the level of the cistern may
be easily computed, such a barometer may give ac-
curate results ; yet the inconvenience is entirely rem-
edied by making the scale movable. In this case the
lower extremity of the scale is made to terminate in
an ivory point, which, by the motion of a screw, D,
may at each observation be brought to exact coinci-
dence with A, the surface of the mercury in the cistern.
14 METEOROLOGY.
In some barometers the scale is fixed, but the level of the
mercury in the cistern may be adjusted to the extremity of the
scale by means of a screw, B.
In order that observations made with different barometers may
be comparable, corrections are required both for temperature and
for capillary action.
13. Correction for Temperature. Heat expands the column of
mercury ; that is, diminishes its specific gravity, and thus a great-
er height is required to produce a given pressure. Now, since
the barometer is daily subjected to changes of temperature, va-
riations in the height of the column do not necessarily indicate
variations of pressure. Before we can decide whether there has
been a change of pressure, we must compute the effect due to the
change of temperature. For this purpose, we must know the tem-
perature of the mercury at each observation ; and, accordingly,
a thermometer always accompanies a barometer, and is techni-
cally called the attached thermometer. At every observation of
the barometer the attached thermometer should also be observed.
For the purpose of comparison, all barometric observations should
be reduced to a standard temperature, and the temperature gener-
ally agreed upon is that of melting ice. The expansion of mer-
cury from the temperature of melting ice to that of boiling water
is -^5- of its volume, which is about Tir.Tnjirth part for one degree
of Fahrenheit's thermometer. In order, therefore, to reduce the
observed height of the barometer to the height which would have
been indicated if its temperature had been 32, we must subtract
the ten thousandth part of the observed altitude for each degree
above the freezing point. If the temperature be below 32, this
correction must be added to the observed altitude. Tables have
been computed, from which we may obtain, by mere inspection,
the correction to be applied to the observed height of the barom-
eter. See Table VIIL, pages 258-259.
14. Correction for Capillary Action. By capillary action the col-
umn of mercury in the tube is depressed below that height which
would just balance the pressure of the air, and a correction must
be added to the observed heights of the barometer in order to
obtain the true pressure of the atmosphere. This correction va-
ries with the diameter of the tube.
CONSTITUTION AND WEIGHT OF THE ATMOSPHERE.
15
' 0.10 inch "
' 0.140 inch.
.20 "
.058 "
.30 "
the depression
.029 "
.40 "
amounts to
.015 "
.50 "
.008 "
.60 .
.004 "
In a tube whose
diameter is
15. The Aneroid Barometer is an instrument for measuring
Fig. 3. the pressure of the atmosphere by
means of the elasticity of a plate
of metal. It consists of a cylin-
drical brass box, about three inch-
es in diameter and half an inch in
height, the sides of which are made
very thin, and which is hermetic-
ally sealed after the air has been
partly exhausted from the interior.
When the pressure of the atmos-
phere increases, the inclosed air is
compressed, the capacity of the box
is diminished, and the two flat ends
approach each other. "When the
pressure diminishes, the ends resume their former position, in con-
sequence of the expansion of the inclosed air. By means of a
combination of levers, this motion of the ends of the box is com-
municated to a pointer, which travels over a graduated dial-plate,
and the original motion is magnified, so that the index travels .
over a space of three inches, while the end of the box only moves
the -zrou-th of an inch. This instrument has the advantage of ex-
treme portability, and, when well made, will measure small devi-
ations from the mean pressure within one or two hundredths of
an inch. In observations requiring great accuracy, it should,
however, be frequently compared with a standard mercurial ba-
rometer.
16. Self -registering .Barometers. In order to diminish the labor
of frequent observations of the barometer, attempts have been
made to render it self-registering. One of the best methods of
accomplishing this object is by means of photography. The light
of a lamp or gas-flame, A, is concentrated by means of a lens, B,
16
METEOROLOGY.
Fig. 4.
so as to strike upon the summit of the column of mercury in the
barometer tube, CD. A sheet of paper suitably prepared for
photographic experiments is attached to a frame, F, placed behind
a screen, G, having a narrow vertical slit placed in the line of the
rays passing through B. The mercury protects a portion of the
paper from the action of the light of the lamp, while above the
mercury the rays of the lamp fall unobstructed upon the paper.
By means of a clock, H, the paper is carried steadily forward at
the rate of about half an inch per hour, and thus the column of
mercury leaves upon the paper a permanent record of its height
for each instant of the day. At the close of the day a new
Fig. 5. sheet of paper must be ap-
plied, and thus the record is
continued. Fig. 5 represents
the appearance of a sheet con-
taining a day's observations,
m't. 2h 4 es 10 noonTh 4 6 8 io m't. A graduation upon the vert-
ical side of the sheet indicates differences of height, while a grad-
uation upon the horizontal side indicates the corresponding hours
of observation.
17. Hardy's Self-registering Barometer is a siphon barometer,
ABC, both ends of the tube having the same diameter. Upon
the surface of the mercury at C rests an iron float, to which is
attached a cord passing over a pulley, P, and from the other end
of the cord is suspended a counterpoise, D D. The float is thus
made to rise and fall with the mercury in the barometer, with-
out interfering with the free motion of the mercury, and this mo-
CONSTITUTION AND WEIGHT OF THE ATMOSPHEKE.
17
tion is copied by the weight. This weight
carries a pencil whose point is very near to
a large vertical cylinder, E E, which turns
uniformly about its axis. This cylinder,
which is covered with a sheet of paper, is
made to revolve by means of the clock, G.
Every half hour this clock moves a ham-
mer, H K, whose head strikes the weight,
D D, by which means the point of the pen-
cil is pressed against the cylinder, and makes
a mark whose position indicates the height
of the mercury in the barometer. On the
sheet of paper is traced a horizontal line di-
vided into equal parts to indicate the hours
of the day. The series of points thus mark-
ed upon the paper shows the movement of the barometer during
24 hours.
18. Hough's Printing Barometer. Mr. G. W. Hough, Director of
the Dudley Observatory at Albany, has invented an instrument
which furnishes automatically a printed record of the pressure of
the atmosphere for every hour of the day. For this purpose he
employs a siphon barometer, and a float resting upon the mercu-
ry in the open arm. This float supports a small platinum disk
which is placed horizontally between the points of two wires
which communicate with a voltaic battery. These wires are sup-
ported by a screw, S, which is attached to a toothed wheel, W..
When the mercury rises in the short leg of the siphon, the plati-
num disk is raised, and touches the upper wire, closing the circuit
through an electro-magnet, advancing the wheel W one tooth, and
raising the screw S ; and so long as the mercury continues to rise,
the screw S rises also. When the mercury in the siphon falls,
the under side of the platinum disk is brought in contact with the
point of the lower wire, closing the circuit through another mag-
net, moving the wheel W one tooth backward, and depressing the
screw S. Thus the screw S is made to rise or fall with the mer-
cury in the barometer. This screw carries a pencil, which traces
upon a revolving cylinder a line showing the minutest move-
ments of the column of mercury during a period of twenty-four
hours. This same screw also gives motion to a series of wheels
B
18
METEOKOLOGY.
which carry types, by which at the end of every hour the height
of the column of mercury is printed on a slip of paper to the ac-
curacy of the thousandth part of an inch.
.19. Mean Height of the Barometer. If we record the height of
the barometer for each hour of the day, after it is corrected for
temperature and capillarity, and divide the sum of the results by
24, we obtain the mean height for the day. If we divide the sum
of the daily means for a month lay the number of days, we ob-
tain the mean height for the month. If we divide the sum of the
monthly means by 12, we obtain the mean height for the year. If
we divide the sum of the annual means for a long period by the
number of years, we obtain the mean height of the barometer for
the place of observation. The mean height of the barometer at
Boston is 29.988 inches.
20. Influence of Latitude. The mean height of the barometer
at the level of the sea varies with the latitude of the place. Near
Fig. 7.
bo.y
^^
^
*"
^
*S
^
--
^
30.0
^^
S
\
L
^-
s
\
29.8
\
t
\
\
oq o
L ^-
\
28 8
E-E
1
D' J
D u
6
) u
4
\
U u
S
u u
1
(j
1
a
o u
O u
4
)"
i
t u
e
o u
7
80
North Latitude.
South Latitude.
the equator the mean height of the barometer at the level of the
sea is 29.927 English inches. In the northern hemisphere this
pressure increases with the latitude up to 32, where the mean
height of the barometer is 30.210 inches ; the pressure thence
diminishes up to latitude 64, where the mean height is 29.652
inches; from which point the pressure slightly increases as we
advance northward, being in latitude 78 equal to 29.775 inches.
In the southern hemisphere the barometer is highest near the
parallel of 25, being there 30.11 inches ; the pressure thence di-
minishes up to latitude 70, where the mean height is only 28.88
inches, and in latitude 76 the mean pressure is 28.95 inches.
This variation of pressure, in different latitudes is shown by Fig.
7. If the atmosphere were at rest, its pressure at the level of the
CONSTITUTION AND WEIGHT OF THE ATMOSPHERE.
19
. 8 .
sea ought to be every where nearly the same. This inequality
of pressure must then be due to the movements of the atmosphere,
as will be explained hereafter, pages 84 and 147.
21. Inequality of the Monthly Means. The mean height of the
barometer is not the same for each month of the year, being gen-
erally less in summer than in winter. At many places the ine-
quality amounts to half an inch, while at other places it almost
entirely disappears. At Pekin, in China, the mean height of the
barometer is least in July, from which time the mean pressure
increases uninterruptedly to January, after which it declines un-
interruptedly to the next July ; the pressure in January exceed-
ing that in July by three fourths of an inch. A similar law pre-
vails throughout a considerable portion of the continent of Asia.
The cause of this fluctuation will be explained on page 63.
In the middle latitudes of Europe and America, the mean height
of the barometer is usually about the same for each month of the
year. At Boston there are no two
months whose mean pressures differ
by more than one tenth of an inch.
A similar remark is applicable to
' London and Paris. These* varia-
tions of pressure are conveniently
represented by means of curve lines.
We draw upon a sheet of paper a
horizontal line J J, and divide it
into twelve equal parts to represent
the different months of the year,
and through these points of divis-
ion we draw a system of vertical lines. Upon each of the verti-
cal lines we set off the mean height of the barometer for the cor-
responding month, and connect all these points by a broken line.
We thus obtain a line whose curvature represents the mean mo-
tion of the barometer for each month of the year. The four
curves of Fig. 8 show the motion of the barometer : P for Pekin,
H for Havana, L for London, and B for Boston. See Table X.
22. Hourly Variations. If we record the height of the barom-
eter for each hour of the day, during a long period of time, and
take the mean of all the observations for each hour, we shall find
\
\
JFMAMJJ ASONDJ
20
METEOROLOGY.
Fig.
that these averages are not equal to each other. The height of
the barometer is greatest about 10 A.M., and least about 4 P.M.
Smaller fluctuations are also observed during the night, the ba-
rometer attaining a second maximum about 10 P.M., and a sec-
ond minimum about 4 A.M. The amount of this diurnal oscilla-
tion is greatest at the equator, where its value is 0.104 inch, and
it diminishes as we proceed toward either pole. In latitude 40
it is reduced to 0.05 inch ; and in latitude 70 it is only 0.003
inch. This oscillation is due partly to changes in the pressure
of the gaseous atmosphere, and partly to changes in the amount
of vapor present in the air, as will be shown on page 62.
These variations of pressure may be represented by curve lines.
We draw upon a sheet of paper several vertical and equidistant
lines to represent the hours of the day.
We set off upon each of the vertical
lines the mean height of the barome-
1#r for the corresponding hour, and
connect all these points by a broken
line. We thus obtain a line whose
curvature represents the mean motion
of the barometer for each hour of the
day, The three curves of Fig. 9 show the motion of the barome-
ter, E for the equator, P for Philadelphia, and S for St. Peters-
burg. These curves are seen to have two daily maxima and two
daily minima. See Table XI.
23. Inequality depending on the Position of the Moon. There is
a small fluctuation in the pressure of the atmosphere depending
on the position of the moon ; but this variation is exceedingly
minute, and can only be detected by taking the mean of the most
accurate observations continued for a long period of time. At
Singapore, latitude 1 18', when the moon is on the meridian, the
pressure of the atmosphere is 0.0057 inch greater than when the
moon is six hours from the meridian. At St. Helena, latitude
15 55', when the moon is on the meridian, the pressure of the
atmosphere is 0.004 inch greater than when the moon is six
hours from the meridian. In higher latitudes the difference of
pressure is still less. These results indicate a feeble tide in our
atmosphere, similar to the tides of the ocean.
m'c
9 m't
CONSTITUTION AND WEIGHT OF THE ATMOSPHERE. 21
24. Accidental Variations. The non-periodic oscillations of the
barometer are far greater than the periodical ones. In the mid-
dle latitudes the barometer is almost constantly in motion, and
the fluctuations are so great and so irregular that the periodical
movements are only detected by taking the mean of a long series
of observations. The difference between the greatest and least
heights of the barometer during a single month is called the
monthly oscillation; and by combining observations extending
over a great number of years, we obtain the mean monthly oscil-
lation. The mean monthly oscillation is least in the neighbor-
hood of the equator, and increases as we approach the poles. At
the equator it is but little over one tenth of an inch ; in latitude
30 it is four tenths of an inch ; in latitude 45, over the Atlantic
Ocean, it is one inch ; in latitude 65 it is one inch and a third ;
and in latitude 78 it is one inch and a fifth. During the three
winter months, the mean monthly oscillation is about one third
greater than the numbers here stated. Ovei Ihe continents of
Europe and America, the oscillations are generally less than over
the Atlantic on the same parallel.
25. Extreme Fluctuations of the Barometer. The ext/eme fluctu-
ations of the barometer are much greater than the numbers here
given. The greatest height which the barometer at Boston has
attained in 37 years is 31.125 inches, and the least is 28.47
inches ; the difference being 2.655 inches, or ^th of the aver-
age height of the column. At London, the greatest observed
range of the barometer is three inches, while at St. Petersburg
and in Iceland it is 3.5 inches. At Christiansborg, near the equa-
tor, the entire range of the barometer in five years was 0.47 inch.
26. Influence of the Wind. The height of the barometer is sensi-
bly influenced by the direction of the wind. At Philadelphia the
barometer generally stands highest when the wind is northeast,
and lowest when the wind is west or southwest, the mean differ-
ence in the height of the barometer for these different winds being
a quarter of an inch. Throughout the northwest part of Europe
the barometer stands highest when the wind is northeast, and
lowest when the wind is south, the mean difference for these two
winds being 0.22 inch.
22 METEOROLOGY.
27. Pressure affected by Height of Station. "When a barometer
is elevated above the surface of the earth, the column of mercury
sinks, because the force which sustains the column, that is, the
weight of the superincumbent air, is diminished. By comparing
the height of the mercury in barometers at two stations, one of
which is above the other, we ascertain the weight of a column of
air extending from the lower to the higher station. For exam-
ple, if the mercury in the lower barometer stands at 30 inches,
and in the higher barometer at 29 inches, it follows that a column
of air extending from the lower to the higher station has a weight
equal to that of a column of mercury one inch high. Now the
density of mercury is 10,464 times that of air ; hence a fall of
one inch in the barometer would indicate an elevation of 10,464
inches, or 872 feet, above the first station, provided the density of
the air were the same at both stations.
28. Heights measured ly Barometer. Since the air is readily
compressed, its density rapidly diminishes as the height increases.
Mathematicians have endeavored to discover the exact relation
between the change of barometric heights and the difference of
level of the two stations of observation. Laplace deduced a
formula which is designed to take account of all the corrections
required for attaining the greatest accuracy, such as the change
of temperature of the air between the two stations, the diminu-
tion of gravity on a vertical line, etc. According to this formula,
in the neighborhood of New York, when the atmosphere is at its
mean state, if we ascend above the level of the sea
917 feet, the barometer sinks 1 inch.
1860 " " " 2 inches.
2830 " " " 3 "
3830 " " " 4 "
4861 " " " 5 "
Table IX, page 260, affords the means of determining the dif-
ference in the heights of any two places by means of barometric
observations.
TEMPEEATUEE OF THE AIR AND OF THE EAETH. 23
CHAPTER II.
TEMPERATURE OF THE AIR AND OF THE EARTH.
29. Climatology. Climatology is the science of climates. By
the climate of a country we understand its condition relative to
all those atmospheric phenomena which influence organized be-
ings. Climate depends upon the mean temperature of the year ;
upon that of each month and each day ; upon the maximum and
minimum temperatures ; upon the frequency and suddenness of
the atmospheric changes ; upon the transparency of the atmos-
phere and the amount of solar radiation ; upon the moisture of
the air and the earth ; upon the prevalence of fogs and dew ; the
amount of rain and snow ; the frequency of thunder-storms and
hail ; the direction, force, and dry ness of the winds, etc. All
these particulars can only be determined by long and careful ob-
servations.
30. Thermometer. The changes of temperature of the air are
measured by means of the thermometer. This instrument gen-
erally consists of a small glass bulb, to which is attached a long
glass tube, having a very small bore. The bulb is rilled with
mercury or alcohol, which also rises somewhat within the tube.
Now mercury and alcohol are very much expanded by an in-
crease of heat, while glass expands very little. If, then, the tem-
perature of the thermometer rises, since the mercury expands
more than the bulb which contains it, it overflows the bulb, and
is forced up into the small tube. If the temperature falls, the
mercury contracts more than the glass bulb, and the mercury in
the tube descends to fill the vacuum created in the bulb. Thus
the changes of temperature to which the thermometer is subject-
ed are indicated by the ascent or descent of the mercury in the
small tube.
31. Graduation of the Scale. In order that we may have an in-
telligible measure of these changes of temperature, the tube must
24 METEOKOLOGY.
be graduated according to some general principles. We need
two invariable temperatures for the determination of two fixed
points upon the scale. The temperatures generally adopted for
this purpose are those of melting ice and boiling water ; and the
interval between these points is variously divided in different
countries. Upon Fahrenheit's thermometer, melting ice is mark-
ed 32, and boiling water 212, the interval being divided into
180 equal parts. The same graduation is extended downward
from 32 to zero, and may be continued below zero as far as is
desired. Degrees below zero are distinguished by the minus sign.
Thus we may have a temperature of 40 above zero, or 40 be-
low zero. Fahrenheit's scale is generally used in England and
the United States.
Upon the Centigrade thermometer, the freezing point is mark-
ed 0, and the boiling point 100. This thermometer is generally
used in France. Upon Eeaumur's thermometer, the freezing
point is marked 0, and the boiling point 80. This thermometer
is generally used in Germany and Kussia.
32. Requisites of a good Thermometer. It is evident that the de-
grees upon the thermometer scale should correspond to equal vol-
umes of mercury. If the tube of the thermometer were through-
out of uniform bore, then the divisions upon the scale should be
throughout of equal length ; but if the tube be not of uniform
bore, these equal volumes -will correspond to unequal lengths
upon different parts of the scale. Now it is impossible to obtain
a glass tube perfectly cylindrical, and therefore when 'an accurate
graduation is required, we proceed as follows : Having selected a
tube whose bore is as nearly uniform as possible, we introduce
Fig . 10. into it a short column of mercury
i i i ~ ' 1 A B, and mark its extremities upon
the tube. Then, by agitating the tube,
we push the mercury along to B C, so that its left extremity may
occupy the same position as the right extremity in the first trial ;
and mark the extremity C. The volumes of A B and B C are
evidently equal. We thus crowd the column of mercury along
from one end of the tube to the other, and divide it into portions
of equal volume. Each of these portions, A B, B C, C D, etc.,
should then be made to contain the same number of divisions of
the scale.
TEMPEKATUKE OF THE AIK AND OF THE EAKTH.
25
When a standard thermometer has been constructed in this
manner, other thermometers are frequently graduated by com-
parison with it at several different points of the scale.
33. Self-registering Thermometers. It is frequently desired to
determine the greatest heat or the greatest cold experienced dur-
ing a day or some longer interval of time. To do this with an
ordinary thermometer, it is necessary that the instrument be fre-
quently observed at short intervals. Such observations are very
laborious ; and in order to diminish this labor, self-registering ther-
mometers have been invented.
The following is one form of a thermometer for registering the
highest temperature. A small piece of steel wire c, about half an
inch in length, and finer than the bore of the thermometer, is in-
troduced into the tube of a mercurial thermometer above the
mercury. The thermometer is placed with its stem A B in a
Fig. 11.
horizontal position, and the steel index is brought into contact
with the extremity of the column of mercury. Now, when the
heat increases and the mercury expands, the index c will be
thrust forward ; but when the* temperature falls, and the mercury
contracts, the index will be left behind. The point of the scale
where the index is found shows therefore the greatest degree of
heat to which the instrument has been subjected since the last
observation.
34. Minimum Thermometer. The lowest degree to which the
thermometer has fallen may be indicated as follows : A spirit
thermometer is placed with its stem D E horizontal, and within
the tube is a very fine glass rod, or index, n, about half an inch
in length, and a little smaller than the bore of the tube. This
index is immersed in the column of alcohol, but must be brought
into contact with the extremity of the column. On account of
the capillary adhesion between the alcohol and the glass, when
the alcohol contracts, it drags along with it the glass index ; but
26 METEOROLOGY.
when the alcohol expands, it passes by the index without dis-
placing it, so that the position of the index shows the lowest tem-
perature to which the instrument has been subjected since the
last observation.
These instruments are especially adapted to indicate the maxi-
mum and the minimum temperature in twenty -four hours. The
steel index being placed in contact with the mercury, and one
extremity of the glass index being made to coincide with the ex-
tremity of the column of alcohol, the position of the two indices
on the following day will show what has been the highest and
what has been the lowest temperature during the last twenty-
four hours.
35. Phillipds Maximum Thermometer. In this thermometer a
small portion of the column of mercury is separated from the re-
mainder of the column by an extremely minute speck of air, so
that this detached column serves the same purpose as the steel
wire in the ordinary maximum thermometer. The end of this
detached column remains at the point of 'maximum temperature,
while the other part of the column retreats toward the bulb when
the temperature declines. By bringing the instrument to a verti-
cal position with the bulb downward, the detached portion de-
scends nearly into contact with the remainder of the column, but
is prevented from uniting with it by the presence of the air speck.
This instrument is susceptible of very great precision.
36. Photographic Register of the Thermometer. In some observa-
tories, the height of the thermometer is registered photographic-
ally, in a manner similar to that described in Art. 16. The light
of a lamp is concentrated by means of a lens, so as to strike upon
the summit of the column of mercury in the thermometer. A
sheet of paper suitably prepared for photographic experiments
is placed behind the thermometer, and receives the shadow cast
by the mercury. By means of clock-work, the paper is carried
steadily forward, and thus the column of mercury leaves upon
the paper a record of its height at each instant of the twenty-four
hours. This is in some respects the best self-registering ther-
mometer known, although the record is usually not very sharp,
and therefore not as accurate as could be desired.
TEMPERATURE OF THE AIR AND OF THE EARTH. 27
37. Cause of the variations of Temperature. The sun is the prin-
cipal cause of the variations of the temperature of the atmosphere.
The amount of heat which the sun communicates in a given time
depends upon the elevation of the sun above the horizon, and
upon the transparency of the atmosphere. The difference be-
tween summer and winter depends upon the time that the sun
remains above the horizon, as well as upon its distance from the
zenith of the observer.
38. How the Atmosphere is Heated. The atmosphere is heated
in three ways : by the direct rays of the sun ; by contact with
the warmer earth ; and by the radiation and reflection of heat
from the earth.
A portion of the rays of heat which are emitted by the sun
are absorbed by our atmosphere before they can reach the earth's
surface. It is estimated that in clear weather the atmosphere
absorbs about one fourth of the rays which traverse the atmos-
phere vertically. The remaining rays are received upon the
earth's surface, by which means the earth is heated. This heat
is thence communicated to the air which rests upon the earth ;
and this air, being thereby rendered lighter, rises and gives place
to colder air from above. This in turn, by contact with the earth,
becomes heated, and rises, and thus there is maintained a con-
tinued circulation between the strata of air in the neighborhood
of the earth.
A portion of the heat whfch the earth receives from the sun
radiates into space. These rays are partly absorbed by the air,
especially by its lower strata, and these strata, in their turn, diffuse
invisible rays of heat in every direction.
The effect of the direct rays of the sun is plainly seen in win-
ter when the ground is covered with snow. In the vicinity of
trees and posts the snow disappears more rapidly than it does
where the surface of the snow is entirely unbroken. This is be-
cause the rays of the sun are absorbed by the dark surface of the
trees more readily than they are by the snow. Thus the trees
are warmed, and these, in their turn, send out rays of heat by
which the adjacent snow is melted.
39. Proper Exposure of a Thermometer. For the purpose of
measuring the temperature of the air, a thermometer should be
28 METEOKOLOGY.
exposed in the open air, where the circulation is unobstructed.
It should face the north, and should be always in the shade. It
should be removed at least a foot from the wall of the building,
and should be elevated about ten feet from the ground. It should
be protected against the heat reflected by neighboring objects,
such as buildings or a sandy soil, and it should be sheltered from
the rain. If the thermometer should happen to become moisten-
ed by rain, the bulb should be carefully dried about five minutes
before making the observation ; since drops of water, by their
evaporation, would lower the temperature of the mercury in the
bulb.
In order to secure all these advantages, it is generally found
necessary to cover the thermometer with a wooden frame of open
lattice- work ; but this covering should be such as to allow a per-
fectly free circulation of air about the thermometer, and it should
rig. 12. be such as readily to acquire the temper-
ature of the surrounding air.
Fig. 12 represents a frame adopted at
Greenwich Observatory for supporting the
thermometers. It consists of two paral-
lel inclined boards, with a small projecting
roof, beneath which the thermometers are
suspended, so that the air circulates free-
ly about the bulbs. The whole frame re-
volves on an upright post, and the inclined
side is always turned toward the sun.
40. Hourly Observations of the Tliermome-
ter. In order to determine the laws which govern the variations
of the temperature of the atmosphere, we require that observ-
ations should be made from hour to hour, both night and day,
throughout a period of several years. Such observations have
been made at many different places. The most extensive series
of this kind in North America was made at Toronto, where bi-
hourly observations were continued for ten years. At Philadel-
phia, hourly observations were made for two and a half years,
and bi-hourly observations for another two and a half years. At
Washington, observations every two hours were continued for
two and a half years. Similar observations upon a less extensive
scale have been made at a few other places in this country.
TEMPERATURE OF THE AIR AND OF THE EARTH.
29
Fig. 13.
58
66
54
52
50
48
46
44
42
* n
/
\
/
\
/
\
/
\
j
' x.
-.
/
^>
"--
<
rt _
h. 4 6 8 10 noon 2h. 4 6 8 10 m
41. Hourly Variations of Temperature. The temperature of a
place changes from one hour to another, according to the distance
of the sun from the horizon. If we take the average of all
the temperatures observed
at each hour of the day for
a long period of time, we
shall find that- the mean
hourly variations of temper-
ature are extremely regular.
Figure 13 shows the general
10 m't ] aw o f the change of tem-
perature at New Haven. The abscissas represent the hours of
the day, and the ordinates the temperatures observed.
We see that on each day there is a maximum and minimum
of temperature. At New Haven, the minimum occurs about an
hour before the rising of the sun, and the maximum about two
hours after noon. In the average of the entire year, the temper-
ature is increasing during nine hours of the day, and decreasing
during the remaining fifteen hours of the day.
The highest temperature of the day should occur when the
amount of heat lost each instant by radiation is just equal to the
heat received from the sun. Before midday, the earth receives
from the sun more heat than it loses by radiation, and its temper-
ature rises. After noon, the earth receives each instant from the
sun less heat than it did at noon ; but the heat received is still
greater than that which is lost by radiation. Hence the maxi-
mum takes place some time after noon. During the night we re-
ceive no direct heat from the sun, and the earth cools by radia-
tion. The lowest temperature should occur when the heat re-
ceived each instant from the returning sun is just equal to the
loss by radiation. This occurs about an hour before sunrise.
42. Mean Temperature of a Day. The mean temperature of a
day is the mean of the twenty -four observations taken at each
hour of the day. Since hourly observations of the thermometer
are very laborious, it is important to discover simpler methods of
ascertaining the mean daily temperature. The following are the
principal methods which have been employed for this purpose.
43. From the Maximum and Minimum Temperatures. The
30 METEOKOLOGY.
mean of the highest and lowest degrees of the thermometer dur-
ing the twenty-four hours differs but little from the mean derived
from hourly observations ; and when self-registering thermome-
ters can be procured, this is a convenient mode of obtaining the
mean daily temperature. This method is not, however, entirely
accurate, since the mean of the two daily extremes is generally a
little greater than the mean for the twenty -four hours. At New
Haven the average difference of the two results for the entire
year is about half a degree ; being nearly an entire degree in
winter, and about zero in summer. When the highest accuracy
is required, a small correction should therefore be applied for the
error of this method.
44. From Observations at a single Hour. When self-registering
thermometers can not be obtained, one of the following methods
may be practiced : Twice during each day the height of the ther-
mometer must coincide with the mean temperature of the day.
At New Haven, this coincidence occurs about a .quarter before
nine in the morning, and also about a quarter before eight in the
evening. We should, therefore, obtain very nearly the mean
temperature by a single daily observation at either of these hours.
Since, however, at these times, the changes of temperature are
quite rapid, a considerable error would result if the observation
were made a little too soon or a little too late. Moreover, these
hours vary at different localities, and they also vary with the sea-
son of the year, so that it is better to deduce the mean tempera-
ture from two or more daily observations.
45. From two Hours of the same Name. It is found that the
mean temperature of any two hours of the same name differs but
little from the mean of the twenty -four hours. Thus the mean
of two observations at 6 A.M. and 6 P.M., is nearly the same as
that of two observations at 7 A.M. and 7 P.M., or 8 AM. and 8
P.M., etc. ; and at New Haven the mean of two observations at
10 A.M. and 10 P.M. differs only about one third of a degree
from the mean of the twenty-four hours. These hours (10 A.M.
and P.M.) are better than any other two hours for furnishing the
mean temperature ; and the mean of these hours is generally
nearer the mean temperature of the day than the mean of the
two daily extremes.
TEMPEEATUKE OF THE AIR AND OF THE EAETH.
31
46. From three Daily Observations. A still more reliable result
may be derived from three daily observations. The mean of ob-
servations at 6 A.M., 2 and 9 P.M., gives very nearly the mean
temperature of the day. The mean of observations at 7 A.M.,
2 and 9 P.M., is a little 'too great; but if we add twice the nine
o'clock observation to the sum of the other two observations, and
divide the result by four, the error of the result for the separate
months at New Haven in only one instance exceeds a quarter
of a degree, and for the entire year differs but one hundredth of
a degree from the true mean temperature. It is found that for
nearly every variety of climate this method furnishes the best
result which can be deduced from any three daily observations,
and these are therefore the three hours to be generally recom-
mended to observers. See Tables XY. and XYI.
47. Mean temperature of the Months. The mean temperature
Fig. -4. of a month is found by
dividing the sum of the
daily means by the num-
ber of days. Figure 14
shows the mean temper-
ature of each month of
the year at New Haven,
and also the mean maxi-
mum and minimum for
the month, according to
86 years of observations.
The months are arranged
upon the horizontal line,
and the temperature for
each month is represented by the corresponding ordinate. The
upper and lower curves pass through the maxima and minima
temperatures for the different months, and the intermediate curve
corresponds to the monthly mean temperature.
We find that at New Haven, 1st. The warmest months of the
year are July and August, and the maximum for the year occurs
near July 24th. 2d. The coldest month of the year is January,
and the minimum for the year occurs near January 21st. 3d. The
difference between the minimum and maximum for each month
is greater in the cold months than in the warm months. 4th. The
70
60
50
40
30
20
10
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XIMl
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JPMAMJJASONB
32 METEOROLOGY.
mean temperature of the month of April is two degrees below the
mean temperature of the year, while that of October is two de-
grees above the mean for the year ; and the mean temperature of
the two months April and October differs less than one tenth of
a degree from the mean temperature of the year.
48. Monthly Change in different Latitudes. At most places in
the northern hemisphere, the change of temperature for the dif-
ferent months follows a law similar to that above described for
New Haven. We find at most places that the average heat goes
on increasing from day to day, uninterruptedly from March until
some time in summer, and after that time the mean heat of each
day decreases uninterruptedly until some time in winter. The
time, however, of the annual maximum and minimum varies with
the latitude of the observer. Near the equator, the entire annual
variation of temperature is very small, and the greatest cold may
occur in any month from November to March, or even from July
to September. Indeed, at some places near the equator there are
two annual maxima of temperature and two annual minima.
But in the extrerrie southern part of the United States, the great-
est cold usually occurs in December; near the parallel of 40, it
occurs about the middle of January ; in the northern part of the
United States, about the first of February ; at Toronto it occurs
as late as the middle of February ; and in latitude 78, the great-
est cold occurs in March.
Throughout most of the United States, the maximum tempera-
ture occurs about the middle of July ; but at some places north
of the United States, the maximum does not occur until the 10th
of August. See Table XVII.
49. Cause of these Peculiarities. If the temperature at any place
depended simply upon the direct momentary influence of the sun,
the maximum would coincide with the summer solstice ; but dur-
ing the most of summer the heat received from the sun during
the day is greater than the loss of radiation during the night, and
the maximum occurs when the loss by night is just equal to the
gain by day. During the autumn, the loss by night is much
greater than the gain by day, and the mean temperature rapidly
falls. The minimum occurs when the gain by day is just equal
to the loss by night, and this generally takes place* .some time
after the winter solstice.
TEMPERATURE OF THE AIR AND OF THE EARTH. 33
The time of maximum or minimum temperature depends not
simply upon the sun's altitude at noon, but also upon the num-
ber of hours during which the sun is above the horizon ; that is,
upon the relative length of the days and nights. The minimum
occurs later in high than in low latitudes on account of the short-
ness of the winter days in high latitudes ; and the maximum oc-
curs later on account of the greater length of the summer days
in high latitudes.
50. Mean Temperature of a Place. The mean temperature of a
year is found by taking the average of all the monthly tempera-
tures for the year. This annual mean is not the same every year
at the same place ; nevertheless, the difference between the cold-
est and hottest years seldom exceeds ten degrees. At New Haven
the hottest year which has occurred in a period of 86 years was
that of 1828, and the coldest year was that of 1836, the extreme
range of the annual temperature in 86 years being 6.3.
At Breslau, in Prussia, the extreme range of the annual tem-
perature in 66 years has been ten degrees.
By taking the average of the mean annual temperatures for a
great number of years, we obtain the mean temperature of a place.
To determine this mean temperature with considerable accuracy
for a variable climate, observations should be continued at least
a quarter of a century, in order that the accidental differences be-
tween successive years may compensate each other.
This mean temperature of a place is sensibly constant from one
century to another, and there is no sufficient reason for believing
that the mean temperature of any place on the earth's surface has
changed appreciably in two thousand years.
51. Non-periodic Variations. Besides the periodic variations of
temperature, there are accidental variations due to causes which
will be mentioned hereafter. These fluctuations of temperature
are frequently experienced simultaneously over large portions of
the globe ; and we frequently find that at the same time in other
parts of the world changes of temperature are observed in the op-
posite direction.
C
34 METEOKOLOGY.
DISTRIBUTION OF HEAT OVER THE EARTH'S SURFACE.
52. Temperature of different Latitudes. If we follow a meridian
from the equator toward either pole, we shall find that the mean
temperature generally decreases, but not uniformly. On the con-
trary, there are places where,' as we proceed toward the pole, the
mean temperature rises instead of falling. These irregularities
are due to local causes, which vary upon different meridians, so
that the points of equal mean temperature are not situated upon
a parallel of latitude.
53. Isothermal Lines. In order to represent all the observa-
tions of temperature conveniently upon a map, we draw a line
connecting all those places whose mean temperature is the same.
Such a line is called an isothermal line. In the neighborhood of
the equator, the mean annual temperature is usually about 80.
In Africa and the Indian Archipelago, the mean temperature near
the equator is about 82, and in a few localities it is still higher.
In a few places the mean temperature of a single year has been
known to rise to 85, and even higher. The area having a mean
temperature of 80 and upward, forms a belt of over 1000 miles
in breadth for more than half the circumference of the globe ; for
about a quarter of the circumference this belt has a breadth va-
rying from 1000 miles to zero ; and for thirty or forty degrees
of longitude, the mean temperature near the equator does not
exceed 79. See Table XYIII.
The isothermal line of 70 is a line gently undulating, but gen-
erally is nearly parallel to the equator. In the northern hemi-
sphere, this line passes through Galveston, New Orleans, Mobile,
and St. Augustine ; through the Island of Teneriffe ; through
Alexandria, in Egypt ; and Canton, in China.
The isothermal line of 60 passes through Sacramento, Cali-
fornia ; Memphis, Tennessee ; Chapel Hill, North Carolina ; Nor-
folk, Virginia ; through the northern part of Spain ; Kome, in
Italy ; a little south of Constantinople ; near the south end of the
Caspian Sea ; and through Shanghai, in China.
The isothermal line of 50 passes through Puget's Sound, on the
Oregon coast ; through Burlington, Iowa ; Pittsburg, Pennsyl-
vania ; New Haven, Connecticut; Dublin, in Ireland ;. Brussels,
TEMPERATURE OF THE AIR AND OF THE EARTH.
35
in Belgium ; and Vienna, in Austria ; near the northern shore of
the Caspian Sea ; and a little north of Pekin, in China.
The isothermal line of 40 passes through the middle of Lake
Superior ; through Hanover, New Hampshire ; through Quebec ;
a little south of Iceland ; through Upsala, in Sweden ; through
Petersburg and Moscow, in Eussia.
36
METEOROLOGY.
GO
.10
The isothermal line of 32 is an undulating oval curve, whose
centre is near the north pole, and which is elongated in the di-
rection of the continents of America and Asia. Over the conti-
Fig 16 nents this line descends
to latitude 52, but on
the coast of Norway it
rises as high as latitude
120 72. The longer diame-
ter of this curve is near-
ly twice that of its short-
er. This line passes a
little south of Behring's
Straits ; near the north-
ern shore of Lake Supe-
rior ; through the south margin of James's Bay ; the southern
part of Greenland; a little north of Iceland; through North Cape,
in Norway ; and through Barnaul, in Siberia. Throughout the
entire area inclosed by this line the mean annual temperature is
below that of melting ice. All these lines are represented on
figures 15 and 16.
It is not claimed that all of these lines have been traced by
actual observation, and the positions assigned them are liable to
some degree of uncertainty ; but the observations are so numer-
ous and so well distributed as to leave little doubt respecting the
approximate position of the isothermal lines.
54. Mean Temperature of the North Pole. At several places in
the Arctic regions the mean temperature has been found to be
but little above zero ; and at Yan Eensselaer Harbor, in latitude
78, the mean temperature is two and a half degrees below zero.
It is probable that near the north pole there is a considerable
area whose temperature is below zero of Fahrenheit. From the
form of the neighboring isothermal lines we conclude that this
area is an oval nearly 2000 miles in length and 700 miles in
breadth, and it lies chiefly on the American side of the north
pole. It is even doubtful whether the north pole is at all in-
cluded in this area. The coldest spot in the northern hemisphere
appears to be north of the American continent in latitude 80 to
85, and its mean temperature is probably at least five degrees
below zero. See Table XIX.
TEMPERATURE OF THE AIR AND OF THE EARTH. 37
55. Two Sides of the Atlantic compared. The mean temperature
of the eastern side of the Atlantic is much warmer than that of
the western, upon the same parallel of latitude. The mean tem-
perature of New York is about the same as that of Dublin, al-
though Dublin is 13 north of New York. Near Lake Superior, in
latitude 50, we find the same mean temperature as at the North
Cape, in latitude 72.
This high temperature of the European coast is due to the high
temperature of the North Atlantic, combined with the prevalent
westerly winds. By means of the Gulf Stream, the waters of the
equatorial regions are conveyed into the North Atlantic, and a
portion of this warm current extends northward between Iceland
and the British Islands, and continues to the Arctic Ocean. The
temperature of the North Atlantic is thus raised much above
what is due to its latitude ; and since throughout the middle lati-
tudes the prevalent winds are from the west, this heat of the
ocean is communicated to places on the eastern side of the At-
lantic, but not to those on the western side.
56. Two Sides of the Pacific Ocean. The currents of the Pacific
Ocean produce an effect similar to the currents of the Atlantic,
and there is a corresponding difference between the temperatures
of places on opposite sides of the Pacific Ocean, and consequently
a marked difference between the temperatures of places on the
Atlantic and Pacific coast of North America, although situated
on the same parallel of latitude. The isothermal line of 50 is
found ten degrees of latitude farther north on the Pacific coast
than it is on the Atlantic coast. Sitka, in latitude 57 3', has
about the same mean temperature as Eastport, Maine, in latitude
44 54'.
57. Northern and Southern Hemispheres compared. The mean
temperature of the northern hemisphere is sensibly higher than
that of the southern.
On the parallel of 10 ] f 2 .1.
" " 20 ! there is an average dif- j 3 A.
" " 30 [ ference amounting to ] 2 .9.
" " 40 j [l.9.
We have not sufficient observations to decide whether this dif-
ference continues in the higher latitudes.
38 METEOROLOGY.
This difference in the temperature of the two hemispheres
probably results from the unequal distribution of land and water.
The northern hemisphere contains much more land than. the
southern. In the southern hemisphere, the sun's rays fall chiefly
upon the water, and are employed in converting water into va-
por, in which. process a large amount of heat is rendered latent.
This heat again becomes sensible when this vapor is condensed
in rain. But rain is much more frequent in the northern hemi-
sphere than in the southern. From a comparison of records,
embracing in the aggregate a period of nearly one thousand
years of observations, it appears that the number of rainy days
in the North Atlantic is fifty per cent, greater than it is in the
South Atlantic. Thus we find that the southern hemisphere ia
cooled by evaporation more than the northern, and the northern
is warmed by the condensation of vapor more than is the south-
ern, by which means the average temperature of the northern
hemisphere is rendered sensibly higher than that of the southern.
58. Hottest and coldest Months compared. The climate and pro-
ductions of a country are very imperfectly indicated by its mean
temperature. Two places may have the same mean temperature,
yet differ greatly in their extreme temperatures, and consequently
also in their vegetable productions. Thus the mean temperature
of New York is the same as that of Liverpool ; yet the difference
between the mean temperature of the three summer months and
that of the three winter months is twice as great in New York
as it is in Liverpool. Throughout England the heat of summer
is insufficient to ripen Indian corn ; while the ivy, which grows
luxuriantly in England, can scarcely survive the severe winters
of New York.
There are some places where the mean temperature of the
hottest month of the year differs less than five degrees from that
of the coldest month. This is true of some of the West India
Islands, and also in the Indian Archipelago. At Singapore, the
mean temperature of January differs but 3-^ from that of July.
On the contrary, there are some places where the mean tem-
perature of the hottest month differs 50, 80, and even 100 from
that of the coldest month. At Quebec, this difference amounts to
60; at Fort Churchill, on Hudson's Bay, the difference is 86;
and at some places in Siberia the mean temperature of January
is more than 100 below that of July. See Tables XX. and XXI
TEMPERATURE OF THE AIR AND OF THE EARTH. 39
59. Climates either Marine or Continental. The most uniform
temperature is found to prevail upon islands, while the greatest
range of temperature prevails in the interior of continents.
Hence climates may be characterized as either marine or conti-
nental. The temperature of the ocean varies but little from sum-
mer to winter, while that of the land may vary more than 100.
Hence those places whose temperature is mainly controlled by
the ocean have an equable climate, while those which are but lit-
tle affected by the ocean have an extreme climate.
The annual range of temperature is much less on the eastern
than on the western side of the Atlantic, because the prevalent
winds are from the west. Hence, on the western coast of the At-
lantic, where the prevalent winds come from the land, the climate
is essentially continental, but upon the eastern side of the Atlan-
tic, where the prevalent winds come from the sea, the climate is
mainly controlled by the ocean.
60. Highest observed Temperature. Although the highest mean
temperature is found near the equator, yet the thermometer fre-
quently rises higher in the middle latitudes than it does at many
places under the equator. Thus, at Singapore, under the equator,
the thermometer never rises above 95, while at New York and
at Paris the thermometer has been known to rise to 104. At
Mosul, in Armenia, the thermometer has been known to rise to
117; at Fort Miller, California, to 121; in India, to 132; and
on the Great Desert of Africa, to 133.
These numbers are supposed to indicate the temperature of the
air where it circulates most freely. A thermometer exposed to
the direct rays of the sun often rises much higher than the pre-
ceding numbers. In India, a thermometer whose bulb was cov-
ered with black wool rose in the sun to 164 ; and a thermome-
ter placed inside of a blackened box, covered with glass, has been
known to rise to 248.
61. Lowest observed Temperature. The lowest temperatures any
where observed have been in North America and Siberia. The
lowest temperature observed at Singapore is 66 ; at Key West,
45 ; at Paris and New York, 10 ; at New Haven, 24 ; and at
Montreal, 38. At New Lebanon, New York, at Franconia, New
Hampshire, and at several places in New England, mercury froze
40
METEOROLOGY.
in January, 1835, indicating a temperature of 40 below zero.
Dr. Kane, in latitude 78, observed a temperature of 67 below
zero ; and Captain Back, at Fort Eeliance, in latitude 62, ob-
served a temperature of 70 below zero ; while in Siberia the
thermometer has been known to fall to 76 below zero.
62. Range of Temperature. By combining these results, we find
that at Singapore the entire range of the thermometer is only
29, while at New York it is 114 ; at Montreal it is 140, and
at Fort Eeliance, in latitude 62, the thermometer in four months
varied from 70 to +81, being a range of 151.
The entire range of the temperature of the air any where ob-
served is from 76 to +133, or 209*.
The range of temperature for a single day in the middle lati-
tudes is often greater than for a whole year in the equatorial re-
gions. At Hanover, New Hampshire, February 7, 1861, at noon,
the thermometer stood at 40 ; the next morning it stood at 32,
making a range of 72 in 18 hours. See Tables XXII. and XXIII.
TEMPEEATUEE OF THE AIE AT DIFFEEENT HEIGHTS.
63. Change of Temperature with Elevation. As we ascend above
the surface of the earth the mean temperature of the air declines.
This depression of temperature is observed when we ascend a
mountain or rise in a balloon. The rate of decrease varies with
the latitude of the place, with the season of the year, as well as
the hour of the day. It is more rapid in warm countries than in
cold countries, and is most rapid during the hottest months. It
is most rapid about 5 P.M., and least rapid about sunrise.
The change is also most rapid near the earth's surface, and di-
minishes as we ascend. From a long series of balloon ascents,
made under the direction of the British Scientific Association, the
following results have been obtained for the vicinity of London :
Elevation.
When the Sky is
clear.
When the Sky is
cloudy.
From Oft. to 5000ft.,tl
" 5000 " 10000
" 10000 " 15000
" 15000 " 20000
" 20000 " 25000
" 25000 " 30000
ie decrea
se is 1
for 239 ft. elev'n.
394
490 "
581 "
877 "
1190 "
1 for 271 ft. elev'n.
u 394 "
" 459 "
" 725 "
" 1111 "
TEMPERATURE OF THE AIR AND OF THE EARTH.
41
64. Cause of this Decrease of Temperature. This decrease of tem-
perature as we rise above the earth's surface is mainly due to the
expansion of the air. The lower strata of the air, being heated
b^ the sun (Art. 38) and expanded, tend to rise in consequence
of their diminished specific gravity. As the air ascends it is sub-
ject to a diminished pressure and expands; its heat is diffused
through a greater amount of space, by which means a part of its
sensible heat becomes latent.
This principle may be proved experimentally by placing a
thermometer under the receiver of an air-pump and rapidly ex-
hausting the air, when the thermometer indicates a diminution of
sensible temperature. Upon readmitting the air the thermometer
rises to its former height.
The atmosphere would be in a condition of equilibrium if a
pound of air at all elevations, whether on the summit of a mount-
ain or at the level of the sea, contained the same amount of heat.
The atmosphere is perpetually seeking to attain to this condition
of equilibrium, but since the sun perpetually acts as a disturbing
force, such an equilibrium is never fully attained.
65. Law of decrease of Temperature with Height. We see from
the observations of Art. 63 that the diminution of temperature is
not proportional to the height ; but we find that the temperature
is intimately connected with the pressure, as shown by the ba-
rometer. The following table presents a summary of these ob-
servations for a clear sky :
Barometer.
Temperature.
Difference.
Barometer.
Temperature.
Difference.
10 inches.
12
-10.9
-6 .1
4.8
20 inches.
22 '
15. 3
21 .0
5. 7
14 <
-1 .7
4 .4
K A
24
26 .8
.0
5 9
16 '
+3 .7
5Q
26 *
32 .7
7 2
18 '
20 *
+9 .5
+ 15 .3
5 .8
28 *
30
39 .9
50 .0
10 .1
Column first shows the pressure indicated by the barometer,
and column second the corresponding temperature when the tem-
perature at the earth's surface was 50. The third column shows
the change of temperature corresponding to a change of two
inches in the pressure. These differences are greatest near the
earth's surface, but after rising one mile they become nearly con-
stant ; that is, the fall of the thermometer is nearly proportional to
the fall of the barometer, the change of the thermometer being
42
METEOROLOGY.
about five degrees for a change of pressure amounting to two
inches.
Fig. IT. The curve in Fig. 17 shows more
12 readily how the temperature de-
pends upon the pressure. The ab-
scissas represent the observed tem-
peratures from 50 to 11, and the
ordinates show the corresponding
pressures from thirty inches to ten
inches. '
50 40 30 20 +10
66. Limit of Perpetual Snow. In consequence of this decrease
of temperature, the summits of high mountains, even within the
tropics, are always covered with snow. The limit of perpetual
snow is not the line whose mean temperature is 32. The snow-
line is determined more by the mean temperature of the hottest
month than by the mean temperature of the year.
The limit of perpetual snow generally descends as we proceed
from the equator toward the poles, but there are many exceptions
to this rule. The height of the snow-line depends upon a variety
of circumstances : not only upon the mean temperature, but upon
the extreme heat of summer ; upon the amount of the annual fall
of snow ; upon the prevalent winds ; and upon the proximity of
mountain peaks or extensive plains. Under the equator the
height of the snow -line varies from 15,000 to 16,000 feet, where
the mean annual temperature is 35. On the Alps, the average
height of the snow-line is 8800 feet, where the mean annual tem-
perature is 25 ; while on the coast of Norway its height is only
2400 feet, where the mean annual temperature is 21. Fig. 18
shows the snow-line on several mountains in different latitudes.
Fig. 18.
Numbers 1, 2, and 3 are the Illimani, Aconcagua, and Chimbo-
TEMPERATURE OF THE AIE AND OF THE EARTH. .43
razo, in South America ; 4, 5, and 6 are the Choomalari, Dhaula-
giri, and Caucasus, in Asia ; number 7 is the Pyrenees, and 8 the
Alps ; number 9 the Sulitelma, in Norway ; and number 10 the
island Mageroe. See Table XXIY.
67. Temperature of the Interplanetary Spaces. The temperature
of the air does not continue to sink indefinitely as we rise above
the earth's surface. Its mean temperature can nowhere fall be-
low the temperature of the interplanetary spaces. The space in
which the planets move has a temperature of its own, due to the
radiation of heat from the stars, each of which is a hot body like
our sun. This temperature of space js necessarily lower than the
mean temperature of the polar regions of the earth, for during six
months of the year these are illumined by the sun, from which
they derive a large amount of heat.
68. Mode of estimating its Amount. The temperature of celes-
tial space must be lower than that of the polar regions during the
coldest months of the. year, for during winter these regions do
not lose all the heat received from the sun during the preceding
summer, and by means of winds there is a constant interchange
of heat between the polar and equatorial regions of the earth.
Now at Jakutsk, in Siberia, the mean temperature of the month
of January is 44 below zero. Moreover, from October to No-
vember, the temperature of that place sinks 34 ; from Novem-
ber to December it sinks 18, and from December to January 6.
If the sun's heat were to be permanently withdrawn, the temper-
ature would doubtless fall still lower than is now observed in
January, probably as low as 60. We can not then suppose
the temperature of space to be higher than 60.
Many different methods have been employed for estimating the
temperature of space. The average of the estimates of several
distinguished philosophers makes it as low as 80.
69. The Atmosphere a regulator of Temperature. The atmos-
phere serves as a regulator of the sun's heat. During the day it
absorbs a portion of the sun's rays, by which it is warmed, and as
it expands a part of the heat becomes latent. During the night
the air intercepts a part of the rays emitted by the earth, and as
it cools it contracts, and restores to the sensible condition the la-
44. METEOROLOGY.
tent heat which it had absorbed during the day. Without an
atmosphere we should experience during the day an excessive
heat from the sun's rays, no portion of which would be inter-
cepted, and during the night an intense cold resulting from the
unobstructed radiation of heat into space.
TEMPERATURE OF THE EARTH AT DIFFERENT DEPTHS.
70. Means of Observation. For the purpose of measuring the
variations of temperature beneath the surface of the earth, ther-
mometers with very long stems have been buried at different
depths in the ground, the stem being of such a length as to rise
above the surface of the e^arth, so that the temperature can be
observed without disturbing the position of the thermometer.
For convenience of comparison, it has generally been agreed to
adopt a uniform system, and thermometers have been buried at
depths of 24, 12, 6, and 3 French feet. [A French foot is about
^th greater than an English foot.] From twenty to thirty years
ago, thermometers were buried at these four depths at Brussels,
Edinburg, Greenwich, and other places ; and other thermometers
were also buried at depths less than three feet. At first, these
thermometers were observed several times each day, but after-
ward once a day or once a week.
71. Range of the fluctuations of Temperature. Since the earth is
a bad conductor of heat, the range of the fluctuations of temper-
ature rapidly diminishes as we descend below the surface. At
a certain depth the diurnal variations of temperature disappear,
and at a greater depth the annual variations also disappear.
These depths vary as the square root of the period compared.
The annual variations disappear at a depth 19 times greater than
the diurnal variations ; 19 being nearly the square root of 365,
the number of days in a year. In Europe generally, the diurnal
variations are not sensible to a greater depth than 3-j feet ; but
the depth varies somewhat with the latitude and the conducting
power of the soil.
At the depth of three feet the annual range of temperature is
less than half what it is at the surface ; at the depth of twelve
feet it is less than one fourth, and at the depth of twenty-four
feet it is less than one tenth what it is at the surface.
TEMPERATURE OF THE AIR AND OF THE EARTH. 45
72. Stratum of Invariable Temperature. At a certain depth the
annual variations- of temperature become insensible ; that is, we
find a temperature which is invariable from summer to winter.
This depth depends upon the extreme range of the temperature
of the air. In Europe it is from 80 to 100 feet beneath the sur-
face. A thermometer which has been kept for 75 years in the
vaults of the Observatory at Paris, at the depth of 91 feet below
the surface, has not varied more than half a degree during the
entire interval.
The annual mean of the temperatures observed at different
depths is very nearly the same as that of the air. Hence we are
furnished with a convenient iteans of determining nearly the
mean temperature of any locality, and this method is one of great
value to scientific travelers.
73. Time of Maximum and Minimum Temperature. Since the
earth is a bad conductor of heat, the heat of the sun penetrates
the ground slowly, and the highest temperature of the year oc-
curs later and later the deeper we descend below the surface. At
the depth of twelve feet the maximum temperature of the year
does not occur until October, and the minimum occurs in April.
At the depth of twenty -four feet, the maximum occurs in Decem-
ber, and the minimum in June or July. These dates vary some-
what in different countries, being dependent upon the conduct-
ing power of the soil.
The maximum of daily temperature also occurs later the deep-
er we descend, requiring three hours to penetrate to a depth of
four inches.
74. Increase of Temperature with the Depth. Below the depth
of 100 feet from the surface, we find an invariable temperature
throughout the year ; but this temperature is not the same as the
mean temperature at the surface. Numerous observations have
been made in different parts of the globe, and they invariably
indicate that the mean temperature increases with the depth.
These observations have been extended to very great depths by
means of mines and artesian wells. An artesian well consists
of a shaft of a few inches in diameter, bored into the earth till a
spring is found. To prevent the water from being carried off by
the adjacent strata,. a tube is generally inserted, exactly fitting the
46- METEOROLOGY.
bore from top to bottom, and through this tube the water rises to
the surface. Artesian borings have been made in Europe to a
depth of more than 2300 feet below the surface, and some of the
mines are more than 2000 feet deep.
In Europe the average increase of temperature deduced from
mines and artesian wells is one degree for a descent of 52 feet.
75. Rate of Increase in the United States. Some very deep bor-
ings have been made in the United States. An artesian well in
Charleston, South Carolina, has a depth of 1000 feet ; one in Lou-
isville, Kentucky, has a depth of 2086 feet ; a third in St. Louis,
has a depth of 2200 feet ; and a fourth in Columbus, Ohio, has a
depth of 2575 feet. The boring at Louisville indicates an in-
crease of temperature of one degree for every 76 feet ; and that
at Columbus gives an increase of one degree for every 71 feet.
The mean of these two experiments gives an increase of one de-
gree for every 73 feet, which is less than the rate of increase in
Europe.
76. Stratum of Frozen Earth. Throughout nearly the whole of
the Arctic circle the mean temperature is considerably below 32,
and this is also the mean temperature of the surface of the earth.
Now, in the polar regions, the earth in summer only thaws to a
depth of three or four feet. Below this line is a stratum of per-
manent frost, whose depth increases as we advance northward,
the lower limit being determined by the increase of temperature
explained in Art. 74. At Jakutsk, latitude 62 2', it has been de-
termined by actual excavation that the earth is frozen to a depth
of 382 feet. In the polar regions, therefore, wells are impossible,
unless they are sunk to a depth below that of the permanent frost.
77. Temperature of the Earth at great Depths. If the tempera-
ture of the earth at great depths increases at the same rate as
near the surface, at a depth of two miles the temperature must
exceed that of boiling water, and at a depth of less than a hund-
red miles the rocks must be in a state of fusion. We are thus
led to the conclusion that, with the exception of a comparatively
thin crust upon the surface, the entire mass of the earth is proba-
bly in a state of igneous fusion.
TEMPERATURE OF THE AIR AND OF THE EARTH. 47
78. Information furnished ~by Volcanoes. This conclusion is con-
firmed by the phenomena of volcanoes. At numerous points
upon the earth's surface we find volcanoes which frequently eject
immense masses of melted rock, and which, without doubt, at all
times contain large quantities of rock in a state of fusion. Vol-
canoes, extinct or active, border the Pacific Ocean from Cape
Horn to the Arctic circle ; thence they extend in a line to Asia,
and along the coast of Japan to the Philippine Islands, New
Guinea, and New Zealand ; and they constitute half of the isl-
ands of the Pacific Ocean. Volcanoes occur also in Central and
Western Asia ; in Southern, Central, and Southwestern Europe ;
in Iceland and the West Indies. Volcanoes therefore are so nu-
merous (their number exceeding 500) as to indicate that a con-
siderable portion of the interior of the earth must be in a state
of fusion.
Some have doubted whether the whole interior of the earth is
in a state of fusion, and are disposed to admit only the existence
of interior seas of liquid rock.
79. Observations of Hot Springs. At many places remote from
any active volcano we find natural springs which emit water of a
very high temperature. Many of the springs of Germany have
a temperature of 140 to 150 degrees, and one has a temperature
of 167. At New Lebanon, N. Y., is a spring whose temperature
is 25 above the mean temperature of the place. A spring in
Virginia has a temperature of 102, another in. North Carolina has
a temperature of 125, while one in Arkansas has a temperature
of 148. Near San Francisco is a spring which perpetually emits
boiling water, and there is a similar one near the eastern bound-
ary of California. These springs probably rise from great depths,
and are proofs of the increasing temperature of the earth as we
descend below the surface.
80. Temperature of Ordinary Springs. The ordinary springs
and wells of a country afford a convenient means of determining
approximately its mean temperature. The mean temperature of
the water proceeding from springs is nearly that of the strata from
which they rise. Hence the water from deep springs preserves
throughout the year a nearly uniform temperature, and this is
generally a little above the mean temperature of the air. This
48 METEOROLOGY.
difference may amount to five or six degrees ; and, on the contra-
ry, the mean temperature of springs is sometimes a little below
that of the air. The temperature of springs is modified by the
temperature of the rain which supplies them. In those places
where the rain falls chiefly in summer, the mean temperature of
springs should be higher than that of the air, but it should be
lower in those countries where the rain falls chiefly in winter.
Hence great caution is required in deducing the mean tempera-
ture of a place from the temperature of its springs.
81. Low Temperature of certain Wells. In some wells the mean
temperature of the water is considerably below the mean temper-
ature of the place. In ordinary wells the water is in continual
circulation, the water of the well flowing off by underground
streams, while fresh water flows in through 'similar channels.
Thus throughout the year the water of the well preserves nearly
the temperature of the earth at the same depth ; and a few ob-
servations of such a well will furnish very nearly the mean tem-
perature of the place. But in some wells there is very little cir-
culation, the same water remaining in the well for a long time
with but trifling change. Now, since cold air is heavier than
warm air, the cold air of winter descends into the well, and com-
municates its own temperature to the water in the well. The
water thus becomes chilled, and it may even freeze, as actually
happens to many wells of New York and New England. When
considerable ice once forms in a well, it must remain for a long
time unmelted, because in summer the warm external air can not
displace the heavier cold air of the well. Under such circumstan-
ces, ice has been known to continue till after midsummer ; and
the mean temperature of such a well may be several degrees be-
low the mean temperature of the place.
82. Remarkable Examples. In Brandon, Yt, is a well 34 feet
deep, in which, during the winter, ice forms six or eight inches
in thickness, and does not entirely disappear until the close of the
succeeding summer.
In Owego, N. Y., was formerly a well 77 feet in depth, where
ice formed during the winter, and has been known to continue
until near the close of July.
TEMPERATURE OF THE AIR AND OF THE EARTH. 49
83. Natural Ice-houses. In hilly countries we sometimes find
secluded spots where the ice which accumulates in winter is so
protected against the action of the sun in summer that it remains
unmelted till August, or perhaps even through the year. The
springs which flow from such places may show a temperature but
little above 32, even in midsummer. Several examples of this
kind are found in New England, and still more remarkable ex-
amples are found in the mountainous districts of Europe. On
the western bank of Lake Champlain, near the village of Port
Henry, is an iron mine in which the ice accumulates in winter,
and does not entirely disappear during the subsequent season.
In Meriden, Conn., is a rocky ledge of little elevation, where the
ice of winter remains unmelted until the succeeding August.
In the eastern part of France (Besangon), at an elevation of
less than 3000 feet above the sea, is a cavern where the ice has
been known to lie unmelted for more than a century.
84. Temperature of the Sea. To determine the temperature
of the sea at different depths we require some kind of self-reg-
istering thermometer. The instrument employed for such ob-
servations in the U. S. Coast Survey** is Saxton's metallic ther-
mometer.
This instrument consists of a compound coil or helix about six
inches in length, formed of two stout ribbons of silver and plati-
num, with an intermediate thin plate of gold, all soldered togeth-
er, the silver being on the inside of the coil. One end of this coil
is firmly attached to the base of a cylinder, while the other end
is fastened to a brass stem passing through the axis of the coil.
When the temperature rises, the curvature of each spiral dimin-
ishes, because the silver expands more than the platinum ; and
when the temperature declines, the curvature of each spiral in-
creases. The coil therefore winds and unwinds with the varia-
tions of temperature, and this motion gives rotation to the brass
stem. This motion is registered upon the dial of the instrument
by an index which pushes before it a registering hand, moving
with friction barely sufficient to retain its place when thrust for-
ward by the index of the thermometer. The instrument may
thus be made to register both the highest and lowest tempera-
tures to which it has been exposed.
D
50 METEOROLOGY.
85. Temperature at the Surface of the Sea. The surface of the
sea becomes heated less readily than the earth : 1st, because the
rays of the sun penetrate the ocean to a considerable depth, and
therefore produce less effect at the surface ; 2d, because water has
a much greater capacity for heat than dry earth ; and, 3d, because,
by the agitation of the sea, there is a perpetual mingling of the
surface water with the lower strata. The surface also becomes
cooled very slowly for the same reasons, and also because, when
the particles of the surface are cooled, they descend, to be re-
placed by warnler particles from beneath.
Hence the diurnal variations of the temperature of the sea are
quite small, amounting to only two or three degrees in the torrid
zone, and 4 or 5 in the temperate zones. The minimum oc-
curs about sunrise, and the maximum about noon.
Near the middle of the Atlantic Ocean, under the equator, the
mean temperature of the sea is 80.4. As we recede from the
equator, the temperature of the sea declines somewhat less rap-
idly than the land, the mean temperature of the middle of the
Atlantic being about four degrees warmer than the western coast
of Africa and Europe.
The entire range of temperature for the middle of the Atlantic
during the year, near the equator, is about 10 ; near latitude
30 it is 15 ; near latitude 40 it is 20, and near latitude 50 it
is 24, which is scarcely one half the annual range of temperature
of the most equable climates in the same latitude on land.
86. Temperature at different Depths. Between the tropics the
temperature of the sea decreases as we descend, at first rapidly,
but afterward more slowly, to the depth of over 1000 fathoms,
where the thermometer has been found to indicate 36. Beyond
latitude 25, the decrease of temperature with the depth is less
rapid ; and beyond latitude 65, during winter, the temperature
sometimes increases as we descend. When the temperature of
the surface-water was 28, the temperature at the depth of 700
fathoms has been found to be 36.
In very deep water, all over the globe, there is found to pre-
vail a uniform temperature of 36 to 39. The depth at which
this temperature is found is about 7200 feet at the equator, and
about 4500 feet in the highest accessible latitudes.
TEMPERATUEE OF THE AIR AND OF THE EARTH. 51
87. Currents of the Sea. On. the surface of the Atlantic Ocean,
near the equator, there is a current setting westward, which di-
vides where it meets the projecting coast of South America, one
portion turning northward and the other southward. The for-
mer gives rise to the Gulf Stream, which travels along the coast
of the United States to latitude 45, whence a portion proceeds
northeastwardly between Iceland and the British Islands, and the
other portion descends along the western coast of Europe and
Africa, and rejoins the equatorial waters.
The Brazil current coasts along the South American shore,
and in the South Atlantic makes a circuit somewhat similar to
that of the Gulf Stream in the north.
In the Pacific Ocean, a current setting westward prevails
throughout the whole of the equatorial belt until near the Asi-
atic coast, where, as in the Atlantic, it divides, and one portion,
called the Japan current, imitates in the North Pacific the course
of the Gulf Stream in the North Atlantic. The larger portion
of the equatorial current is, however, carried southward to sweep
the northern and western coast of Australia.
At the bottom of the ocean there prevail counter-currents, which
carry from the poles toward the equator the cold waters of the
Arctic Seas. The existence of these currents is perceived when-
ever we sink a weight to a great depth by means of a long cord.
This is the cause of the low temperature prevailing in tropical
regions near the bottom of the ocean.
88. Temperature of Banks. Where the sea is shallow the water
is generally found somewhat colder than in the adjacent open
ocean, the difference frequently amounting to ten degrees or
more. This change of temperature is very noticeable over the
Banks of Newfoundland, in contrast with the Gulf Stream, which
flows near their eastern margin, where we frequently find a
change of temperature of 33 within a distance of 300 miles.
Thus a thermometer may frequently give warning of approach-
ing land in the darkness of night, when nothing else would indi-
cate it.
This low temperature over banks has been ascribed to the un-
der-current from the polar regions toward the equator, which in
deep water is only found at great depths, but which in shallow
water is partially forced upward, so as to affect somewhat the
temperature at the surface.
52
METEOROLOGY.
89. Polar Ice. From latitude 40 to 50, during winter, the
water of the ocean freezes somewhat near the shore ; but it is
only in the polar regions that we find firm ice at a great distance
from the land. Sea water freezes at a temperature of 27-iy , and
since, during winter, the mean temperature of the polar regions
is considerably below zero of Fahrenheit, ice forms even in the
open sea with great rapidity, and sometimes attains a thickness
of twenty -five feet. In the spring of the year this ice is partially
dissolved ; it is then broken up by tides and currents, and by
northerly winds is driven into the open sea, sometimes forming
a field of ice 100 miles in length and 50 miles in breadth, with a
thickness of 20 or 25 feet. During the months of May and June
this ice is annually encountered in immense fields off the coast
of Newfoundland, near the track of vessels from New York to
Liverpool.
In connection with these immense fields of comparatively thin
ice are generally found some masses of ice called icebergs, some-
times rising 200 feet above the water, and descending to a depth
of 1000 feet beneath the surface. These masses are detached
from the coasts, around which, in winter, the ice accumulates in
cliffs of vast height and extent. The largest of them are de-
tached portions of vast glaciers, such as abound on the precipi-
tous coast of Greenland and Spitzbergen, which were broken off
either by their own weight or the action of the waves, and then
transported by winds and currents to the lower latitudes. Fig.
Fig. 19.
19 represents an iceberg encountered some years since near the
Cape of Good Hope.
TEMPERATURE OF THE AIR AND OF THE EARTH. 53
90. Temperature of Lakes and Rivers. The temperature of lakes
exhibits changes much greater than those of the ocean. The sur-
face may freeze in winter, while in summer the temperature may
rise to 77. In deep lakes, at a certain depth, we find a constant
temperature of about 39, this being the temperature of water at
its maximum density. Since the warm water of the surface de-
scends as fast as it becomes cooled, the surface of a lake can not
freeze until the entire mass has fallen to the temperature of 39,
unless under the influence of very sudden and severe cold.
In rivers the constant agitation of the water tends to render
the temperature uniform throughout. Hence the temperature at
the surface would not change very greatly during the year were
it not for the diminished flow in summer, which leaves but a thin
stratum of water to be acted upon by the sun. During winter
congelation can not take place until the entire mass is cooled to
32, with the exception perhaps of deep cavities.
91. Anchor Ice. Ice sometimes forms upon stones and other
objects at the bottom of rivers when the surface water is not
frozen, and this is called anchor ice. Such ice may form under
the following circumstances. During a period of severe cold the
water of a river may sink below 32 from top to bottom through-
out, and the surface water not congeal, because it is kept in con-
stant agitation, while the water at the bottom, being more quiet,
may congeal. The ice thus formed at the bottom forms a nucle-
us about which the congelation continues and extends. When
the ice becomes quite thick, its buoyant force may overcome its
adhesion to objects at the bottom, and it rises to the surface. A
slight elevation of temperature, causing a partial fusion, may also
detach it from the bottom.
Anchor ice never forms at the bottom of tranquil water, be-
cause congelation commences at the surface, while the tempera-
ture of the bottom is above 32.
54 METEOROLOGY.
CHAPTER III.
THE MOISTUKE OF THE AIR.
92. How Water is converted into Vapor. If during summer we
expose to the sun's rays a vessel containing water, we find that
the water rapidly diminishes, and in a few days entirely disap-
pears. The water seems to have been annihilated, but in fact it
has been converted into vapor, which is diffused through the at-
mosphere. This vapor is entirely invisible, but by the applica-
tion of cold we may condense it, and reduce it again to the form
of water. Thus in summer, if we pour cold water into a metallic
vessel, we find that the outside of the vessel, which was previous-
ly quite dry, soon becomes covered with moisture. This moist-
ure does not come from the inside of the vessel. It is simply the
vapor of the air, condensed by coming in contact with a cold
surface.
The vapor of the air may be -condensed in a similar manner at
.all seasons of the year. The phenomenon is most frequently no-
ticed in summer, because then the temperature of the air rises
highest above that of the water which we are accustomed to use.
But at any period of the year, if the water be not already cold
enough, by adding to it ice, and if necessary salt, we may con-
dense the moisture of the air even in the coldest weather.
93. How Vapor is sustained in the Air. The atmosphere always
contains vapor of water. This vapor is not sustained in the air
like water in a sponge, nor does it float in the air like small par-
ticles of dust, but it penetrates between the particles of the per-
manent gases which compose the atmosphere, and sustains itself
precisely in the same manner as they do. If we exhaust all the
air from a close vessel, and introduce into it a quantity of water,
a portion of the water will immediately pass into the state of va-
por, which will fill the entire vessel. Indeed, with the exception
of the facility with which it is reduced to the liquid state, vapor
of water has precisely the same properties as oxygen or nitrogen.
THE MOISTURE OF THE AIR. 55
If into a close vessel containing atmospheric air perfectly dry
we introduce a quantity of water, vapor will be formed of the
same tension as if the vessel were previously void. The only
difference will be that in a vacuum the maximum tension of the
vapor will be attained instantly, while in a vessel filled with gas
a certain time will be required to produce the same result.
94. Amount of ^Evaporation Measured. The amount of evapo-
ration from the earth's surface is measured by placing a vessel of
water in the open air, and determining the loss of water from day
to day. The vessel usually employed for this purpose is a cylin-
der from six to twelve inches in diameter. It is nearly filled
with water, the quantity having been previously weighed or
measured ; it is then placed out of doors, freely exposed to the
action of the atmosphere. At the end of twelve or twenty-four
hours the water is again measured, and the loss of water shows
Fig. 20. the amount of evaporation that has
taken place. If rain has fallen
between the two observations, the
amount collected in the rain gauge
must be deducted from the quantity
in the evaporating gauge. The wire
cage around the gauge, Fig. 20, is to
prevent animals, birds, etc., from
drinking the water.
From observations continued for nine years at London, How-
ard determined that the average amount of evaporation was thirty
inches annually, although the annual fall of rain at that place is
only twenty-five inches.
95. Rate, of Evaporation Variable. The rate of evaporation de-
pends greatly upon the exposure of the evaporating dish. If the
vessel be freely exposed to the sun and wind, the amount of evap-
oration will be greater than that which takes place from the sur-
face of the earth ; but if the vessel be very much sheltered, the
result will be too small. The total evaporation from the earth's
surface in a year must be equal to the total precipitation in the
form of rain, snow, dew, etc.; but hitherto the relative amount of
evaporation from the ocean and from the land has not been ac-
curately determined.
56
METEOROLOGY.
Evaporation is accelerated by a brisk wind. The vapor which
rises from water and pervades the surrounding air, is carried off
by a wind which brings a fresh body of air in contact with the
water.
96. Evaporation at all Temperatures. Evaporation proceeds at
all temperatures, even the lowest. If during the coldest weather
of winter we weigh a lump of ice, and then expose it in the open
air on a clear day upon the north side of a building, we soon find
that the ice has lost weight. So also in winter a large mass of
snow often disappears without any appearance of liquefaction.
Evaporation proceeds, although at a diminished rate, even when
the thermometer stands below zero of Fahrenheit.
HYGROMETERS.
97. Any instrument adapted to measure the amount of moist-
ure in the air is called a hygrometer. An instrument which sim-
ply indicates changes of humidity is called a hygroscope. All or-
ganic substances are affected by moisture, which generally in-
creases their dimensions. Thus porous wood expands with an
increase of moisture, and contracts when deprived of moisture.
A strip of such wood may be employed as a hygroscope, but it is
not sufficiently sensitive for any useful purpose. A thin shaving
Fie. 21. f whalebone, or a single hair, is much more
sensitive. A hair will vary to the amount
of one fiftieth of its entire length by simple
change of moisture.
98. Saussurds Hygrometer. This instru-
ment consists of a metallic frame, to the top
of which is attached one extremity of a
hair, E F, whose lower extremity is wound
around a small wheel. To the axis of this
wheel is attached an index, C, whose ex-
tremity traverses a graduated arc. When
the moisture of the air increases, the hair
lengthens and the index descends; when
the moisture decreases, the index rises.
To graduate the instrument, we determine
two fixed points, viz., that of saturation and
THE MOISTURE OF THE AIR.
57
that of extreme dryness. To obtain the first point, we place the
instrument under a close vessel containing water, and mark the
position of the index. For the point of absolute dryness, we place
the instrument in a dry vessel containing quick-lime. The in-
terval between these fixed points is divided into one hundred
parts, which are called the degrees of the hygrometer.
In Babinet's hygrometer, the variations in the length of the
hair from day to day are measured by means of a microscope
attached to the frame of the instrument.
The hair hygrometer is a very imperfect instrument. It is
essential to a perfect hygrometer that two instruments made in-
dependently in distant countries should agree with each other.
But it is found that two instruments made with different hairs,
or with hairs differently prepared, may differ in their indications
five degrees. Even the same hygrometer undergoes a gradual
change, since the length of the hair increases from the continued
pressure of the weight which it supports. This instrument is
therefore so unsatisfactory that it has been entirely discarded in
scientific researches.
99. Dew-point Defined. The amount of vapor in the air may
be measured with great accuracy by noting the temperature at
which moisture begins to be condensed on a cold vessel. The
moisture thus deposited is called dew, and the temperature at
which this deposition begins is called the dew-point. The dew-
point, then, may always be determined by cooling a metallic ves-
sel until dew begins to appear upon its surface, and noting by a
thermometer the temperature of the vessel. This experiment,
however, requires considerable time, and various contrivances
Fig 22. h ave k een proposed to facili-
E tate it.
100. Bachds Hygrometer.
When it is required to determ-
B ine the dew-point frequently at
short intervals, the following
apparatus, invented by Profess-
or Bache, is very convenient.
A small metallic box, A, is fill-
ed with a mixture of salt and
58
METEOROLOGY.
snow, by which means its temperature is reduced to about zero.
From the side of the box projects a polished metallic bar, B,
having on its upper side a groove, C, containing mercury, in
which is immersed the bulb of a thermometer, D, which is sus-
pended from a support, E, so that the thermometer is movable
along the groove. One end of the bar, B, has a very low temper-
ature, while the other is but little below that of the surrounding
air. That portion of the bar whose temperature is below the
dew-point will be covered with moisture, while the other part
will be dry, and the two portions will be separated by a well-
defined bounding line. By placing the bulb of the thermometer,
D, opposite to this line, we may immediately determine the tem-
perature of the dew-point. When only an occasional observa-
tion of the dew-point is desired, this instrument is inconvenient,
because it requires considerable time to prepare it for experi-
ment.
101. Darnell's Hygrometer. This instrument is more convenient
than Bache's when only an occasional observation is to be made.
It consists of two glass bulbs, A and B, about
three fourths of an inch in diameter, con-
nected by a small tube, which is bent in two
places at right angles, and the whole is her-
metically sealed. The lower bulb, A, which
is made of dark-colored glass, is about half
filled with ether, and -contains a small ther-
mometer, T. The upper bulb, B, is covered
with a piece of fine muslin. If we pour
ether upon the ball B, the ether will rapidly
evaporate and produce cold, condensing the
vapor of ether which, previously filled the
ball B. The ether in the ball A, being relieved from the press-
ure of the vapor upon it, now rapidly evaporates, and its tempera-
ture falls, as is shown by the "depression of the thermometer, T.
If this depression be sufficient, the vapor of the atmosphere will
be condensed on the outside of the ball, and the state of the ther-
mometer, T, at that instant will indicate the dew-point.
This instrument is ordinarily very convenient for use, but when
the atmosphere is very dry it requires ether of the best quality,
and some dexterity in manipulation, to obtain a deposit of dew.
Fig. 23.
THE MOISTUEE OF THE AIK.
59
Fig. 24.
102. Wet-bulb Thermometer The hy-
grometer which, on accotfnt of its con-
venience, is now most generally used, is
the wet-bulb thermometer. It consists
of a common thermometer, with its bulb,
B, Fig. 24, covered with a piece of thin
muslin, and kept constantly moistened
with water by means of loose cotton
threads communicating with a cup of
water, A. The evaporation of the water
produces cold, and this thermometer ha-
bitually stands lower than a dry ther-
mometer similarly exposed. This de-
pression strictly measures only the evap-
orating power of the air ; yet, as the lat-
ter depends upon the amount of moist-
ure present in the air, the depression
of the wet-bulb thermometer indirectly
measures the humidity of the air.
103. Dew -point deduced from the Wet
Bulb. The difference between the tem-
perature of the air and that of the dew-point is called the comple-
ment of the dew-point. When the air is saturated with moisture
the complement of the dew-point is zero.
From the comparison of a great number of observations with.
Daniell's hygrometer, combined with simultaneous observations
of the dry and wet bulb thermometers, a method has been dis-
covered by which the dew-point may be deduced from the read-
ings of the wet-bulb thermometer. The ratio of the complement
of the dew-point to the depression of the wet-bulb thermometer
is a variable one. When the temperature of the air is 53, the
difference between the readings of the dry and. wet bulb ther-
mometers is one half the complement of the dew-point ; at 33 it
is one third; at 26 it is one sixth; and at lower temperatures
the ratio is still less. Table XXV., p. 273, furnishes the factors
by which the dew-point may be deduced from the readings of the
wet-bulb thermometer for any temperature of the external air.
60 METEOROLOGY.
104. Weight of Vapor determined. The elastic force of the va-
por present in the air, that is, the pressure which it exerts, is in-
dicated by the dew-point. Dalton constructed a table showing
for every degree of temperature the corresponding elastic force of
vapor, and this table has since been brought to great perfection.
When the
dew-point
is at
f32<
40 C
50 C
60 C
70 C
80 C
the pressure f 0.181 inch in height,
of the vapor
in the air
will sustain
a column
of mercury
A more extensive table is given on page 276. With the as-
sistance of such a table, from the indications of either of the hy-
grometers already described, we can deduce the elastic force of
the vapor present in the air, and hence we may determine its
weight.
105. How the Humidity of the Air is denoted. The character of
a climate, whether it is to be regarded as dry or humid, does not
depend simply upon the absolute amount of vapor present in the
air. Its humidity is expressed by the ratio which the amount of
vapor actually present in the air bears to the amount which the
air would contain if it was saturated. Thus, suppose the temper-
ature of the air to be 60, while the dew-point is 50. The press-
ure of the vapor in the air according to the table in Art. 104, is
-.36 inch ; but if the atmosphere were saturated with moisture,
that is, if the dew-point were 60, the pressure of the vapor would
be .52 inch. Hence the air contains 70 per cent, of the amount
of vapor which it would contain if it were saturated, and its hu-
midity may be represented by the number 70. See Table XXVI.
In this manner we find that at Philadelphia the average hu-
midity of the 'air is 73 ; that is, the air, on an average, contains
about three fourths of the vapor required for its saturation. At
St. Helena, the mean humidity of the air is 88, while at Madrid
it is only 62. Near great bodies of water the atmosphere gen-
erally contains more moisture than it does over the interior of
continents.
106. Extremes of Humidity. In different localities and at dif-
ferent times we meet with every variety of condition, from per-
THE MOISTURE OF THE AIR.
61
feet humidity to almost absolute dryness. In ordinary pleasant
weather, the complement of the dew-point is from 10 to 15.
Occasionally, at Philadelphia, it amounts to 25 or 30, and it has
been observed as high as 45. ' In India, the temperature of the
air has been known to rise 61 above the dew-point; and it is
said that in California the temperature has been observed 78
above the dew-point, in which case the atmosphere contained
only six per cent, of the vapor required for its saturation.
107. Diurnal Variation in amount of Vapor. The amount of
vapor present in the air is subject to great fluctuations, some of
which are periodical. One of these fluctuations has a period of
one day. At Philadelphia, the amount of vapor present in the
air is least about an hour before sunrise, from which time the
amount increases uninterruptedly until a little before sunset,
rig. 25. after which it decreases un-
interruptedly until the next
morning. The mean diur-
nal variation amounts to one
eighth part of the average
amount of vapor. Fig. 25
shows the diurnal variation at Philadelphia, the numbers on the
left indicating, in inches of mercury, the pressure of the vapor at
the hours given at the bottom of the figure.
The cause of this variation is obvious. As the sun rises and
the heat of the day increases, more water is evaporated from the -
ocean and the moist earth, and the amount of vapor in the air in-
creases. During the night a portion of this vapor is condensed
in the form of dew and hoar-frost ; that is, the amount of vapor
present in the air is least a short time before sunrise, and greatest
a short time before sunset.
108. Annual Variation in amount of Vapor. There is an an-
nual variation in the amount of vapor present in the air. At
Philadelphia the vapor present in the air is least in January and
greatest in July ; the amount in July being more than four times
as great as in January. This is evidently the effect of the sun's
heat producing a more rapid evaporation in summer than in
winter.
m't 2h 4 6 8 10 noon. 2h 4 6 8 10 m't
62
METEOROLOGY.
109. Influence of Elevation. The humidity of the air generally
diminishes as we rise above the surface of the earth. From a
large number of balloon ascensions near London, it has been
found that when the sky is clear {here is a slight increase of hu-
midity until we reach an elevation of 3000 feet, and afterward a
gradual decrease to 23,000 feet, where the humidity is expressed
by 16. When the sky is overcast the increase of humidity up
to the height of 3000 feet is very slight, after which there is gen-
erally a decrease, but very irregularly up to 23,000 feet. At the
highest elevations at which observations have been made, the air
has never been found entirely free from vapor of water.
110. Diurnal Variation of the Barometer explained. We have
seen, Art. 22, that the height of the barometer is subject to a di-
urnal fluctuation. This fluctuation is a complex effect, depend-
ing partly upon a change in the amount of vapor, and partly upon
a change in the weight of the gaseous atmosphere. It is only
when we separate these two effects that their cause can be clearly
understood. We have seen that there is a diurnal variation in
the amount of vapor present in the air, and that this variation de-
pends upon the heat of the sun. If from the entire height of the
barometric column we subtract the pressure of the vapor, the re-
mainder will represent the pressure of the gaseous portion of the
atmosphere.
).C2
Fig. 26.
111. Diurnal Variation
of Pressure of the Gaseous
Atmosphere. At Philadel-
phia the pressure of the gas-
eous atmosphere is greatest
1 t about an hour after sun-
s
-^
|
^
\
.55
.56
.54
.52
n
\
".
~/"
\
j
\
_/
\
/
\
>
i't 2h 4 6 8 10 noon. 2h 4 6 8 10 m
rise, from which time the pressure diminishes uninterruptedly un-
til about 4 P.M., after which the pressure increases uninterrupt-
edly until the next morning, as shown by the curve in Fig. 26.
This fluctuation is evidently the effect of the sun's heat. As
the heat of the day increases, the atmosphere becomes warmed, it
expands in volume, and swells up to a height greater than it had
during the night. The upper portion therefore flows off laterally
in all directions to places where the height of the atmosphere is
lessj by which means the pressure of the air is diminished, and
THE MOISTURE OF THE AIR.
63
the barometer falls. During the night the temperature declines,
the air contracts in volume, its height sinks below that which
existed during the day, and the defect is supplied by air which
flows in from regions where a higher temperature prevails. The
pressure of the air is thereby increased, and the barometer again
rises.
112. Why the Barometer shows two daily Maxima. The press-
ure of the vapor and that of the gaseous atmosphere have each
but one daily maximum and minimum. But the motions of the
vapor and of the gaseous atmosphere following different laws,
and their maxima occurring at nearly opposite hours of the day,
the sum of their effects, or the total pressure as shown by the ba-
rometer, exhibits two daily maxima and minima, which occur at
different hours from the maximum and minimum of tempera-
ture.
113. Annual Variation of Pressure of the Gaseous Atmosphere.
At Philadelphia the pressure of the gaseous atmosphere is great-
est in January, from which time it diminishes uninterruptedly
until July, after which it increases uninterruptedly until the suc-
Fig.2t. ceeding January. A similar re-
mark is applicable to nearly ev-
ery part of the globe, with this
exception, that the difference be-
tween the winter and summer
pressures is very unequal in dif-
ferent countries. At Philadel-
phia and Boston this difference
amounts to half an inch, but
throughout nearly the whole
FMAMJJASOND of Central Asia the difference
amounts to an entire inch and upward, while under the equator
it is scarcely appreciable.
Fig. 27 shows the annual curve of pressure of the gaseous at-
mosphere at Pekin, in China.
This fluctuation in the weight of the gaseous atmosphere is
due to the influence of the sun's heat, combined with the effect of
the excessive rains on the mountain ranges of Central Asia. As
the sun advances from the southern to the northern hemisphere,
30.2
30.0
29.8
29.6
29.4
29.2
29.0
9 R
-v
^
\
/
\
,
\
1
\
i
-
\
i
\
/
\
I
\
l
\
\
/
\
/
2
6-i METEOROLOGY.
the latter is heated and its atmosphere expands, while the former
is cooled and its atmosphere contracts. The atmosphere in the
northern hemisphere being thus rendered higher than in the
southern, the excess of air in the northern hemisphere flows over
to the southern ; in other words, the barometer is lowest in the
hemisphere where summer prevails, and highest in that where
winter prevails. The amount of this effect depends partly upon
the annual range of the thermometer. Over the great desert of
China the air in summer becomes unusually heated, the air above
it is expanded to a corresponding height, and flows off to the
colder portions of the southern hemisphere.
The remarkably low state of the barometer which prevails in
summer throughout a large part of Asia is. probably due, in a
great degree, to the excessive rains on the mountain ranges of
Central Asia, in accordance with a general principle which will
be developed in Chapter VI.
In the temperate zones of Europe and America during sum-
mer, the increase in the amount of vapor is nearly equal to the
loss of weight sustained by the gaseous atmosphere, so that the
absolute height of the barometer remains nearly the same through
every month of the year.
CHAPTER IV.
THE MOTIONS OF THE ATMOSPHERE.
114. WIND is air in motion. The movements of the air are
proverbially variable and seemingly capricious, and it has been
supposed that they are not subject to any law. "We shall find,
however, that the winds are subject to laws as definite as those
of the barometer or thermometer.
The direction of the wind is designated by the point of the ho-
rizon from which it blows. This direction is commonly indicated,
as in navigation, by the terms north, north by east, north-north-
east, etc. If we wish to indicate the direction with greater pre-
cision, we may employ degrees of azimuth, as in astronomy ; thus
a wind designated by 1ST. 13 E. comes from a point 13 degrees to
the east of north. Sometimes it is found convenient to designate
THE MOTIONS OF THE ATMOSPHERE.
65
the direction by degrees of the horizon reckoned continuously
from up to 360.
For the purpose of investigating the laws which govern the
movements of the atmosphere, we require some means of measur-.
ing both the direction and velocity of the wind.
115. How to determine the Direction of the Wind. Any instru-
ment for measuring the direction of the wind near the earth's
surface is called an anemoscope. The simplest anemoscope is the
common vane. In order that the vane may give reliable results,
particular care is required in its construction. A vane usually
consists of a flat vertical plate, turning freely about an upright
spindle. That part of the vane which is before the spindle, and
is turned toward the wind, is called the head ; the rest of the
vane is called the tail. If a vane were made in the form of a
rectangular plate of uniform thickness, and balanced upon its
centre of gravity, the action of the wind upon the head would
just neutralize its action upon the tail, and the vane would have
no directive power. The directive power of the vane depends
simply upon the difference of the wind's action upon the head and
tail. The tail should therefore present a large amount of surface,
and the head a small surface. Moreover, in order to maintain
the spindle in an upright position with the least friction against
its supports, the two ends of the vane should exactly balance
each other.
The vane represented in Fig. 28 is designed to fulfill these
Fig. 28. conditions. It
consists of a rod
of iron, A B,
three fourths of
an inch in di-
ameter, to one
end of which is
attached a pine
board about
half an inch thick, one foot wide, and eleven feet long, and bal-
anced by a sphere of iron or lead, A, attached to the other end
of the rod. To give to the instrument more steadiness, the wood-
en part is made to consist of two boards inclined at a small angle,
as shown in the section E G. The vane is attached to an upright
E
METEOROLOGY,
spindle, H K, which revolves freely, and the direction of the wind
is measured by a graduated circle attached to the spindle.
116. Self-registering Anemoscope. An anemoscope may be ren-
dered self - registering in the following manner: Place a cylin-
drical vessel beneath the revolving shaft C C x ,
which carries the vane AB, and let it be di-
vided into a large number of equal compart-
ments, as shown in Fig. 29. Attach to the
shaft a funnel, D, filled with sand so arranged
that in every position of the funnel the sand,
as it flows out, shall fall into one of the com-
partments of the cylindrical vessel. The
amount of sand which collects in the several
compartments will indicate how long the
vane is maintained in the corresponding po-
sitions. If there are eighteen compartments,
each will correspond to an arc of twenty de-
grees.
A second series of compartments may be arranged in the same
cylindrical vessel, and a second funnel, D', be arranged like the
first, for the purpose of balancing the weight of D.
117. Woltmanris Anemometer. An instrument designed to
measure the velocity or force of the wind is called an anemometer.
Woltmann's anemometer consists of a small wind -mill, to
whose axis is attached an endless screw, which imparts motion to
a toothed- wheel, while the number of revolutions is shown by an
index. An observation consists in determining the number of
revolutions made in one minute, when the sails are exposed to
the action of the wind. In order to deduce the wind's velocity
from such an observation, upon a calm day we travel with the
apparatus on a carriage or a rail-car, and observe the number of
revolutions made in going a known distance in a given time.
The effect will be the same as if the instrument was at rest and
the air in motion. In this manner we may construct a table
showing the velocity of the wind corresponding to a given num-
ber of revolutions of the sails per minute.
118. WheweWs Anemometer. Whewell's anemometer, Fig. 30,
THE MOTIONS OF THE ATMOSPHERE.
67
V
consists also of a
small wind -mill,
with complete ap-
paratus for regis-
tering the total
effect of the
wind. The mill
is mounted upon
a vertical cylin-
der, C, about two
feet high, and
four inches in
diameter, and
around the cylin-
der is coiled a
sheet of paper,
ruled vertically,
to indicate the
points of the
compass. The
revolution of the
arms of the wind-
mill, F, gives mo-
tion to an end-
less screw, which
causes a pencil, P, to descend along a vertical rod, and traces an
undulating line upon the paper cylinder. When the pencil has
reached the bottom of the paper (which ordinarily requires an in-
terval of a 1 ", least twenty -four hours), a new sheet of paper must
be applied to the cylinder, and the pencil set back again at the
top. The direction of the wind is indicated by the portion of
the sheet upon which the pencil line is traced, and its velocity
by the rate of motion of the pencil. Thus this instrument regis-
ters the amount of the wind's progress for every point of the
compass.
119. Robinson? s Anemometer. Robinson's anemometer, Fig. 31,
consists of four equal metallic cups, A, B, C, D, in the form of
hemispheres, attached to two arms which cross each other at right
angles, and are supported so as to turn freely about a vertical
METEOROLOGY.
Fig. 31.
axis, E. The
base of each
hemispherical
cup is in a ver-
tical position ;
and since the
action of the
wind upon the
concave side
of one of these
cups is greater
than its action upon the convex side, a moderate breeze is suffi-
cient to maintain the arms in continuous rotation. Dr. Eobinson
has proved that (making no allowance for friction) the centre of
each hemisphere moves with one third the velocity of the wind,
and thus this instrument measures directly the wind's velocity.
The axis E carries an endless screw, which gives motion to a
series of wheels which register the wind's progress up to 1000
miles.
Fig. 32.
120. Osier's
Anemometer.
Osier's anemom-
eter, Fig. 32, reg-
isters both the di-
rection and force
of the wind. It
consists of a large
vane,Y, support-
ed upon a revolv-
ing spindle. At-
tached to the
lower extremity
of this spindle
is a small pinion
working in a
rack, ef, which
slides backward
and forward as
the wind turns
THE MOTIONS OF THE ATMOSPHERE.
69
the vane, and to this rack is attached a pencil, A, which presses
against a horizontal sheet of paper, ruled to indicate the points of
the compass. This sheet of paper is moved forward uniformly
by clock-work, C, at the rate of half aa inch per hour, so that
while the vane oscillates to and fro, the direction is registered on
the sheet of paper, which also indicates the time at which each
change took place. Fig. 33 shows the register made by the pen-
cil in one day.
Fig. 33.
Mel
121. How the Wind's Force is Measured. In order to measure
the wind's force, a brass plate, T, two feet square is attached to
the vane, so as always to be presented perpendicularly to the ac-
tion of the wind. To the back of this plate is attached a spiral
spring, which is compressed by the pressure of the wind against
the plate, and the degree of compression of the spring affords a
measure of the wind's force. By means of a connecting wire this
square plate gives motion to a second pencil, B, which at each in-
stant registers upon the same sheet the wind's force. At the end
of twenty -four hours a new sheet must be applied, and thus each
sheet indicates the direction and force of the wind for each in-
stant during a period of twenty -four hours. The undulating line
at bottom of Fig. 33 shows the register of the wind's force. The
irregular line at the top of the same figure shows the amount of
rain registered by an arrangement not here represented.
122. How Velocity is deduced from Pressure. The indications
of Osier's anemometer are expressed in pounds of pressure per
square foot. In order to deduce from these results the velocity
of the wind in miles per hour, we require a table showing the
velocity of the wind corresponding to different pressures. The
following table shows the velocity of the wind in miles per hour,
corresponding to the pressure upon a square foot of surface, ac-
cording to the experiments of Smeaton.
70
METEOROLOGY
Velocity.
Miles.
Pressure.
Pounds.
Velocity.
Milea.
Pressure.
Pounds.
Velocity.
Miles.
Pressure.
Pounds.
Velocity.
Miles.
Pressure.
Pounds.
1
2
3
4
5
0.005
.020
.044
.079
.123
6
7
8
9
10
0.177
.241
.315
.399
.492
11
12
13
14
15
0.595
0.708
0.831
0.964
1.107
16
17
18
19 '
20
1.260
1.422
1.594
1.776
1.968
It will be seen that the wind's force varies as the square of its
velocity. Thus, when the wind's velocity is 20 miles per hour,
its force is four times as great as that of a wind blowing 10 miles
per hour.
123. Wind's Force represented ly a Scale. When an observer
has no anemometer, he should estimate the force of the wind as
accurately as he is able, and it is recommended to indicate the
wind's forge by a series of numbers from 1 to 10, according to the
following scale :
No.
Character.
Velocity
in Miles
per
Hour.
Force in
Pounds
per square
Foot.
No.
Character.
Velocity
in Miles
pei-
Hour.
Force in
Pounds
per square
Foot.
1
2
3
4
5
Just perceptible.
Gently pleasant.
Pleasant brisk.
Very brisk.
High wind.
2
4
121
25
35
0.02
0.08
0.75
3.00
6.
6
7
8
9
10
Very high wind.
Strong gale.
Violent gale.
Hurricane.
Most violent hurricane.
45
60
70
80
100
10
18
24
31
49
The numbers in the preceding table have been deduced from
a great variety of experiments. One mode of experimenting
consists in noting the effects produced by a motion of the ob-
server at different velocities during a clear day, as, for example,
upon a railway train. Another method consists in measuring
the velocity with which light objects, like a lock of cotton, are
carried along by the wind.
124. Average Velocity of the Wind. Observations with accurate
anemometers have been made at several places in Europe, and at
a few stations in America. It is found that at Philadelphia the
mean velocity of the wind during the entire year is eleven miles
per hour, being least in summer, when it is nine miles per hour,
and greatest in winter, when it is fourteeen miles per hour. At
Toronto, the average velocity of the wind is nine miles per hour.
At Plymouth, England, the average velocity of the wind is
nine miles per hour; at Oxford and Greenwich it is ten miles
per hour; and at Liverpool it is thirteen miles per hour. On
THE MOTIONS OF THE ATMOSPHERE.
71
the ocean, the mean velocity of the wind, as deduced from the
average rate of sailing of ships, is estimated at eighteen miles per
hour.
According to observations at Philadelphia, the mean velocity
of the wind is least about sunrise. After sunrise the velocity
rapidly increases, and becomes greatest at 2 P.M., after which it
rapidly declines till 8 P.M.,
from which time it changes
but little until sunrise, the
pressure at noon being fully
double that at midnight.
m'tau 4 G * iuuoou..u 4 6 6 luiat Fi & 34 ^presents the av-
erage force of the wind at
Philadelphia for each hour of the day, expressed in pounds press-
ure per square foot, as shown on the left of the figure.
125. Mean Direction of the Wind. Suppose a current of air
coming from the north passes the point
C, Fig. 35, with a velocity v, continued
for a time 2, the amount of air which
passes will be measured by vt. If an-
other current subsequently coming from
the south moves with a velocity v' dur-
ing a time 2', the amount of air which
passes will be measured by v't' t and the
resulting motion will be the same as if a
mass of air vtv't' passed the point C
during the time t+t'. If then N and S represent masses of air
coming from the north and south, the resulting motion will be
represented by N S. In like manner, if we consider two winds
coming successively from the east and west, the resulting motion
will be represented by E W.
If we represent these results N S and E W by the lines
C A, C B, we may easily determine their resultant, C D. The
angle 'V which it makes with the meridian N S is given by the
formula
DA CB E-W
A wind blowing from any intermediate point may be resolved
into two others, one of which coincides with a meridian, and the
72 METEOKOLOGY.
other is perpendicular to it. A wind from the northeast may be
resolved into two others, one in the direction of C S, represented
by NE cos. 45, and the other in the direction of C W, also equal
to NE cos. 45. A wind from the northwest, southeast, or south-
west may be resolved in a similar manner. If then we consider
.the winds from the eight principal points, and regard motion from
N" to S, or from E to W as positive, while we regard motion from
S to N, or from "W to E as negative, we shall have
E-W+(NE + SE-NW-SW) cos. 45
~ N-S + (NE + NW-SE-SW) cos. 45 '
The mean velocity of the resulting wind is given by the
formula
_ CB _E- W+(NE + SE-NW-SW) cos. 45
: r^p : =p= .
sin. V sin. V.
In most meteorological registers the velocity of the wind is
not measured, or perhaps not even estimated, and we are obliged
to assume that the average velocity of the wind is the same for
all points of the compass, in which case N, S, E, etc., in the pre-
ceding formula, represent simply the number of times that the
wind has blown from each of these points.
The assumption that the winds from the different points of the
compass blow with the same average velocity is not entirely cor-
rect, and the error which may result from its adoption can only
be determined by careful observations with an anemometer.
When the direction of the winds is given for more than eight
points of the compass, we may resolve each wind separately into
two rectangular components by means of a traverse table, in the
same manner as we resolve a traverse in navigation. We then
subtract the sum of all the southerly motions from the sum of all
the northerly, and represent this difference by C A. We also
subtract the sum of all the westerly motions from the sum of all
the easterly motions, and represent this difference by C B. The
resulting direction will then be given by the equation
tang.V=g|.
126. Wind's Progress represented ~by a Polygon. A geometrical
figure to represent the total progress of .the wind for an entire
year may be constructed as follows : Draw the line A B, Fig. 36,
to represent a northwest direction, and, assuming any convenient
THE MOTIONS OF THE ATMOSPHERE. 73
Fig. se. scale, make the length of A B to cor-
respond to the northwest motion of the
wind for the given time. Draw B C
to represent a west direction, and make
its length to correspond to the west
motion of the wind upon the same scale
as the preceding. In the same manner
draw C D for the southwest wind, and
so on for each of the other directions of the wind, and suppose
the last line representing the north winds reaches to I. Then
join A I, and this line will represent the direction and rate of the
wind's total progress during the period embraced in the observ-
ations. The annexed figure represents the relative frequency
of the different winds, according to observations made during
twenty-five years, at about thirty academies in the State of New
York.
By a series of observations with Osier's anemometer, it is found
that at Philadelphia the actual progress of the wind is toward a
point a little north of east, and at the average rate of about four
miles per hour, or one hundred miles per day.
127. Observations of the Wind's Direction. Although observa-
tions with accurate anemometers are not very numerous, yet ob-
servations of the common vane have been made to such an ex-
tent as to determine (if not the velocity of the wind) at least its
average direction for nearly every part of the globe. In the
northern hemisphere we have observations from about six hund-
red stations on land, at which the wind's direction has been re-
corded for periods varying from a few months to more than half
a century, and amounting in the aggregate to nearly three thou-
sand years of observations. We have also the log-books of ships
which have penetrated nearly every sea, and which have been
collected at the Observatory of Washington, furnishing more than
three millions of observations, and embracing in the aggregate a
period of more than three thousand years of observation. These
materials are sufficient to indicate with considerable precision the
average direction of the wind for every part of the northern hem-
isphere, whether over the continents or the ocean, at least as far
as latitude 60. Beyond latitude 60 observations are much less
numerous ; nevertheless, the observations which we have from
74 METEOROLOGY.
this region are pretty uniform in their indications. In the south-
ern hemisphere our materials from the continents are less abund-
ant than in the northern hemisphere, but observations from the
ocean are very numerous.
128. Three Systems of Winds. When we project all these ob-
servations upon a map of the earth, we find that the winds are
naturally divided into three grand systems.
1. The equatorial system.
2. The winds of the middle latitudes ; and,
3. The polar winds.
129. The Trade Winds. Throughout nearly the entire equato-
rial region of the globe, whether over the land or on the ocean,
the winds preserve a remarkable uniformity; on the northern
side of the equator blowing almost invariably from some north-
east quarter, and on the southern side of the equator from a
southeast quarter. This system of currents is called the trade
winds.
In the Atlantic Ocean, the N. E. trades extend on an average
from about latitude 7 to latitude 29 1ST., while the S. E. trades
extend to latitude 20 S. Between the 1ST. E. and S. E. trades is a
belt of calms or variable winds, extending at different seasons
from 150 to 500 miles in breadth, and the centre of this belt is
about five degrees north of the equator.
Throughout the northern half of the belt of the N. E. trades,
the average direction of the winds is from N. 60 E. ; but near
latitude 10 they veer more to the east, and near their southern
limit their direction is almost exactly east. The average direc-
tion of the S. E. trades is from S. 54 E.
The boundaries of the trade winds vary somewhat with the
season of the year. During the summer they advance a few de-
grees farther toward the north, while in winter they recede some-
what toward the south. In spring, the centre of the belt of calms
is only 1 or 2 north of the equator, while in summer it rises to
latitude 9 or 10.
130. Winds in the Middle Latitudes. Beyond the borders of the
trade winds in either hemisphere we find the prevalent winds at
the earth's surface are from the west. In the northern hemi-
THE MOTIONS OF THE ATMOSPHERE.
75
sphere they blow from a point a little south of west, and in the
southern hemisphere from a point a little north of west. This
zone of westerly winds is from 25 to 30 in breadth ; the west-
erly motion being most decided in the middle of the belt, but
gradually diminishing as we approach the limit on either side.
Throughout the middle latitudes of the United States, the aver-
age direction of the wind is from S. 80 W. ; and the easterly
winds are to the westerly in about the ratio of 2 to 5.
So, also, between the parallels of 40 and 60, in the southern
hemisphere, the prevalent direction of the surface-winds is from
about N. 73 W. ; and the easterly winds are to the westerly as
1 to 5.
131. Direction of the Polar Winds. Beyond the parallel of 60
the general tendency of the winds is almost, without exception,
toward the equator ; but in some places the inclination is toward
the west, and in others toward the east. In the northern hemi-
sphere, beyond the parallel of 60, northeast winds generally pre-
76 METEOROLOGY.
vail, but in many districts the prevalent winds are from the
northwest. Fig. 37 represents for every latitude the prevalent
direction of the winds at the earth's surface.
132. The Surface Winds. The winds here described are the
winds which prevail at the earth's surface. They also extend to
a considerable height, as is shown by observations on the sum-
mits of mountains, and by the observed direction of the clouds.
It is believed that the directions here given are the average
directions of the wind, as high as two miles from the earth's
surface, and perhaps somewhat higher, including nearly (and
perhaps fully) one half the weight of the entire atmosphere.
Above this height we find an entirely different system of winds
to prevail.
133. Motion of the Upper half of the Atmosphere. It is evident
that over any parallel of latitude, the northerly motion of the en-
tire mass of the atmosphere must be exactly equal to its south-
erly motion, otherwise the atmosphere would be gradually with-
drawn from certain portions of the earth, and would accumulate
over other portions. If, then, in the equatorial regions, we find
the average motion of the lower half of the atmosphere is toward
the equator, the average motion of the upper half must "be from
the equator; and we actually find that in the northern hemi-
sphere, within the region of the trade winds, the upper half of
the atmosphere moves from the southwest. This is proved by
the eruptions of volcanoes, and by observations on the summits
of mountains.
134. Evidence derived from Volcanoes. Within the limits of the
trade winds are several volcanoes, which sometimes eject ashes to
a great height, and these ashes indicate the direction of the stra-
tum of air into which they rise. In the West Indies, in latitude
15, on the island of St. Yincent, is a volcano which in 1812
emitted a vast quantity of ashes. A large mass of ashes fell
upon the island of Barbadoes, which is ninety miles east of St.
Yincent, although between the two islands the trade winds con-
tinually blow with such force that it is only by making a very
long circuit that a ship can sail from the latter to the former.
The ashes were doubtless transported by an upper current blow-
THE MOTIONS OF THE ATMOSPHERE. 77
ing in a direction contrary to that which prevailed at the surface
of the sea.
A similar phenomenon was observed in January, 1835, on the
great eruption of the volcano of Coseguina, in latitude 13 north,
on the shores of the Pacific. Some of the ashes fell upon the
island of Jamaica, at the distance of 700 miles in a direct line
northeast from the volcano. At the same time, ashes were car-
ried in the contrary direction westward, and fell upon a ship in
the Pacific more than 1200 miles distant.
135. Fine Dust transported l>y Winds. At several places in
Southern Europe, Lyons, Genoa, etc., there has repeatedly fallen
a fine dust, which was once supposed to come from the sandy
plains of Africa ; but Ehrenberg, by examination with the mi-
croscope, has shown that this dust contains microscopic organ-
isms and dried infusoria. Among them he has found several
South American species belonging to the valleys of the Oronoco
and the Amazon, and which have not been found in any other
part of the world.
We must then conclude either that this dust came in part from
South America through the upper regions of the atmosphere, or
these species exist in some other part of the world hitherto un-
discovered. There is little doubt that the former is the true ex-
planation, and we conclude that this dust from South America was
elevated into the upper regions of the atmosphere, where it met
a current from the southwest, in which it was transported a dis-
tance of over five thousand miles before it fell again to the earth.
136. Winds on the Summits of Mountains, Observations on
the summits of mountains indicate the same westerly current in
the upper regions of the atmosphere. Upon the summit of
Mauna Kea, on one of the Sandwich Islands, at the height of
13,951 feet, there is uniformly found a blustering wind from the
southwest, while the regular trade wind from the northeast is
blowing at its base.
The Peak of TenerifTe (12,205 feet in elevation) does not reach
the limit of the lower half of the atmosphere, yet the wind here
often blows from the southwest, and the clouds over the peak
constantly move from the southwest in a direction opposite to
the trade winds below. The traveler Bruce noticed a similar
fact on the mountains of Abyssinia.
78 METEOROLOGY.
137. Upper Current in the Middle Latitudes. Over the middle
latitudes, at an elevation of about 10,000 feet above the earth's
surface, we find a stratum of air uniformly moving from the
northward. This is indicated by the following considerations :
1. In May, 1783, the famous volcano Hecla, in Iceland, com-
menced vomiting out smoke and ashes, which continued for a
period of more than two months. This smoke rose to a great
height in the atmosphere, and spread over nearly the whole of
Europe, forming what was called a dry fog. It appeared first in
the northwest part of Europe, gradually extending southward
and eastward into Italy and even into Syria, which seems to in-
dicate that during these two months there was an upper current
of atmosphere moving from the northwest, all the way from Ice-
land to Syria.
During the same period a similar dry fog extended over a great
part of North America, which seems to indicate the existence of
another and probably higher current blowing steadily from the
northeast.
During another eruption of this volcano in 1845, great quanti-
ties of the ashes fell on the Orkney Isles, and the ships navigating
the neighboring seas.
2. Aeronauts who have ascended to the height of 10,000 feet
in the middle latitudes, usually find the wind blowing from the
west ; and if they rise still higher, generally find the wind blow-
ing from a point somewhat to the north of west.
3. Clouds chiefly prevail in the lower half of the atmosphere,
and their average direction is about the same as that of the air at
the earth's surface ; but if, during a specially dry time, clouds are
observed at a great elevation, they are generally found to move
from a point north of west. According to six years' observations
at Philadelphia, when the dew-point was 25 below the tempera-
ture of the air, the mean direction of the clouds was from N.55 W.
138. Upper Current in the Polar Regions. It is evident that if
in the polar regions the general progress of the surface current is
toward the equator, there must be an upper current directed from
the equator.
139. Entire System of Atmospheric Circulation. We hence
conclude that a section of the atmosphere made by a meridian
THE MOTIONS OF THE ATMOSPHERE.
79
Fig. 38. would exhibit the system of
currents represented in Fig. 38,
where N denotes the north pole,
S the south pole, and E the
equator. Within the tropics
we find the surface current
moving toward the equator, and
the upper current from the
equator. In the middle lati-
tudes the surface current is
moving from the equator, and
the upper current toward the
equator. In the polar regions
the surface current is from the
poles, and the upper current
must therefore be toward the
poles.
This diagram merely indi-
cates whether the wind is mov-
ing to or from the equator.
Its easterly or westerly motion
could not be exhibited without
a modification of the diagram.
Throughout the equatorial belt of winds in the northern hemi-
sphere the surface current is from the northeast and the upper
current from the southwest ; between the parallels of 30 and 60
the surface current is from the southwest and the upper current
from the northward, while beyond the parallel of 60 the surface
current is toward the equator, and the upper current is from the
equator.
It is required to explain this system of atmospheric circulation.
140. Causes of the Winds. There are three important causes
which contribute to the production of wind.
1. Unequal atmospheric pressure.
2. Unequal specific gravity of the air; and,
3. The rotation of the earth.
Unequal pressure tends to produce motion in the atmosphere
For conceive of two vertical columns of air extending to the top
of the atmosphere, and imagine them to be connected near the
80
METEOROLOGY.
earth by a horizontal tube. If the weight of one column exceeds,
that of the other, the air must flow from the heavier to the light-
er column, in the same manner as when water stands at unequal
heights in the two arms of a recurved tube. The wind must
therefore blow from places where the barometer is highest toward
places where it is most depressed.
141. Unequal Specific Gravity of the Air. Unequal specific
gravity of the air may result from unequal temperature or from
unequal humidity.
Let ACB, Fig. 39, represent
an extended region of country,
a portion of which, near C, is
covered with sand, and becomes
intensely heated by the rays of
the sun, while at A and B the
earth is covered with vegeta-
tion. The air which rests upon
C, being more expanded than
the surrounding air, rises, and
its place is supplied by air flow-
ing horizontally from A and B
in the direction of the arrows. At the same time, tne column of
air, DEFGr, being expanded, and rising above the surrounding
atmosphere, overflows on each side in the direction of the arrows
HK, producing upper currents moving in a direction contrary to
the winds at A and B, and at a certain distance give rise to de-
scending currents to supply the place of the air which near the
earth's surface flows toward the heated region.
The motion here described may be illustrated by the following
experiment : If in winter we partially open a door communica-
ting between a hot and a cold room, and hold a lighted candle
near the top of the crevice, the flame will be bent outward from
the warm room, indicating a current of air from the hot to the
cold room ; but if we hold the candle near the bottom of the
crevice, the flame will be bent inward, indicating a current from
the cold to the hot room. "We thus discover that the air flows
out at the top of the heated room, while the cold air enters near
the floor. In a similar manner, the unequal warmth of the earth's
surface gives rise to currents of air of immense extent, the denser
air flowing under and displacing the lighter.
THE MOTIONS OF THE ATMOSPHERE. 81
The specific gravity of the vapor of water is only about two
thirds that of dry air at the same temperature and pressure;
and since it requires time for vapor to diffuse itself through the
atmosphere, an excess of aqueous vapor must give rise to cur-
rents in the atmosphere in the same manner as inequality of
temperature.
Even then, though the barometer may every where indicate the
same pressure, the wind at the surface of the earth will tend from
the colder to the warmer region, from the place where the atmos-
phere contains the least vapor to that where there is the most
vapor.
142. Mode of Propagation of Winds. The win4 is first noticed
near the heated column of air, and gradually extends to a greater
distance from it. As the air moves from A and B toward the
ascending column DEFGr, the air at A and B is rarefied, and
this rarefaction is communicated to the more distant air, and so
on ; that is, the wind is propagated in a direction contrary to that
in which it blows. Winds thus propagated are called winds of
aspiration. Winds which are propagated in the same direction
as that in which they blow are called winds of impulsion. Ex-
amples of both of these classes of winds are found in all great
storms, as will be shown in Chapter VI.
143. Rotation of the Earth. The rotation of the earth would
alone produce no permanent wind, because, if there were no other
disturbing causes, the atmosphere would, by friction upon the
earth's surface, soon acquire the same velocity of rotation as that
of the portion of the earth upon which it rested ; but the earth's
rotation materially modifies the operation of other disturbing
causes.
Since the earth is nearly a sphere, rotating upon its axis once
in twenty-four hours, the velocity of rotation of different parallels
of latitude is very different.
In latitude the velocity eastward is 1036 miles per hour.
u it ^50 a it it ti 1000 it a tt
ti ti g0o tt tt n it 397 ti ti tt
450 it tt tt it 732
g0 a ti it it 513
750 a (t tt t{ 263
F
82 . METEOROLOGY.
144. Relative Motion resulting from this Rotation. If a mass of
quiescent air from the parallel of 30 could be suddenly trans-
ported to the parallel of 15, it would have an easterly motion
103 miles per hour less than that of the parallel arrived at ; that
is, it would have a relative motion westward of 103 miles per
hour. So also, if a mass of air from the parallel of 15 could be
suddenly transported to the parallel of 30, it would have an east-
erly motion 103 miles per hour greater than that o'f the parallel
arrived at. That is, in general, if air is transported from the
equator toward the poles, it will have a relative motion eastward ;
and if air is transferred from a higher latitude toward the equa-
tor, it will have a relative motion westward.
145. Surface Winds in the Equatorial Regions. We have seen,
Art. 20, that near the parallel of 32 the mean height of the ba-
rometer is greater than in any other part of the earth, and is .283
inch greater than it is near the equator. Also, the mean temper-
ature of the surface air -at the equator is about 12 higher than it
is over the parallel of 32. For both of these reasons, the air
must tend from the parallel of 32 toward the equator; and if no
other force acted upon it, the motion of the air in either hemi-
sphere would be along a meridian toward the equator. But
while the air from the parallel of 32 in the northern hemisphere
flows toward the equator, it retains the easterly motion of the
place from which it started, and in its progress southward reaches
in succession parallels moving eastward more rapidly than itself.
It therefore drags continually behind ; that is, its motion with
reference to the earth's surface is toward the west. Under the
action of these two forces the progress of the air is toward the
southwest, and the exact path described will depend upon the
relative magnitude of the southerly and westerly motions.
A similar result must be produced on the south side of the
equator, and thus originates a system of currents flowing from
the northeast in the northern hemisphere, and from the southeast
in the southern hemisphere.
146. Upper Current in the Equatorial Regions. The mean tem-
perature of the surface air at the equator is considerably higher
than it is over the parallel of 32, while near the upper limit of
the atmosphere the temperature must be nearly the same in all
THE MOTIONS OF THE ATMOSPHERE. 83
latitudes. Now air is expanded by heat to the amount of -^ th
part of its bulk for each degree of the thermometer. The atmos-
phere over the equator must therefore rise somewhat higher than
it does over the parallel of 32, notwithstanding the difference in
the height of the barometer. If the earth were at rest, the air
thus expanded at the equator would flow over at the top, and
descend as along an inclined plane toward the middle latitudes.
But while in 'the northern hemisphere an upper current flows to-
ward the poles, it crosses in succession parallels of latitude whose
easterly motion is less than its own ; and since it retains the east-
erly motion which it had at the equator, it has a relative motion
from the west, which, combined with the first northerly motion,
carries it toward the northeast. Thus above the northeast trade
winds we find an upper current moving from the southwest.
. For a similar reason, in the southern hemisphere, above the
southeast trades, the upper current moves from the northwest.
147. The Surface Wind in the Middle Latitudes. Over the par-
allel of 32 the mean pressure of the air is .558 inch greater than
over the parallel of 64, and therefore at the earth's surface the
air tends from the parallel of 32 toward the pole. The air in
latitude 32 is indeed warmer, and therefore lighter than it is near
the poles, and this creates a tendency of the surface current from
the poles toward the equator ; but the effect of the . increased
pressure of the air near the parallel of 32 is greater than that
of its diminished density, and the air actually moves toward the
poles.
But, while in the northern hemisphere the air from the parallel
of 32 moves northward, it crosses successively parallels of lati-
tude whose easterly motion is less than fits own; and since it re-
tains the easterly motion which it had. at starting, it has a relative
motion from the west, which, combined with the first northerly
motion, carries it toward the northeast. Thus throughout the
middle latitudes of the northern hemisphere the prevalent mo-
tion of the lower portion of the atmosphere is from the southwest,
and, for like reasons, in the southern hemisphere the lower por-
tion of the atmosphere moves from the northwest.
148. The Surface Wind in the Polar Regions. It is believed
that in the neighborhood of either pole the mean pressure of the
84 METEOROLOGY.
atmosphere te somewhat greater than it is near the parallel of
64, and, since the air is colder, it has a greater density. Both
causes, therefore, conspire to impel the air toward the lower lati-
tudes ; and this force, combined with the effect of the earth's ro-
tation, produces a northeast wind within the Arctic circle, and a
southeast wind within the Antarctic circle.
149. Ascending Current near the Parallel 0/"64. "We thus find
that in the northern hemisphere the surface wind from each side
of the parallel of 64 blows toward that parallel. This wind here
rises from the earth's surface as it does near the equator, and be-
comes an upper current receding on either side from the parallel
of 64 ; but this upper current will not continue its course ex-
actly in the direction of a meridian. As the air advances south-
ward, it crosses successively parallels whose easterly motion is
more and more rapid, so that, after some time, the direction of
the upper current should be from the northeast. The northwest
current, which seems generally to prevail at the height of two
or three miles, may result from the partial mingling of this north-
east current with the westerly wind which prevails near the earth's
surface.
150. Cause of the High Barometer near the Parallel o/32. If
the pressure of the barometer were the same at all points of the
earth's surface, in consequence of the greater heat of the equato-
rial regions, there would be a general tendency of the surface air
toward the equator, and of the upper air from the equator. This
upper current could not, however, proceed on uninterruptedly to
the pole, because the meridians converge, and their distance from
each other continually diminishes, until they all meet at the poles.
As the upper current of air recedes from the equator, it crosses suc-
cessively parallels of less and Ifiss circumference, by which means
the atmosphere is forced up to a corresponding height, and its
pressure upon the earth's surface thereby increased. In latitude
32, the distance between the meridians is nearly one sixth less
than it is at the equator. This increased pressure of the air in
the middle latitudes arrests the farther progress of the polar cur-
rent, and a calm ensues. The upper air descends to the earth's
surface, and joins the surface current toward the equator, where
it again ascends, and thus maintains a perpetual circulation.
THE MOTIONS OF THE ATMOSPHERE.
85
Pig. 40.
The high barometer near the parallel of 32 forces a surface
current northward in opposition to the increased density of the
air arising from a diminished temperature. Beyond the parallel
of 64 the latter tendency is stronger
than the former, and the surface cur-
rent is from the poles.
The cause of the low barometer
near the parallel of 64 will be con-
sidered in Chapter VI, page 149.
There is some reason for supposing
that in the most elevated regions of
the atmosphere, where the atmosphere
has nearly reached its limit of tenui-
ty, the current over the equatorial
regions, instead of descending to the
earth's surface near latitude 32, may
continue on the same course over the
middle latitudes to the polar regions,
as represented in Figure 40 ; but the
principal mass of the atmosphere is
believed to circulate as represented in
Fig. 38.
151. The Monsoons. In mid-ocean the direction of the trade
winds is quite uniform, but in the neighborhood of the conti-
nents great irregularities are observed. The most remarkable of
these occur in the Indian Ocean, and are known by the name of
monsoons. During the cooler half of the year, from October to
March, the regular trade winds prevail here as in other parts of
the northern hemisphere ; but during the warmer half of the
year, from April to September, the prevalent wind blows in the
contrary direction, viz., from the southwest.
152. Cause of the Monsoons. This change of wind results from
the influence of the sun's heat upon the continent of Asia. In
summer, the southern part of Asia becomes warmer than the In-
dian Ocean near the equator, and the cooler air from the ocean
rushes northward toward the land to displace the heated air.
This air, coming from a lower latitude, has an excess of motion
toward the east, which, combined with the motion from the south
86 METEOROLOGY.
due to the influence of heat, produces a wind from the southwest.
This southwest wind sweeps over the high range of mountains
north of Hindostan, by which means its vapor is condensed, form-
ing excessive rains, by which means a vast amount of latent heat
is liberated, and the surrounding air is still more expanded, thus
adding to the force of the previous southwest current in a man-
ner which is explained in Chapter VI., page 147.
During winter the Indian Ocean is warmer than the southern
part of Asia, and the air from the land flows toward the equator,
producing the usual northeast trade wind.
153. Influence of the Seasons. Similar phenomena are noticed
in every part of the world near the coasts of extensive conti-
nents. The continents being colder than the ocean during the
winter and warmer during the summer, the winds tend from the
land in winter and from the sea in summer. At most places this
tendency serves simply to modify the direction of the prevalent
winds. At ISTantucket the winds blow from the northwest in win-
ter and from the southwest in summer. Throughout the State of
New York the average direction of the wind is 18 more south-
erly in summer than it is in winter. As we proceed southward
this difference increases. At Washington the mean direction of
the wind is northwest in winter and southwest in summer, while
at many places on the coast of Florida the winds blow from the
south in summer and from the north in winter, constituting well-
marked monsoons.
Similar phenomena are observed on the Pacific coast of the
United States. At San Francisco, during winter, the winds blow
most frequently from the northwest, while in summer they blow
from the southwest with a steadiness equal to that of the trade
winds within the tropics. At San Diego, lat. 32 42', throughout
most of the year the wind blows steadily from the southwest, but
during the winter months easterly winds are very prevalent, and
sometimes for a month or two the average direction of the wind
is from the northeast.
154. Land and Sea Breezes. The diurnal change of tempera-
ture has a sensible effect upon the direction of the wind. This
is seen in land and sea breezes which prevail on the coasts of
continents and islands, particularly in tropical countries. During
THE MOTIONS OF THE ATMOSPHERE. 87
the day the land is heated more rapidly than the sea, and during
the night it is more rapidly cooled. In the morning, the air in
immediate contact with the land, being heated, is displaced by the
cooler air in contact with the sea, and thus arises a breeze from
the sea to the land. In summer this breeze usually springs up
soon after 8 A.M., and attains its greatest intensity about the time
of highest temperature. About sunset the breeze ceases entirely.
During the night the land becomes colder than the sea, and a
breeze springs up from the land to the sea, which attains its
greatest force about the time of lowest temperature. This breeze
extends only to a short distance from the coast.
If no other cause operates to produce a wind, the direction of
the land and sea breeze will be perpendicular to the coast ; but
if some other cause operates at the same time, the actual direction
of the wind will be such as results from the composition of the
two forces.
155. Sea Breeze in the Temperate Zones. In the temperate zones
the diurnal change of temperature produces a sensible effect in
modifying the direction of the prevalent wind, and sometimes
entirely reverses its direction. At New Haven the average di-
rection of the wind throughout the year is 20 more southerly
at noon than it is at sunrise, and from March to September the
average change amounts to 35. This effect is so uniform that
sometimes every day, without exception for an entire month, the
wind at noon is more southerly than it was at sunrise. Fre-
quently the change amounts to 180, the wind blowing from the
north at sunrise and from the south at noon, and this phenome-
non is rarely observed except during clear and pleasant weather,
indicating that the change of wind is not the result of a great
storm in progress.
156. Temperature of the Wind. The temperature of the wind
depends upon the quarter from which it blows and the countries
which it has traversed. Generally in the northern hemisphere
the winds from the south are warm, while those from the north
are cold ; but the precise point of the horizon corresponding to
the greatest heat and the greatest cold varies considerably. The
following table shows how much the temperature of each wind is
above or below the mean at New Haven, as deduced from a com-
parison of several years of observations.
88
METEOROLOGY.
Wind.
Temperature.
Wind.
Temperature.
N.
N.E.
E
S.E.
-2. 7
-0 .6
+ .5
+ 1 .2
s.
s.w.
w.
' N.W.
+ 3. 2
+ 4 .0
-1 .1
-4 .5
Fig. 41.
-1-4
-2
4
If we represent these differences of temperature by the ordi-
nates of a curve, we shall obtain a
diagram like Fig. 41, which indicates
that at New Haven the highest tem-
perature accompanies a wind from
south 33 west, and the lowest tem-
perature corresponds to a wind from
north 40 west, the mean difference in the temperature of these
two winds being 8.7.
In many parts of Europe the coldest wind comes from a quar-
ter somewhat east of north, and the hottest wind generally comes
from a point a little west of south.
E SE S SW W NW N NE E
157. Hot WMs (/Zteserfe. On the deserts of Africa and Ara-
bia there sometimes prevails a wind extremely dry and intensely
hot, which raises clouds of sand, and transports it to a great dis-
tance. These winds are known by the name of simoon, harmat-
tan, etc., according to their locality. Plants are withered by this
wind ; men and animals suffer intensely from the heat and dry-
ness of the air ; and entire caravans have been buried in the drift-
ing sand. This dust is sometimes transported across the Medi-
terranean into Spain, Sicily, and Italy, where the wind which
brings it is known by the name of sirocco. In Sicily, during its
continuance, the thermometer sometimes rises to 110 degrees in
the shade.
158. Cold Winds from Mountains. In mountainous countries
the winds from certain quarters are celebrated for their low tem-
perature, and frequently for their dryness. The westerly winds
which cross the range of the Kocky Mountains, being cooled by
elevation, deposit most of their moisture upon the west side of
the mountain, and when they descend upon the eastern side they
are cold and dry winds. Hence along the eastern margin of the
Rocky Mountains rain seldom falls, and ordinary agricultural
products can not be raised without artificial irrigation.
PRECIPITATION OF THE VAPOR OF THE, AIR. 89
The high mountains of South America produce effects still
more remarkable. In Peru, between two great chains of the
Fig. 42. Andes, Fig. 42, in latitude 16 S., at
the height of 13,000 feet, is a deso-
late table-land called the Punos, ex-
tending about 500 miles in length
by 100 in breadth. The trade wind,
by passing over the eastern chain
of mountains, is reduced to a very
low temperature, and nearly all its
vapor is condensed in the form of rain or snow. When the
air descends upon the western side of the mountain it is so cold
and dry that the bodies of dead, animals exposed to it are dried
up like mummies, without any signs of putrefaction. Prescott
states that the ancient Peruvians preserved the bodies of their
dead for ages by simply exposing them to the cold dry air of the
mountain.
CHAPTEE Y.
PRECIPITATION OF THE VAPOR OF THE AIR.
SECTION I.
DEW.
159. Effect of Radiation of Heat. All bodies on the surface of
the earth send out rays of heat toward the sky, and when they
radiate more heat than they receive, their temperature falls below
that of the surrounding air. In order to study these effects, we
place a number of thermometers upon the ground on substances
of different kinds, and suspend other thermometers in the air at
various elevations, and compare the readings of these thermome-
ters simultaneously at all hours of the day and night. Very care-
ful observations of this kind were made at Greenwich, England,
for two years, and it was found that a thermometer placed on
grass fully exposed to the sky frequently sinks ten degrees below
a thermometer suspended four feet from the ground. On nine
nights the difference of temperature was more than 15, and in
one instance a thermometer placed on raw wool sunk 28.5 below
one suspended eight feet from the ground.
90 METEOKOLOGY.
Eadiation of heat from the earth to the sky takes place at all
times, both day and night, and in all states of the sky. Generally,
when the sun is above the horizon, the heat received from it by
the earth exceeds that which is radiated from the earth. Some-
times, however, in places sheltered from the sun, but open to a
considerable portion of the sky, the amount of heat radiated ex-
ceeds that received from the sun and all other sources, so that
grass may continue colder than the air during the day as well as
the night. This difference at midday has been known to amount
to ten degrees.
160. Effect of Partial Exposure to the Sky. Whatever impairs
the free exposure of an object to the sky causes its temperature
to decrease less than it would if the exposure was complete. This
effect is produced by spreading a sheet of cloth over the ground,
even though it be at a considerable elevation. The thinnest cam-
bric handkerchief produces a decided effect. Trees and buildings,
and whatever conceals a part of the sky, diminish the effect of
the radiation of heat from the surface of the earth.
Clouds produce the same effect as an artificial covering. From
an average of all the Greenwich observations, it was found that a
thermometer placed on grass fully exposed to the sky sunk below
a thermometer suspended four feet from the ground as follows :
On cloudless nights, ^ 9.3 degrees.
" nights half cloudy, ' 7.3 "
" " principally cloudy, 6.8 "
" " entirely cloudy, SA "
161. Eadiation from different Substances. Thermometers placed
on different substances exhibit very unequal reduction of tem-
perature on the same night. When a thermometer placed on
grass sinks ten degrees below one suspended four feet from the
ground, a thermometer placed on raw wool will sink 12 or 15 ;
a thermometer placed on copper will sink 8 ; on paper, 6 ; and
on brick, only 3 or 4. Tab. XXXII. shows the average results
found for a great variety of substances. These numbers indicate
the comparative radiating power of different substances for heat.
162. Increase of Temperature with Elevation. By suspending
thermometers at different elevations above the earth from one or
PRECIPITATION OF THE VAPOR OF THE AIR. 91
two inches up to 200 feet, it is found that the loss of heat by noc-
turnal radiation is quite sensible at the elevation of 50 feet, and
does not entirely disappear at the height of 150 feet. During the
night, therefore, the temperature of the air increases as we rise
above the earth's surface. In England, according to the average
of observations continued throughout the year, if a thermometer
placed on grass fully exposed to Jhe sky be taken as the zero, a
thermometer one inch above it would read 3 higher ;
" six inches " " 6 "
one foot " " 7 "
" twelve feet "' " 8 "
fifty feet " " 10 "
one hundred and fifty feet 12 "
and the effect is appreciable at still greater elevations.
163. Origin of Dew. When, in consequence of radiation, ob-
jects near the earth's surface, such as grass and leaves of vegeta-
bles, become cooled below the dew-point in the vicinity, they con-
dense upon themselves a portion of the vapor which is present
in the atmosphere, in the manner explained in Art. 99, and this
moisture is called dew. The amount of dew thus deposited is
greatest upon those substances whose temperature is the lowest,
being proportional to the amount of depression of their tempera-
ture below that of the dew-point. Dew, therefore, does not fall
from the sky like drops of rain, as was formerly supposed, but
the vapor of the air is condensed by coming in contact with the
cold surface of the object upon which the dew collects.
In some parts of the world, nearly all the moisture which the
earth ever receives comes in the form of dew. This is particu-
larly true of some parts of Egypt and Arabia.
164. Circumstances favorable to Dew. The circumstances most
favorable to the production of dew are mainly those which are
most favorable to the loss of heat by radiation. These are,
1st. A cloudless night and unobstructed exposure to the sky.
The deposition of dew is immediately checked by clouds which
reflect back to the earth the heat radiated from it. The same
effect is produced by any artificial covering, even though of the
thinnest texture. Hence, also, plants placed beneath a tree or near
a building collect much less dew than those which are freely ex-
posed to the sky.
92 METEOROLOGY.
2d. A nearly tranquil atmosphere. A slight breeze, by renew-
ing the air which has deposited its excess of vapor, renders the
dew more abundant ; but a fresh breeze, by agitation of the air,
produces a mingling of the air at different elevations, equalizing
the temperature throughout, so that the air at the earth's surface
can not become much colder than the superincumbent atmos-
phere. Little dew is therefore deposited on windy nights.
3d. A moist atmosphere. Wiien the atmosphere is most humid,
a given reduction of temperature will sooner reach the dew-point,
at which the deposition of moisture begins. An abundant dew
is regarded as an indication of approaching rain, because it proves
that the air contains a large quantity of vapor.
4th. Good radiators and bad conductors of heat are required
for collecting the dew. Different substances, having the same
exposure, do not collect the same amount of dew. Wool radiates
heat freely, and, being a bad conductor, collects a large amount of
dew ; while but little dew is deposited on polished metals, since
they are good conductors of heat, and must be reduced through-
out to a low temperature before any dew can be deposited upon
them. If similar plates of polished glass and steel are exposed
alike upon the ground during a favorable night, in the morning
the glass will be drenched with dew, while the brightness of the
metal will be scarcely dimmed. The glass radiates heat more
rapidly than the metal, and, being a bad conductor, draws but little
warmth from the earth to supply its loss ; while the metal, being
a good conductor, readily derives heat from the warm soil below.
165. Dew during the Day. The deposition of dew sometimes
commences before sunset. It continues at all hours of the night,
provided the weather remains favorable ; but more dew is formed
after midnight than before ; and the deposition sometimes con-
tinues after sunrise.
In places sheltered from the sun, but open to a considerable
portion of the sky, dew is sometimes deposited on grass even at
midday.
166. Where there is no Dew. Dew is not deposited on the sur-
face of large bodies of water whose temperature is above 40, for
as soon as the particles at the surface are cooled they become
heavier and sink, while warmer and lighter particles rise to the
PKECIPITATION OF THE VAPOR OF THE AIR. 93
top, by which means the surface of the water is maintained at
nearly the same temperature as the surrounding air.
In the midst of sandy deserts, on account of the dryness of the
atmosphere, dew is almost entirely unknown. Travelers upon
the deserts of Africa and Asia are notified of their proximity to
lakes or rivers by the appearance of dew.
But little dew is deposited in cities, because most of the objects
there found are poorer radiators than the leaves of vegetables,
and because the heat of the city is always greater than that of the
surrounding country.
167. Amount of Dew determined. Attempts have been made to
determine the total amount of dew annually deposited in differ-
ent countries. This is sometimes done by exposing a plate of
glass or some other substance to the sky, and carefully weighing
the amount of moisture deposited upon it. In this way it has
been concluded that in Italy and the south of France the annual
deposit of dew amounts to a little more than a quarter of an inch.
Such results, however, are not very reliable, since tliey are great-
ly influenced by the radiating power of the plate employed, and
also by its position.
SECTION II.
HOAR-FROST.
168. Formation of Hoar-frost. Hoar-frost is formed under the
same circumstances as dew, with the exception of a lower tem-
perature. When the temperature of plants falls below 32 the
moisture of the air is condensed upon them in the solid state, and
forms a layer of spongy ice. Hoar-frost, therefore, is not frozen
dew, but the moisture of the air is deposited in the solid form, with-
out having passed through the liquid condition. Hoar-frost, like
dew, is deposited chiefly upon those bodies which radiate best,
such as plants and the leaves of vegetables, and the deposit is
made principally on those parts which are turned toward the sky.
Since plants sometimes become cooled by radiation from 12
to 15 below the temperature of the surrounding air, a frost may
occur although a thermometer a few feet above the ground does
not sink to 32. During a clear and still night, when a thermom-
eter six feet above the ground sinks to 36 P , a very heavy frost
METEOROLOGY.
may be expected, and a slight frost may occur when the same
thermometer sinks only to 47.
169. How Plants are protected from Frost. Whatever prevents
the radiation of heat serves also to check the formation of hoar-
frost. During the cold nights of spring, plants which are shel-
tered by trees are less liable to be injured by frost than those
which are fully exposed, and a thin covering of cloth or straw
will generally afford entire protection. A garden may frequent-
ly be saved from injury by kindling a small fire, which shall en-
velop the plants in a cloud of smoke. Fogs and clouds also pro-
tect vegetation from the effects of frost.
170. Frost in Valleys. Plants are often killed by frost in the val-
leys and up to a certain height upon the hills, while above this
limit they entirely escape injury. It has been found by observ-
ation that a thermometer attached to a high' tower in a valley in-
dicates at night the same average temperature as a thermometer
on the side of a neighboring hill upon the same level. This in-
dicates that during a tranquil night the cold air resulting from
radiation at the surface of the earth settles in. the valleys in con-
sequence of its greater density, and the warm and cold air are ar-
ranged in nearly horizontal strata like liquids of different densities.
171. Crystalline Structure of Hoar-frost. Hoar-frost generally ex-
hibits a crystalline structure, and consists of long spicula3, which
are found to be hexagonal prisms with angles of 120. These
spieulse are frequently seen in great perfection in the frost which
forms on wooden fences, on the decayed branches of trees, etc.
When a thin film of
water freezes upon a
flat surface of glass or
stone, it often forms a
great variety of beau-
tiful figures, some-
times resembling the
leaves of certain
plants, the leaves of
the palm-tree, or the
feathers of birds, Figs.
Fig. 43.
Fig. 44.
PRECIPITATION OF THE VAPOR OF THE AIR. 95
43 -and 44. In cold weather, smooth and flat stones upon the
side- walk are often covered with these figures, which, upon ex-
amination, are found to consist mainly of spiculas more or less
perfectly formed.
A species of hoar-frost occurs when a warm wind succeeds a
period of severe cold weather. Stone buildings are then often
covered with an incrustation of minute crystals caused by the
low temperature of the stone, which condenses and congeals the
moisture of the air.
SECTION III.
FOG.
172. Condensation of the Vapor of the Atmosphere.' The vapor
in the atmosphere is nearly or quite transparent; but when, from
any cause, the air becomes cooled below the dew-point, a portion
of its vapor is precipitated in the form of drops of water ex-
tremely minute, which affect the transparency of the air, and
form fog or cloud according as it occurs near the surface of the
earth, or in the upper regions of the atmosphere. If we com-
press moist air in a close vessel and allow it suddenly to escape,
the air, by its expansion, will be cooled, and a slight fog be pro-
duced ; but the air soon regains its warmth, the drops of water
return to the state of vapor, and the fog is dissipated.
When steam rises from a vessel of warm water and mingles
with a cold atmosphere, a portion of the vapor is condensed and
a mist is formed. This mist is sometimes, but improperly, called
vapor. Vapor of water is a gaseous body, while mist is a liquid
body. A similar condensation often takes place in nature upon
a large scale, and the mist is then called a fog.
173. Fogs over Rivers in Summer. At certain seasons of the
year, especially during the latter part of summer, upon nearly
every clear and still night, fogs form over rivers and lakes. At
night the temperature of the air over the lanct becomes cooler
than the water of lakes and rivers. The vapor which rises at
such a time from the warm water is condensed by contact with
the cooler air from the land, and a fog is formed, which seems to
rest upon the water.
On a clear and quiet morning in the month of August, an ob-
server on the summit of Mount Washington sees the bed of the
96 METEOKOLOGY.
Connecticut Kiver distinctly traced by a long line of fog, and the
position of a multitude of surrounding lakes is indicated in the
same manner, while other portions of the country are entirely
free from fog.
That this fog is formed by the vapor of the warm water rising
into an atmosphere which is cooler than the water, is proved by
observations of the thermometer. On a morning in July, when
the Connecticut Kiver was covered with a thick fog, the tempera-
ture of the water was found to be 73, while the temperature of
the air over the neighboring land was only 68.
Such fogs generally disappear soon after sunrise. Sometimes,
from the effect of the sun's heat, they are seen to ascend and rise
above the hills, forming clouds, which soon disappear with the
increasing heat of the sun.
Fogs are often formed in a similar manner over harbors, bays,
etc., and these fogs, by a gentle current, are often drifted over the
land. In this manner a sea fog sometimes spreads over the city
of New York, and extends several miles up the Hudson Kiver.
174. Fogs in Spring and Winter. During the spring of the
year, fogs are sometimes formed over rivers, when the tempera-
ture of the water is colder than that of the surrounding air. In
this case the warm and moist air of the neighboring land is chill-
ed by coming in contact with the cold water, and a portion of its
vapor is condensed.
In the same manner, after a warm rain in mid-winter, dense
fogs are sometimes formed by a warm and moist air flowing over
a country which is covered with snow ; or the fog may result
from the moist air becoming cooled by contact with a frozen soil.
Indeed, a fog may be formed at any time at a distance from large
bodies of water, when the vapor which rises from a very moist
soil mixes with a cold atmosphere.
In the same manner, fogs are often formed on the sides of
mountains. The warm air from the valleys being forced up the
sides of the mountain, its vapor is condensed, partly by the cold
of elevation, and partly by contact with the cold surface of the
mountain.
175. Where Fogs are most Prevalent. On the Atlantic Ocean,
from 30 south to 35 north latitude, fogs are almost unknown.
PRECIPITATION OF .THE VAPOR OF THE AIR. 97
On the northern side of the Gulf Stream they are of common
occurrence, but they are most prevalent near the Banks of New-
foundland. These fogs occur in every month of the year, but they
are most prevalent in summer, when the Banks are enveloped
in fog nearly half the time. The vapor which causes these fogs
is furnished by the warm air of the Gulf Stream, and it is con-
densed by the cold air of the Banks, the contrast of temperature
being here more sudden than is found in any other part of the
Atlantic Ocean. During the month of July the water on the
Banks frequently has a temperature of 45, while within a dis-
tance of less than 300 miles the Gulf Stream has a temperature
of 78. The contrast of temperature is almost equally great in
January, but fogs are less frequent in winter, because at that pe-
riod the air is more agitated by storms, which tend to equalize the
temperature over different parts of the ocean.
In the South Atlantic, beyond the parallel of 30, fogs are of
common occurrence, but they are nowhere so prevalent as on the
Banks of Newfoundland.
176. Fogs of Polar Regions, etc. Fogs are very prevalent in the
Arctic regions, particularly in summer. During an Arctic sum-
mer the temperature of the earth rises much above that of the
sea, portions of which are covered with immense fields of ice.
The air resting upon the earth partakes of its temperature, and
when this warmer air is brought in contact with the colder ocean,
a portion of its vapor is condensed, and a heavy mist is formed.
During winter, England and the neighboring portions of the
Continent are frequently enveloped in dense fogs, and in those
towns where bituminous coal is used abundantly the sky is some-
times so darkened by the mixture of fog and smoke that locomo-
tion even at midday becomes almost impossible. In London,
during winter, the streets are sometimes lighted with gas all day,
and travel through the city is attended with serious danger.
This fog results from the warm air of the sea spreading over the
cold land.
177. Where Fogs do not Prevail. Fogs are never formed when
the air is very dry, and therefore they are never known in deserts.
Fogs are not common in tropical countries except in the neigh-
e
98 METEOROI^)GY.
borhood of mountains ; but the summits of mountains, even un-
der the equator, are habitually shrouded in fog or cloud.
178. The Vesicular Theory. Since fog consists of particles of a
liquid which is nearly eight hundred times denser than the air,
it has been thought difficult to explain how it can be sustained in
the atmosphere. Some have supposed that the particles of fog
are hollow, each consisting of a sphere of air surrounded by a
thin envelope of water like a soap-bubble. Such a hollow sphere
is called a vesicle, and this theory of the constitution of fog is call-
ed the vesicular theory.
179. Argument from the appearance of Mist. Some observers
who have examined with a magnifying-glass the particles of mist
rising from hot water, have detected on their surface colored rings
like those seen on soap-bubbles, indicating that their structure
was analogous to that of soap-bubbles, and it has been inferred
that the particles of fog generally have a similar constitution.
Water ordinarily contains minute bubbles of air, and when the
water is warmed these bubbles expand and rise to the surface.
They often rest upon the surface of warm water, and, being sur-
rounded by a thin film of water, they should exhibit colored rings
like soap-bubbles, but there is no evidence that the particles of
fog are generally so constituted. A fog is formed from the vapor
of water previously existing in the air in the gaseous state, and
when this vapor returns to the liquid condition there is no evi-
dence that it assumes the form of hollow spheres.
180. Argument from the absence of a Rainbow. It is contended
that the particles of fog can not be solid spheres of water, because
if they were, a rainbow should be seen whenever the spectator is
situated between the sun and a fog. A rainbow is formed by the
reflection of the sun's rays from falling drops of water. These
drops are spheres of water ; and since a fog does not form a rain-
bow, it is contended that the particles of the fog can not be solid
drops. But it has been shown that when the spheres of water
are very small, as is the case with a fog, a bow should indeed be
formed, but the different colored bands are very broad, and their
light is proportionally feeble. Moreover, if the spheres are not
all sensibly of the same diameter, there will be formed simultane-
PRECIPITATION OF THE VAPOR OF THE AIR. 99
ouslj bands of different breadths, which will be superposed upon
each other in such a manner that the different colors will be very
much blended, and produce a light which is nearly white, form-
ing thus a very faint and nearly white rainbow, or rather fog-low,
which corresponds exactly with the facts. When a spectator is
situated between the sun and a bank of fog, a white bow is often
seen with but little appearance of prismatic colors, and the breadth
of the bow is about double that of an ordinary rainbow.
Thus the absence of the common rainbow in fogs not only
does not' establish the vesicular theory, but the existence of the
white fog-bow positively refutes this theory.
181. Argument from the Constitution of Clouds. Fogs evidently
have the same constitution as clouds. Now, when clouds are
formed at a low temperature, their particles are solid, consisting
of spiculas of ice, which, united, form flakes of snow. But we find
nothing of the vesicular constitution in snow-flakes ; nevertheless,
clouds composed of spiculaB of ice remain suspended in the air for
hours, and sometimes days in succession. The vesicular hypothe-
sis, therefore, is not necessary to account for the permanence of
clouds and fogs, and there is no evidence that they are ever thus
constituted.
182. How Fog is sustained in the Air. The particles of fog are
sustained in the air in the same manner as a cloud of dust is
sustained. A cloud of dust remains for a long time suspended
in the air, although each particle of dust may consist of matter
two thousand times as dense as the air in which it floats. When
the air is perfectly tranquil these particles do indeed fall, but they
descend so slowly that their motion is only perceptible after the
lapse of a considerable interval of time.
183. Diameter of Particles of Fog. The diameter of particles
of fog is very variable, being sometimes so small that the indi-
vidual particles can not be separately seen, and it is only in mass
that they make any impression upon the eye, and they are found
increasing in size until they fall with considerable velocity, when
they are called rain -drops.
The diameter of the smallest visible particles of fog has been
estimated at -^^ inch ; and when the diameter of the particles be-
100 METEOROLOGY.
comes equal to V inch, they fall with an appreciable velocity, and
are called rain-drops.
184. Indian Summer. At certain seasons of the year there
occurs a peculiar phenomenon called a dry fog. In the United
States this frequently occurs in November, or the latter part of
October, and this period is commonly known by the name of In-
dian Summer. This period is characterized by a hazy condition
of the atmosphere, a redness of the sky, absence of rain, and a
mild temperature. This appears to result from a dry and stag-
nant state of the atmosphere, during which the air becomes filled
with dust and smoke arising from numerous fires, by which its
transparency is greatly impaired. A heavy rain washes out these
impurities and effectually clears the sky.
This phenomenon is not peculiar to the United States, a simi-
lar condition of the atmosphere being frequently observed in
Central Europe. Moreover, this dry and stagnant state of the
atmosphere is not limited to a single season of the year. The
long periods of drought which frequently prevail in summer are
characterized by a like condition of the atmosphere.
185. Prevalence of Volcanic Ashes, etc. Sometimes a dry fog
continues for several weeks, and prevails over a vast area, ex-
hibiting very peculiar characteristics. These fogs have been as-
cribed to the presence of fine volcanic ashes in the atmosphere,
and perhaps also of substances foreign to the earth.
In 1783 such a fog prevailed over all Europe, and continued
for more than a month. It was preceded by a remarkable erup-
tion of the volcano Hecla, in Iceland, which for a long time emitted
smoke of unusual density.
In 1831 a similar fog prevailed in the United States, in Europe,
and on the coast of Africa. It obscured the air to such an extent
that the sun could be observed all day with the naked eye, with-
out the interposition of any colored glass. At night the fog
seemed decidedly phosphorescent, and emitted an appreciable
amount of light, which could not be ascribed to the reflected light
of the stars.
PRECIPITATION OF THE VAPOR OF THE AIR. 101
SECTIOK IV.
CLOUDS.
186. Clouds differ from fogs only in their elevation above the
earth. A fog resting on the top of a mountain is called a cloud.
A cloud resting on the surface of the earth is called a fog.
187. Classification of Clouds Cirrus. Clouds present an in-
finite variety of forms, yet they may be divided into six classes,
each presenting -characteristics tolerably distinct. Three of these
modifications are primary, and three are compound.
The cirrus cloud consists of long, slender filaments, either par-
allel or diverging from each other, and often presents the appear-
ance of a lock of cotton whose fibres are electrified so as power-
fully to repel each other. These clouds appgar to have the least
density, the greatest elevation, and the greatest variety of form.
They are generally the first to make their appearance after a pe-
riod of perfectly clear weather. Indeed, in fair weather, the sky
is seldom entirely free from small groups of cirrus clouds. They
are believed to be composed of spiculae of ice or flakes of snow,
floating at a great height in the air. At the height at which they
prevail, the temperature of the air is below 32 even in midsum-
mer. It is among clouds of this variety that halos and parhelia
are formed, phenomena which are ascribed to the refraction of
light by minute prisms of ice.
188. Cumulus. The cumulus cloud usually consists of a hemi-
spherical or convex mass, rising from a horizontal base. It is
much denser than the cirrus, and is formed in the lower regions
of the atmosphere. In fair weather the cumulus often forms a
few hours after sunrise, goes on increasing until the hottest part
of the day, and disappears about sunset. "We often see near the
horizon large masses of cumulus clouds, which resemble lofty
mountains covered with snow.
The rounded top of the cumulus results from the mode of its
formation. When the surface of the earth is heated by the rays
of the sun, currents of warm air ascend, and as soon as they reach
a certain height a portion of their vapor is condensed, and forms
cloud ; and since the upward motion is greatest under the centre
of the cloud, the vapor is there carried up to the greatest height.
102 METEOROLOGY.
In like manner, when steam escapes in large quantities from the
boiler of a steam-engine, especially in a damp atmosphere, it forms
a rounded mass of mist.
189. Stratus. The stratus cloud is a widely-extended, continu-
ous, horizontal sheet, often covering the entire sky with a nearly
uniform veil. This is the lowest of the clouds, and sometimes
descends to the earth's surface.
190. Compound Modifications. The cirro-cumulus consists of
small, well-defined, rounded masses, in close proximity. These
little rounded clouds, on account of their fleecy appearance, are
sometimes called woolly clouds. The cirro-cumulus is frequent
in summer, and is attendant on warm and dry weather.
The cirro-stratus consists of delicate fibrous clouds spread out
in strata, which are either horizontal, or but slightly inclined to
the horizon. This cloud appears to result from the subsidence
of the fibres of the cirrus to a horizontal position. Sometimes
the whole sky is so mottled with this kind of cloud as to resem-
ble the back of a mackerel, and is hence called the mackerel-sky.
The cirro-stratus precedes wind and rain, and is almost always to
be seen in the intervals of storms.
The cumulo- stratus consists of the cumulus blended with the
stratus, and is formed in the interval between the first appear-
ance of the fleecy cumulus and the commencement of rain. On
the approach of a thunder-storm the cumulo-stratus clouds are
often seen in great magnificence, and present those peculiar forms
kn6wn in some places by the name of thunder-heads. All these
varieties of cloud are represented in Plate II.
191. Best Mode of observing the Clouds. In order to be able to
distinguish well the form of clouds, it is often necessary to di-
minish their brilliancy by viewing them through a glass of a
deep blue color, or by reflection from a mirror of black glass.
We are thus able to detect peculiarities which entirely escape
observations with the unassisted eye.
The appearance of a cloud often changes greatly with its
change of position in the heavens. The peculiarities of clouds
are generally more noticeable when they are near the zenith
than when they are near the horizon.
PRECIPITATION OF THE VAPOR OF THE AIR. 103
Besides the six modifications of clouds above enumerated,
Howard admitted a seventh, which he called the cumulo-cirro-
stratus, or nimbus, to denote a cloud or system of clouds from
which rain is falling ; but it is often so difficult to distinguish be-
tween the stratus and nimbus that it seems inexpedient to retain
the last division.
192. Average degree of Cloudiness. Clouds are more prevalent
in some parts of the world than in others. Throughout New
England, on an average for the whole year, -n^ tDS f tn sky are
covered with clouds, while in the Southern States the average
is truths. Near the equator, between the K E. and S. E. trade
winds, there are places where the sky is almost constantly cov-
ered with clouds. At St. Helena, at an elevation of 1764 feet, on
an average for the whole year, i^ths of the sky are covered with
clouds, and on the tops of high mountains the sky is seldom free
from clouds.
Throughout most of Great Britain the average cloudiness is
nnjths, while at Bombay it is only -^-ths, and at Sacramento,
California, it is only
193. Height of Clouds. The height of a cloud may sometimes
be measured in the same manner as the height of any other in-
accessible object, by simultaneous observations of its direction at
two stations. More satisfactory results may, however, be obtain-
ed by ascending in a balloon, and noting the height of the ba-
rometer at the instant of entering a cloud, and again when emerg-
ing from it ; the barometer affording the means of computing
the corresponding altitudes. In mountainous countries we may
sometimes determine the height of a cloud by comparing it with
some peak ^of known elevation near which the cloud is carried
by the wind.
The height of clouds is very variable, and their mean eleva-
tion is not the same in different countries. The stratus cloud
often descends to the earth's surface. In pleasant weather, the
lower limit of cumulus clouds varies from 3000 to 5000 feet ele-
vation, and their upper limit from 5000 to 12,000 feet. Cirrus
clouds are never seen below the summit of Mount Blanc, which
has an elevation of 15,744 feet.
Clouds are sometimes seen above the summit of Chimborazo,
104 METEOROLOGY.
which has an elevation of 21,424 feet. Gay-Lussac and Glaisher,
in their different balloon ascents to the height of 23,000 feet, saw
cirrus clouds which appeared considerably above them. It is
estimated that the greatest height at which visible clouds ever
exist does not exceed ten miles.
194. Vertical Thickness of Clouds. The vertical thickness of
clouds does not generally exceed half a mile, but cumulus clouds
are sometimes formed of enormous magnitude and height. It
has been computed that the tops of cumulus clouds sometimes
attain the height of four miles, while their bases are not more
than half a mile above the earth's surface.
195. Formation of Clouds. The vapor generated at the surface
of the earth by the heat of the sun tends, by its expa-nsive force
to spread in all directions, but especially upward, and forms an
atmosphere of vapor whose density decreases with the elevation,
Since the temperature of the air sinks as we rise above the earth,
it may happen that at a certain height the tension of the vapor
is greater than corresponds to the temperature which prevails at
that elevation, in which case a portion of the vapor will be pre-
cipitated and form a cloud.
Yapor may also be transported to a great height by the ascend-
ing currents of air caused by the heat of the sun. These currents
ordinarily give rise to cumulus clouds, and it frequently happens
that the sky, though clear in the morning, is filled with those
clouds at noon.
Any cause which chills a humid air determines the formation
of cloud. A cold wind penetrating a humid air, or a warm and
humid wind penetrating a cold air, causes the precipitation of
vapor and the formation of cloud. At the close of a warm day,
especially after a rain, clouds are frequently formed, which in-
crease during the night, and are dissipated the next day by the
effect of the sun's heat.
196. Summits of Mountains enveloped in Cloud. The summits
of high mountains are almost always enveloped in clouds, even
though every other portion of the sky is perfectly clear. This is
not due to any attraction between the mountain and the cloud,
but rather the mountain causes the cloud. The effect of an inter-
PRECIPITATION OF THE VAPOR OF THE AIR. 105
posed mountain is to force a horizontal wind up to an unusual
height where the temperature is low, and when the temperature
of the air is reduced below its dew-point, a portion of its vapor
Pig 45 must be condensed and
form cloud. Thus, let
ABC be a mountain
interposed in the path
of a horizontal current
of air. The air is by
this means forced up-
ward, and made to glide
along the side of the
mountain. If DE rep-
resents the height at which the temperature of the air is just equal
to the dew-point of this current, then, as soon as the wind passes
above the line DE, a portion of its vapor will be condensed, and
a cloud will be formed which will envelop the summit of the
mountain. But when the air, descending from the mountain on
the opposite side, passes below the line DE, it attains a tempera-
ture which is above the dew-point, and the cloud is redissolved.
It is sometimes thought strange that the strong wind which
usually prevails on the summits of mountains does not blow away
the cloud. Undoubtedly the cloud is drifted off with the wind, but
its place is instantly supplied with new cloud. Thus, although
the cloud on the summit of the mountain appears perfectly sta-
tionary, the particles which compose the cloud are continually
changing. A somewhat similar effect often takes place over
countries which are tolerably level. The sky does not become
overcast solely from clouds which are drifted by the wind from
places beyond the horizon ; but new clouds frequently form di-
rectly in sight of an observer. On the contrary, a cloudy sky
sometimes clears up, not because the clouds are drifted off by the
wind, but because they are converted into vapor by the increas-
ing heat of the air.
197. How Clouds are Sustained. Since clouds consist of parti-
cles which are heavier than the surrounding air, they must sink,
even though it be slowly, and we might conclude that in calm
weather they must at length fall to the ground. The particles of
a cloud, however, in pleasant weather can not reach the ground,
106 METEOKOLOGY.
because in descending they meet a warmer stratum of air which
is not saturated with vapor, when the lower part is again con-
verted into vapor and disappears. This explains why the base
of the cumulus cloud is uniformly horizontal. While, however,
the particles on the lower side of the cloud are dissolved, the up-
per part of the cloud is continually increasing by the condensa-
tion of new vapor, which is carried upward by ascending currents
of air, by which means the cloud appears to maintain a constant
elevation above the earth.
198. Currents in the Air. Observations of the clouds often dis-
close the existence of currents in the atmosphere flowing in vari-
ous and perhaps opposite directions. We sometimes notice a
stratum of clouds moving nearly in the direction of the air at the
earth's surface, while at a greater elevation we observe a stratum
moving in a different direction, and sometimes a third and per-
haps a fourth moving in still other directions. Such cases are of
frequent occurrence near the commencement or during the prog-
ress of a great storm.
199. Peculiar Arrangement of Clouds. Clouds sometimes assume
remarkable forms, which we can not ascribe to chance. Some-
times cirro-cumulus clouds arrange themselves in long lines,
Fis.46. stretching quite across the horizon. Some-
times several such lines stretch across the
sky in nearly parallel directions, while oc-
casionally the whole heavens are covered
with such bands, which seem to diverge
from one point of the horizon, and con-
verge to the opposite point. Such bands
generally point from southwest to north-
east, as shown in Fig. 46.
The apparent curvature of the lines is the effect of perspective,
the bands being in fact parallel to each other. The direction of
these lines generally coincides with that of the wind, and it has
been suspected that these lines of cloud serve as conductors of
currents of electricity, and this may be the agent which causes
the clouds to assume such artificial forms.
200. Shadows of Clouds. When the atmosphere is filled with a
PRECIPITATION OF THE VAPOR OF THE AIR.
107
Fig. 4T.
dense haze, the shadows of houses and trees are often distinctly
depicted upon the haze. So also when the sky is somewhat hazy,
the shadows of clouds can be
distinctly traced in the sky by
dark lines proceeding from the
sun. Such a haze most fre-
quently prevails near the hori-
zon, and hence these shadows
are most noticeable in that quar-
ter of the heavens which is be-
low the sun. This effect is of
common occurrence in summer,
and is known by the name of "the sun's drawing water." Oc-
casionally we notice these shadows diverging in every direction
from the sun, not only downward, but also laterally and even up-
ward. These shadows are parallel bands, and the apparent di-
vergence is the effect of perspective.
201. 'Shadows after Sunset. A similar phenomenon is fre-
quently noticed about fifteen minutes after sunset, when the
shadows of clouds near the horizon are projected upon the west-
ern sky in the form of radiant beams diverging from the sun.
These beams are parallel lines of indefinite length, but from the
effect of perspective they seem to diverge from the sun, and if
they could be traced entirely across the sky they would, for the
Fig.4s. same reason, con-
verge to a point
directly opposite
to the sun. Such
cases are sometimes,
though not very
frequently noticed.
Similar shadows are
sometimes seen in
the morning before sunrise, and form a conspicuous feature of
the morning twilight. This effect is sometimes noticed in nearly
every part of the world. It must- have attracted the attention of
the ancient Greeks, and is thought to explain that poetic expres-
sion, " the rosy fingered morn."
108 METEOKOLOGY.
SECTION V.
RAIN.
202. Origin of Rain. When a portion of the vapor which ex-
ists in the air is condensed, a mist or cloud is formed. Generally
this condensation proceeds slowly, and the clouds which result
do not furnish rain. But when this condensation takes place
with sufficient rapidity, the small particles of mist increase in di-
ameter by the condensation of more vapor, and, forming drops
of considerable size, they descend to the earth in a shower of
rain.
203. Diameter and Velocity of Drops of Rain. Drops of rain
vary in diameter from a quarter of an inch to -^-th and even T ^th
of an inch. The velocity which they acquire in their descent is
very small. A drop falling in a vacuum would be continually
accelerated, and at the end of one minute would have the veloci-
ty of a cannon ball ; but, falling through the atmosphere, the re-
sistance increases with the velocity until this resistance becomes
equal to the weight of the drop. When this result takes place
there can be no farther increase of velocity, and the drop after-
ward descends with a uniform motion. A drop of rain ^th of an
inch in diameter, by falling through the atmosphere, can not ac-
quire a velocity exceeding 34 feet per second ; a drop ^th of an
inch in diameter can only acquire a velocity of 13 feet per second;
a drop ^th of an inch in diameter, a velocity of 8 feet per second ;
and a globule of water 10 1 o0 th of an inch in diameter can not ac-
quire a velocity so great as two inches per second.
204. To Measure the Amount of Rain. The amount of rain which
falls from the sky is measured by a pluviometer, or rain-gauge. The
object of the rain-gauge is to determine the average depth of rain
which falls in a given neighborhood. For this purpose we catch
in a vessel the rain which falls upon a limited space, as a square
foot, and hence infer the amount which falls in the neighbor-
hood. It is, then, essential to the accuracy of our conclusion that
we catch all the rain which falls within the prescribed limits, and
no more ; and also that this amount be equal to the average
PRECIPITATION OF THE VAPOR OF THE AIR.
109
Fig. 49.
depth which falls in the vicinity. To secure the
first object, the edge of the vessel should be
sharp and its sides upright. If the edge of the
vessel be thick, or the sides be much inclined,
the rain which falls upon the edge, or upon the-
sloping sides, will be scattered in various direc-
tions, and a part will be wasted. A cylinder
several inches deep is the most convenient form
of gauge. This cylinder may be large or small.
They have generally been made about ten
inches in diameter; but a cylinder two inches
in diameter, if carefully made, may yield very
accurate results. Fig. 49 shows the gauge em-
ployed by the Smithsonian Institution. The
cylinder AB is two inches in diameter, and the
tube CD is about half an inch in diameter.
205. Amount of Rain determined. The amount of rain col-
lected in the gauge may be measured in a tube properly gradu-
ated by comparing the area of a section of the gauge with that
of the tube. Suppose the gauge to be a cylinder ten inches in
diameter. Take a glass tube exactly one inch in diameter, and
graduate its side to inches and tenths, and measure the rain in
this tube. One inch of water in the tube will correspond to one
hundredth of an inch in the gauge, and a tenth of an inch in the
tube to one thousandth in the gauge. "We may thus easily meas-
ure the depth of the fallen rain to the accuracy of one thousandth
of an inch. In a similar manner we may measure the depth of
the rain, whatever be the diameter or form of the gauge.
206. Proper Exposure of the Gauge. In order that the amount
of rain collected in the gauge may be equal to the average depth
which falls in the vicinity, a proper exposure is indispensable,
and this is sometimes difficult to be attained. If the gauge be
erected near a building it is liable to be affected by eddies or
currents of air, causing more rain to fall on one side of the build-
ing than on the other. The most suitable place for a rain-gauge
is in an open field remote from all obstructions ; or, if it must be
near a building, a position should be selected which is least ex-
nosed to the influence of eddies.
110 METEOROLOGY.
207. Influence of Height of the Gauge. Two gauges placed near
each other, at different elevations, do not generally collect the
same quantity of rain, the lower gauge usually showing the most
water. At the Observatory of Greenwich, a gauge at the sur-
face of the ground annually collects two thirds more rain than a
gauge elevated fifty feet above it. Similar differences, but less in
amount, have been observed at other places in England, as well
as in Paris and Philadelphia.
This result has been ascribed to an increase in the size of the
drops as they descend through a humid atmosphere, the drops
being generally colder than the surrounding air. But so rapid
an increase in the size of a drop, amounting to two thirds in a
fall of fifty feet, is altogether incredible. Moreover, it ought
sometimes to happen that the drops should diminish in size by
evaporation in traversing a warmer stratum of air, while observ-
ation always indicates the greatest amount of rain in the lower
gauge.
This difference is probably caused by eddies formed in the air
about the gauge. A portion of the air which strikes against the
gauge glances up over it and spreads out laterally, carrying along
with it the descending drops of rain, thus dispersing the drops
which would otherwise fall into the gauge, and diminishing the
quantity of water which it collects. These eddies are strongest
where the velocity of the wind is the greatest; that is, they pro-
duce the greatest effect at a considerable elevation above the
ground, where the course of the wind is unobstructed by oppos-
ing buildings.
Hence we conclude that the best location for a rain-gauge is to
bury it in the earth, making its top just even with the surface of
the ground.
208. How Rain is Caused. Kain is but the condensed vapor
of the air, and this condensation can only be caused by cooling
the air below the temperature of the dew-point. This reduction
of temperature may be effected by radiation, or by the contact
of warm air with the cold surface of the ground, especially the
surface of an elevated mountain ; or by the mingling of warm air
with colder air ; but these processes are so gradual, or limited in
extent, that they probably never result in any thing more than a
fog or a cloud. In order to produce an abundant rain, the air
PRECIPITATION OF THE VAPOR OF THE AIR. Ill
must be suddenly cooled below the dew -point, and there is no
mode in which this can be so readily accomplished as by forcing
it up to an elevation of one or two miles above the earth's sur-
face. The temperature of the air sinks about thirty -five degrees
in two miles of elevation; and if air from the earth's surface should
be forced up to this height, a large portion of the vapor which is
carried up with the air must be condensed. Such an effect may
result from an interposed mountain, or from the opposition of two
currents of air. Examples of both of these methods are of daily
occurrence.
209. Huttoris Theory of Rain. In 1784, Dr. Hutton, of Edin-
burg, proposed a theory of rain, which has acquired great celebri-
ty. This theory is founded upon the following principle : When
two masses of air of different temperatures, and both saturated
with vapor, are mingled together, the temperature of the mixture
is too low to contain all the vapor of the combined masses. This
excess of moisture must therefore be discharged in the form of
rain.
Suppose, for example, there are two masses of air having the
temperatures of 60 and 80, and that each is saturated with
moisture. The elastic force of vapor at these temperatures is
0.518 and 1.023; the mean of the two being 0.770 inch. Sup-
pose the mixture to have a temperature of 70, at which tempera-
ture the elastic force of vapor is 0.733 inch. The difference is
0.037 inch of mercury, or 0.503 inch of water, and this, it is claim-
ed, is the amount of water that should be precipitated the moment
these two masses of air are perfectly mingled. A similar result
should take place if the two masses of air contain considerable
moisture, but without being saturated.
It is objected to this theory that it is impossible to mingle to-
gether two large masses of air of different temperatures, except
very slowly, and hence the resulting precipitation can not be con-
siderable. Moreover, the latent heat evolved in the condensation
of the vapor would raise the temperature of the mixture, so that
a less quantity of water than that above supposed would be pre-
cipitated. Such a mixture might, therefore, give rise to a cloud,
but never to a copious shower.
210. Distribution of Rain over the Earth's Surface. The fall of
112
METEOROLOGY.
rain is very unequally distributed over the earth's surface, vary-
ing from zero to a depth of fifty feet in a year. The amount of
rain is affected by the latitude of the place ; by its elevation above
the sea ; by the proximity and course of chains of mountains $
the proximity and configuration of the coast, as well as by the
direction of the prevalent winds. Fig. 50 is designed to show the
PRECIPITATION OF THE VAPOR OF THE AIR. 113
distribution of rain over the earth's surface ; a deep shade indi-
cating a great fall of rain, and a light shade indicating a scarcity
of rain.
211. Influence of Latitude. The average fall of rain is greatest
at the equator, and diminishes as we proceed toward the poles.
The following table shows the average annual fall of rain for
every ten degrees of latitude as far as 60.
At the equator, 104 inches.
In latitude 10, 85 "
" " 20, 70 "
In latitude 40, 30 inches.
" " 50, 25 "
" " 60, 20 "
30, 40 "
That more rain should fall at the equator than in high lati-
tudes might be expected from the greater amount of vapor con-
tained in the air. The -average amount of vapor present in the
air is about five times as great at the equator as in latitude 60;
and an equal reduction of temperature must precipitate more
moisture from the air in a warm than in a cold climate. If the
causes which produce rain acted with equal intensity in all lati-
tudes, we might expect that the average amount of rain in each
latitude would be proportional to the quantity of vapor con-
tained in the atmosphere. In this case, if we assume the mean
fall of rain at the equator to be 104 inches, as has been determ-
ined by observation, the annual fall in other latitudes would be
us follows :
In latitude 10, 101 inches.
" " 20, 90 "
" " 30, 70 "
In latitude 40, 45 inches.
50, 27 "
" " 60, 18 "
We thus see that while in latitude 60 the actual fall of rain is
fully equal to what might be expected from the amount of vapor
present in the air, there is a great deficiency of rain in the inter-
mediate latitudes, which is most decided from latitude 10 to 30 ;
in other words, we must conclude that the causes which produce
rain act with less intensity near latitude 30 than they do near
the equator, or latitude 60.
212. Number of Rainy Days. We shall arrive at the same con-
clusion if we compare the number of rainy days in a year in dif-
ferent latitudes. The following table, deduced from a comparison
of the log-books of a large number of vessels navigating the
H
114 METEOKOLOGY.
Atlantic Ocean, shows the average number of rains which occur
during a hundred days on different parallels over the Atlantic :
Between
latitude and 10, 45 rains.
" 10 " 20, 18 "
" 20 " 30, 21 " '
Between
latitude 30 and 40, 25 rains.
40 " 50, 34 "
" 50 " 60, 40 "
We thus see that near latitude 60 the number of rainy days
is about the same as at the equator, and is about double what it
is from latitude 10 to 30. This result accords with that de-
duced in the preceding article, viz., that the causes which produce
rain act with diminished intensity between the parallels of 10
'and 30.
The increased fall of rain near the equator is ascribed to the
ascent of a vast column of air due to the meeting of the northern
and southern trade winds, and there is a similar meeting of op-
posing winds near the parallel of 60. On the other hand, be-
tween the parallels of 10 and 30, the winds are more uniform
in their direction than in any other part of the world.
The frequent rains near the equator and the parallel of 60 ex-
plain the diminished height of the barometer in those localities,
for we find the barometer always stands lowest near the centre
of a great rain-storm.
213. Influence of Elevation above the Sea. The annual fall of
rain is uniformly greater on mountains of moderate elevation
than it is at the level of the sea ; and at a certain height the fall
is from two to three times as great as it is near the base of the
mountain. On the island of Guadeloupe, in latitude 16, near
the summit of a mountain of 5000 feet elevation, the fall of rain
in 1828 was 292 inches, while near the base of the mountain the
fall was only 127 inches- This difference is not due to the cold-
ness of the mountain. An equal and probably a greater effect
would be produced by a volcanic mountain whose surface- was
covered with melted lava. When a current of air meets an in-
terposed mountain, it is forced up the side of the mountain ; that
is, it is elevated above the earth's surface into a colder region, and
its vapor is precipitated by the cold of elevation.
We find the same principle exemplified wherever there are
high mountains. Along the western coast of Hindostan runs a
range of mountains whose summits are deluged with rain, while
PRECIPITATION OF THE VAPOR OF THE AIR. 115
near their western base the amount of rain is by no means ex-
traordinary, and on their eastern side the fall is less than one third
of the average for the same latitude. - At Bombay, on the western
Fig 51 side of the mountain,
the average annual fall
of rain is 78 inches ; at
x___3,._^ the elevation of 4500
^ v - feet the average fall is
254 inches, and in 1842
the fall amounted to 305 inches ; while at Poonah, on the eastern
side of the mountain, the average fall is only 23 inches. This
rain falls almost wholly from June to October, during the preva-
lence of the southwest monsoon. The warm and moist air from
the ocean, encountering this range of mountains, is elevated high
above the surface of the sea, by which means it is cooled, and its
vapor is condensed over the summit of the mountain. When
this air descends on the eastern side of the mountain it is a dry
air, and has but little vapor remaining to be precipitated.
A similar effect takes place on the southern slope of the Him-
alaya Mountains, about 300 miles north of Calcutta, where, at an
elevation of 4500 feet, the fall of rain in 1851 was 610 inches, all
of which fell from April to October, during the prevalence of the
southwest monsoon.
Similar effects take place in Oentral America, and on several
of the West India Islands, where the prevalent winds come from
a warm sea, and contain a large amount of vapor.
214. Maximum Fall of Rain. The increased fall of rain upon
mountains attains its maximum at a certain elevation, and above
that point the fall of rain decreases as we ascend. The elevation
at which the fall of rain is greatest is not every where the. same.
In India it is about 4500 feet, while in Great Britain it is about
1900 feet.
215. Influence of Proximity to a Mountain. Sometimes the mere
proximity to a mountain causes more rain to fall at the level of
the sea than is usually found in the same latitude. Thus, at Yera
Cruz, 278 inches of rain have been known to fall in a single year;
and the mean annual fall is 185 inches, which is fully double the
average amount for the Gulf of Mexico. This result is to be as-
116 METEOROLOGY.
cribed to the high mountains on the west coast of Vera Cruz, by
which the warm and moist air from the Gulf is forced up to a
great height, and its vapor is condensed by the cold of elevation,
and this influence is not confined to the immediate vicinity of the
mountain, but extends to some distance beyond its base.
So, also, on the Northwest Coast of America, near latitude 60,
for a similar reason, the annual fall of rain is 90 inches, which is
at least four times the average for other parts of the globe in the
same latitude.
For a like reason, on the coast of Norway, in latitude 60, the
annual fall of rain is more than 80 inches.
216. Influence of Proximity to the Ocean. An increase of rain
usually results from mere proximity to the ocean, even where
there are no mountains, especially if the prevalent winds come
from the sea. This effect is most noticeable near the coast, and
goes on diminishing as we proceed toward the interior of a conti-
nent. Thus, in Europe, near the Atlantic coast, the fall of rain
varies from 30 to 40 inches ; in Central Europe it seldom exceeds
20 inches ; while throughout a large part of Eussia it is only 15
inches, and in Northern Asia it is still less.
Similar results, but somewhat more complicated, are found in
the United States. On the Atlantic coast, near the parallel of
45, the annual fall of rain is 40 inches ; in Michigan it is about
30 inches ; in Minnesota 25 inches ; and near the Missouri River,
on the same parallel, it is only 15 inches.
217. Influence of Winds. Along the Atlantic coast of the
United States, rain occurs most frequently with the wind from
the northeast. Out of one hundred cases of rain or snow re-
corded at New Haven, the average number occurring with the
different winds is as follows :
N. N.E. E. S.E. S. S.W. W. N.W.
8 37 6 19 7 15 1 7
Storms at New Haven generally begin with an easterly wind and
end with a westerly wind, so that the same storm is attended by
both winds ; but as the rain or snow with the first wind general-
ly continues longest, the easterly wind is recorded as accompany-
ing rain at a greater number of the regular hours of observation.
Throughout most of the interior of the United States, the prin-
PRECIPITATION OF THE VAPOR OF THE AIR. 117
cipal part of the rain comes with a westerly wind. At Cincin-
nati, out of one hundred cases of rain or snow, the average num-
ber occurring with the different winds is as follows :
N. N.E. E. S.E. S. S.W. W. N.W.
2 10 19 10 25 18 25
In Central Europe about three fourths of all the rain occurs
with a westerly wind.
218. Annual Fall of Rain at different Places. To obtain the
mean fall of rain at any place requires observations continued
for a considerable number of years, for it not unfrequently hap-
pens that the rain of one year is double that of some other
year at the same place. The following table shows approxi-
mately the average annual fall of rain for different parts of the
United States:
Inches.
Alabama and Louisiana . 56
Oregon 49
Florida .48
Virginia and the Carolinas 48
Tennessee and Kentucky 48
Georgia 44
Arkansas and Missouri . 42
Maryland and Pennsylvania 41
Inches.
Ohio 40
New England .... 40
New York ..... 37
Michigan and Wisconsin 32
Iowa and Kansas ... 31
Texas 29
California 18
New Mexico . 13
219. Distribution of Rain throughout the Year. Throughout
most of the United States east of the Eocky Mountains, the rain
is pretty equally distributed through the different months of the
year, but the rain of summer is every where somewhat greater
than the rain of winter, including the melted snow. In New En-
gland the difference between the rain for these two seasons is less
than 10 per cent. ; in the State of New York it is nearly 50 per
cent. ; in Virginia and the Carolinas it is 100 per cent. ; in Flori-
da it is 200 per cent. ; in Texas it is 75 per cent. ; in Ohio it is 25
per cent. ; in Michigan and Wisconsin it is 140 per cent. ; while in
Iowa and Kansas it is 300 per cent ; that is, the fall of rain in
summer is four times as great as it is in winter. On the Pacific
coast this law is reversed. In California the rain of winter is
more than twenty times as great as that of summer, and in Ore-
gon it is seven times as great. See Table XXIX.
118 METEOROLOGY.
220. Rainy Season and Dry Season. When the rain is very
unequally distributed through the different months, the year is
naturally divided into the rainy season and the dry season.
Throughout most of California but little rain falls- except during
the six colder months, and during the four months from June to
September rain is almost unknown. No rain falls during the
summer months, when the wind blows almost uninterruptedly
from the southwest, because this air comes from a colder ocean,
and, passing over the heated land, its vapor is not condensed until
it meets the Nevada Mountains, on the eastern margin of Cali-
fornia.
Wherever the direction of the prevalent wind changes greatly
with the season of the .year, we generally find the rain unequally
distributed through the different months. On the west coast of
Hindostan, nearly all the rain falls from April to September, dur-
ing the prevalence of the southwest monsoon ; but during the
other half of the year, the winds coming from the northeast have
already passed over high mountains, where they have lost their
moisture, and descend to the earth as dry winds, which often fur-
nish no rain for months in succession.
On the east coast of Hindostan, almost no rain falls during the
prevalence of the southwest monsoon, but abundant rains occur
during the prevalence of the northeast monsoon, when the warm
air from the Bay of Bengal has a higher temperature than the
land.
A similar inequality occurs at many places in tropical America.
At Vera Cruz almost the entire fall of rain occurs from May to
October, when the winds are easterly ; but during the rest of the
year the winds are northwesterly, and several months will some-
times pass without a drop of rain.
At some places near the equator there are tivo rainy periods of
the year, the maxima occurring in June and December.
221. Greatest Fall of Rain. There are certain portions of the
globe which are habitually, and others occasionally deluged with
rain. On the southern slope of the Himalaya Mountains, at the
height of 4500 feet, in latitude 25, there have been registered in
a single year 610 inches of rain ; and of this, 147 inches fell in
the month of June. At a station in latitude 18, near the west-
ern coast of Hindostan, the average fall for fifteen years has been
PRECIPITATION OF THE VAPOR OF THE AIR. 119
254 inches. In the northwestern part of England, at the height
of 1300 feet, the average annual fall of rain is 146 inches, while
at London the annual fall is only 20 inches. At Vera Cruz the
annual fall is 183 inches, and 60 inches have been recorded in a
single month. See Table XXXI.
222. Remarkable Showers. Throughout most of the United
States the rain which falls in one day rarely amounts to one inch,
but occasionally the fall is much more remarkable. Thus, at Flat-
bush, Long Island, on the 22d of August, 1843, nine inches of rain
fell in eight hours; at Catskill, New York, on the 26th of July,
1819, fifteen inches of rain fell in six hours ; at Wilmington, Del-
aware, on the 29th of July, 1834, five inches of rain fell in two
and a half. hours; and at Fairfield, Ohio, on the 12th of August,
1861, eight inches of rain fell in eleven hours.
In India fifteen inches of rain have fallen in a single day, while
at several places in the vicinity of Switzerland thirty inches of
rain have been reported to fall in a single day.
It is not supposed that in a*ny of these cases the amount of rain
was measured with absolute precision ; but that the fall was very
unusual was evident from the aspect of the country after the
storm.
Kains so remarkable are necessarily quite limited in extent, for,
if every particle of moisture in the atmosphere were precipitated,
it would cover the entire globe* to a depth of less than four inches.
This result is obtained as follows : The average temperature of
the entire surface of the globe is estimated at 58, and the aver-
age dew-point at 51. At this temperature vapor will sustain a
column of mercury 0.374 inch in height. The weights of equal
volumes of aqueous vapor and air at the same temperature and
pressure are as 5 to 8 nearly, and the specific gravity of mercury
is 13.6. Hence vapor at 51, reduced to water, becomes 0.374 x
13.6x0.624, which equals 3.17 inches.
At the close of a long rain-storm it is not uncommon for the
air to contain more,. moisture than it did at its commencement.
Hence we must conclude that the rain which falls in these re-
markable showers is derived from moist air drawn from remote
places.
223. Deserts. There are large portions of the earth's surface
120 METEOKQLOGY.
where rain is almost entirely unknown, viz., the interior of Africa
between the parallels of 20 and 30, including most of Egypt;
also a considerable portion of Arabia and Persia ; the great des-
ert of Gobi, on the northeast side of the Himalaya Mountains,
with portions of Peru and California.
There are also other districts where the amount of rain does
not exceed one tenth of that which is found elsewhere in the same
latitude, such as Lower California, where the annual fall of rain is
only three inches ; also the northern coast of Africa, Lower Egypt,
and Persia. See Table XXX.
224. Cause of the African Desert. The Great Desert of Africa
lies near the northern limit of the trade winds, where, as we have
already seen, the causes which produce rain act with the least en-
ergy. This desert is an immense sandy plain, with a range of
mountains near its northern, as well as its southern border.
When the N.E. trade wind first strikes the continent of Africa, a
portion of the vapor is condensed on the northern mountains. As
the wind proceeds southward, it advances toward a warmer lati-
tude, which has a greater capacity for moisture ; and there are no
mountains or opposing winds to force the air up above the earth's
surface until we approach the parallel of 10, where we find a long
chain of mountains, over which the vapor is condensed in copious
rains. The heat which is liberated in the condensation of this
vapor is one cause of the steady trade winds, and the absence of
rain over the Desert. Here and there in the midst of the Desert
is found a high peak, or small mountain, and here rain is occasion-
ally seen.
Similar considerations explain the small amount of rain in
Egypt and Arabia.
225. Great Desert of Gobi, etc. The great desert of Gobi is
caused by the Himalaya Mountains. Here the prevalent winds
are from the S.W., and, having just passed over the mountains,
they have lost nearly all their vapor, that is^ they are extremely
dry winds, having little moisture to be precipitated.
Peru is situated within the region of the S.E. trade winds, which,
on meeting the Andes, are forced up to such an elevation that
their moisture is nearly all condensed, and they descend on the
Pacific side as dry winds, and have no moisture which can be con-
PRECIPITATION OF THE VAPOR OF THE AIR. 121
densed at the temperature which prevails in Peru. The princi-
pal tributaries of the Amazon are fed by the rains % which fall on
the windward side of the Andes.
Between the two great mountain ranges, the Sierra Nevadas
and the Eocky Mountains, comprehending portions of Utah, New
Mexico, and California, is a region which is almost entirely desti-
tute of rain. Throughout this region, whether the wind blows
from the east or the west, it has lost most of its vapor by passing
over the mountains. It is therefore a dry air, and has but little
vapor to be precipitated.
So, also, on the east side of the Eocky Mountains, the prevalent
winds, being westerly, have lost their vapor by passing over the
mountains, and the country is a barren desert, almost without rain.
226. Rain without Clouds. Ordinarily clouds seem to be the
reservoirs from which the rain descends, but rain has been known
to fall when no cloud could be seen near the zenith, and even at
times when no cloud appeared above the horizon. Thus, on the
23d of April, 1800, at 9 P.M., rain fell for twenty minutes at Phil-
adelphia, although the heavens immediately overhead appeared
perfectly clear, and the stars shone with undiminished lustre. Not
a cloud could be seen within 15 of the zenith. Also on the 9th
of August, 1837, a shower fell at Geneva, Switzerland, which last-
ed two or three minutes, although the sky was cloudless. Many
similar cases have been observed in other parts of the world.
227. Rain from Clouds not in the Zenith. It is probable that in
some cases rain reaches the earth's surface from clouds removed
several degrees from the zenith. The path of a rain-drop often
makes an angle with the vertical greater even than 45, and rain
might therefore reach the earth from a cloud removed 20 or 30,
and perhaps even farther from the zenith, especially if there pre-
vailed near the earth's surface a fresh breeze, blowing in a direc-
tion different from that of the current which conveys the cloud.
This principle will probably explain some of the cases which
have been reported ; but there are other cases in which it is said
that rain has fallen, although no cloud was visible above the hor-
izon.
228. Rain from Translucent Clouds. It is probable that, in
122 METEOROLOGY.
these cases of remarkable rain-falls, although the sky was free
from dense cjouds, such as entirely conceal the stars, it was not
entirely free from a haziness, which is, indeed, nothing else than
a cloud so thin as to allow the brighter stars to shine through it.
The partial transparency of such a cloud may be due to the small
number and large size of the rain-drops.
Pure water is nearly transparent, and a fog is opaque, simply
on account of the minuteness and consequent multitude of the
condensed particles. A certain amount of light is reflected from
the surface of each particle, and in a fog the number of reflecting
surfaces is so great that a beam of light is wholly reflected before
it can penetrate through the mass. But if the amount of water
which composes a fog were all collected in a few large drops, the
number of reflecting surfaces would be comparatively small ; that
is, they would but slightly affect the transparency of the air. It
is probable, therefore, that when rain falls from a cloudless sky,
the vapor is condensed in a few large drops, instead of a multi-
tude of minute ones. This condensation probably takes place
with great suddenness in the lower strata of the air, which was
previously saturated with moisture.
229. Snoio from a Cloudless Sky. In the polar regions a fine
snow sometimes falls from a cloudless sky. So, also, in New En-
gland, during a period of intense cold, we sometimes see flakes of
snow descending from the sky, although there is no cloud suffi-
cient to obscure the sun or moon, or even the light of the bright-
er stars. In such cases, the vapor rising from the earth is prob-
ably condensed before it attains a great elevation, and both the
thickness and density of the cloud are quite small.
SECTION VI.
SNOW.
230. How Flakes of Snow are formed. When the vapor of the
air is precipitated at a very low temperature, the vapor is con-
densed in the solid state, without passing through the condition
of a liquid, and generally assumes the crystalline form. These
minute crystals of ice attach themselves to each other and form
flakes of snow, which descend very slowly to the earth. When
the lower stratum of the air is much above 32, the flakes of snow
PRECIPITATION OF THE VAPOR OF THE AIR. 123
melt before they reach the ground, so that rain may frequently be
seen falling on an open plain, while from the same cloud snow is
falling upon a neighboring mountain.
During the severe cold of winter we may frequently witness
snow produced artificially. When a large number of people are
assembled in the same hall, and the room being uncomfortably
warm, a window is opened, the warm air of the room is frequent-
ly condensed by the cold external air, and falls to the ground in
the form of flakes of snow of extreme delicacy.
231. Where Snow Falls. Within the torrid zone snow is al-
most never seen, except on elevated mountains, because near the
level of the sea the temperature is above the freezing point. For
a similar reason, in the middle latitudes, the fall of snow occurs
only in winter, while in the polar regions nearly all the moisture
which is precipitated descends to the earth in the form of snow.
The zone within which snow never falls is determined not so
much by the mean temperature of the year, or the mean temper-
ature of the coldest month, as by the temperature of the coldest
day of winter. At all places where the thermometer in winter
sinks much below 32, snow may occasionally fall. The bounda-
ry of the zone within which snow does not fall, except in a few
very rare cases, is an undulating line crossing the Pacific coast
of America near lat 39, and the Atlantic coast near lat. 35 ; it
passes near Gibraltar in lat. 36, and on the coast of China de-
scends to lat. 24, which is but a little north of Canton.
A slight fall of snow occasionally occurs at San Francisco, Cali-
fornia ; it occasionally falls at New Orleans, and also at Galves-
ton, lat. 29; and snow sufficient for sleighing has been known at
Charleston, S. C. Snow has also been known to fall at Canton,
within the torrid zone, to the depth of four inches.
232. Annual Amount of Snow. The amount of snow which
falls in a year varies in different localities from zero to twelve feet
In Spitzbergen the annual fall of snow is from three to five fet.
In the State of Maine the average annual fall of snow is seven
and a half feet, and the amount in a single year has been known
to exceed twelve feet ; but this amount is not all seen at the same
time. In Vermont and New Hampshire the annual fall is six feet.
In Central Massachusetts the annual fall is four and a half feet, and
124
METEOROLOGY.
the snow has been known to lie five feet on a level. In Connecti-
cut the average fall is three and a half feet ; in New Jersey, two
and a half feet; in Southern Ohio, one foot and a half; and in
Iowa, one foot.
Snow recently fallen has a very small specific gravity, for a
foot of snow, when melted, furnishes only one inch of water.
233. Form of Snoiv-flakes. Crystals of ice generally exhibit
the form of long needles or spiculae, each being a slender prism
with angles of 120. These crystals are often seen in great per-
fection in hoar-frost. Flakes of snow generally consist of combi-
nations of spiculas and of thin plates or laminaB of ice, which usu-
ally present angles of 60 or 120. Sometimes we find simply six
spiculas combined in angles of 60, forming a star with six rays.
Sometimes to each of these spicula3 are attached shorter spicula3,
also inclined at angles of 60, in number amounting to 2, 4, 6, etc.,
up to a dozen or more, forming a perfectly symmetrical figure
bearing some resemblance to a flower of great complexity. See
the first six forms in Fig. 52.
Fig. 52.
PRECIPITATION OF THE VAPOR OF THE AIR. 125
Sometimes we find a simple lamina of ice, in which case the
form is usually that of a regular hexagon, which sometimes has
the appearance of being composed of equilateral triangles. Some-
times ice spiculae are attached to the angles of the hexagon ; some-
times attached to the angles of a central hexagon we find six
smaller hexagons, or perhaps rhomboids composed of two equi-
lateral triangles. Sometimes the central figure consists of six
such rhomboids, with ice spiculse or other rhomboids attached to
the angles.
Sometimes the flakes present forms which can not apparently
be resolved into any of the preceding elements.
Several hundred different forms of snow crystals have been ob-
served and figured. Fig. 52 presents a specimen of the simplest
forms, and also of the most complicated. These crystals are seen
in their greatest perfection when the air is tranquil, cold, and dry.
234. Size of Snow-flakes. Snow-flakes vary in size, according
to the temperature at which they are formed. If formed at a very
low temperature, their diameter is often less than one tenth of an
inch ; when formed near the temperature of 32, they are some-
times found one inch in diameter.
235. Natural Snow-lolls. Sometimes a vast number of snow-
flakes attach themselves together, and descend to the earth as a
loose snow-ball one or two inches in diameter. Sometimes, after
the snow has fallen, it is driven along by the wind, and is rolled
into balls of vast size. These balls are usually cylindrical, some-
what hollowed in the centre, and they have been known to attain
a diameter of three feet. They are of common occurrence on the
slopes of the Alps, in Switzerland.
. m.
236. Snow White and Phosphorescent. Since snow is but frozen
water, it might be expected that it would be transparent like wa-
ter, or large blocks of pure ice. Its brilliant whiteness is due main-
ly to the number of reflecting surfaces arising from the small size
of the spiculaB of ice. In the same manner, the most transparent
glass loses its transparency when pulverized.
Snow is feebly phosphorescent. This is proved by the fact that
in the darkest nights, when the ground is covered with snow, the
snow appears more luminous than the sky. Its light can not,
126 METEOROLOGY.
therefore, be simply the reflected light of the sky. This phos-
phorescence appears to be in part acquired by exposure to the
rays of the sun during the preceding day. If, on the morning of
a. clear day, we cover a portion of the snow with an opaque screen,
and uncover it at evening, we shall find that this portion is some-
what less luminous than the surrounding snow. Snow, like many
other substances, after being exposed to a bright light, retains a
portion of the light for some time after the source of light is with-
drawn.
237. Red Snow in the Polar Regions. In those places where
snow lies unmelted from one year to another, it sometimes ac-
quires a ruddy color, and occasionally becomes red like blood.
This occurs in the polar regions, and also on the mountains of
Southern Europe. In Spitzbergen the snow sometimes appears
of a green hue. It has been discovered that these colors are due
to a vegetable production resembling a mushroom, which is exces-
sively minute, not exceeding Trnro tn ^ ncn i n diameter. There is,
then, a species of vegetation which may* flourish at a temperature
which never exceeds that of melting ice.
238. Glaciers. The summits of high mountains, even under
the equator, are covered with perpetual snow. Within the trop-
ics the limit of perpetual snow varies from 16,000 to 18,000 feet,
while on the Alps of Switzerland it varies from 8000 to 9000 feet.
On these mountains the snow accumulates from year to year, and
in sheltered ravines, where it can not be blown away by the wind,
acquires an immense thickness. Under continued pressure this
sriow becomes solidified, so as to acquire the density of compact
ice. The gorges of the Alps are filled with ice of this descrip-
tion, which is known by the name of glaciers. These glaciers are
from five to ten or more miles in length, and they follow the
gorges from the summit of Mount Blanc down to the base of the
mountain. They are frequently half a mile or more in breadth,
and have a thickness of 200 to 5000 feet. This ice, sustaining the
pressure of a long column, rising to the height of 10,000 or 12,000
feet, is crowded down into the valleys, so that the entire glacier
has a descending motion like a river. The principal glacier of
Switzerland has a descending motion which in some places
amounts to 876 feet in a year, and in other places only 274 feet.
PRECIPITATION OF THE VAPOR OF THE AIR.
127
This motion is continuous, and is probably never wholly inter-
rupted. Nevertheless, the motion is greatest in summer and
least in winter, and the velocity increases with the angle of de-
scent. The middle of the glacier generally moves faster than the
sides. These glaciers extend down into the valleys, where the
temperature is such as to allow wheat and potatoes to come to
maturity, and a traveler may sometimes stand upon the edge of a
glacier and pick ripe cherries from a tree. The ice melts, indeed,
under a summer's sun, but the waste of summer is supplied by the
slow motion of the descending mass, so that the lower end of the
glacier remains nearly stationary from age to age. Fig. 53 rep-
resents one of the most remarkable glaciers of the Alps. It is
seen to be intersected by numerous fissures, caused by its mo-
tion down an irregular valley.
Fig. 53.
^=
The total number of glaciers among the Alps is estimated at
between 500 and 600, and they cover an area of nearly 1500
square miles. The lowest of the glaciers of the Alps descends to
the level of 3400 feet above the sea,
239. Geographical Distribution of Glaciers. No glaciers have
128 JVIETEOKOLOGY.
been found within the tropics, but they are common on the high
mountains of the middle latitudes, and especially in the polar re-
gions. The glaciers of the Himalayas are very numerous and of
immense extent, and are the sources of large rivers. In lat. 27
they descend to the level of 13,000 feet, and in lat. 36 they de-
scend to the level of 9000 feet.
The Pyrenees are nearly destitute of true glaciers.
The elevated mountains of Greenland are covered with perpet-
ual snow and ice, which in many places extends to the sea-shore.
The snow of winter becomes solidified by the warmth of summer,
acquiring in time the density of ice. This ice is crowded down
by its own weight into the sea, and sometimes extends several
miles beyond the original shore-line. By the buoyant power of
the water the outer end of the glacier is lifted, and after a time a
mass, perhaps a mile or more in diameter, is cracked off. This
mass is drifted southward to the middle latitudes, and is called
an iceberg. An iceberg has been measured three fourths of a
mile square, and 315 feet high. Large icebergs continue unmelt-
ed for many weeks, and sometimes advance to lat. 36.
In Norway the glaciers are numerous, and near lat. 60 one of
them descends to within 150 feet of the sea level, while in lat. 70
they descend into the sea.
In Spitzbergen one glacier presents a front of eleven miles to
the sea, with a cliff 400 feet high, and extends backward to the
mountains.
The interior of Iceland is covered with glaciers.
On the west coast of Patagonia glaciers are numerous, and in
lat. 46 S. they descend to the sea.
The glaciers of Victoria Land, lat. 70 to 79 S., are even more
extensive than those of Greenland.
240. Avalanches of Snow. The snow which during winter ac-
cumulates on the sides of the Alps and other mountains, becomes
softened during the summer, and frequently descends into the val-
leys in large masses called avalanches. During summer these
avalanches are of hourly occurrence on some parts of the Alps,
sweeping down a slope of several miles into the valleys, and are
among the chief dangers encountered by travelers who attempt
to climb the mountains.
PRECIPITATION OF THE VAPOR OF THE AIR. 129
SECTION VII.
HAIL.
241. Sleet. In the middle latitudes, in the cold months of the
year, during gusty weather, there often fall from the sky small
spheres of ice, having a diameter of one twelfth to one sixth of
an inch. They are generally soft, opaque, and of a whiteness ap-
proaching that of snow. The largest are sometimes surrounded
with a slight film of ice. Sometimes small hailstones consist en-
tirely of transparent ice, and these are probably rain-drops falling
from clouds brought by south winds, which freeze in traversing
cold strata of air near the earth.
The small hailstones of winter are termed sleet, to distinguish
them from large hail, which falls under different circumstances.
242. Large Hail. Large hail seldom if ever falls except during
thunder-storms. It falls at the commencement of the storm or
during its continuance. It very rarely follows rain, especially if
the rain has continued for some time. The area covered by the
rain-storm is much larger than that covered by the hail, and the
hail at any one place continues but a very short time, generally
only five or ten minutes, seldom so long as fifteen or twenty min-
utes.
In the United States large hail falls chiefly in summer and the
latter part of spring. In India hail falls chiefly in the four months
from February to May.
Hail falls at all hours of the day and night, but large hail is
most common about the hottest part of the day, that is, about 2
P.M. The fall of large hail is commonly preceded by an unusual
degree of heat. An extraordinary rise of the thermometer in
April or May affords reason to anticipate a hail-storm.
243. Size of Hailstones. The size of hailstones varies from one
tenth of an inch or less in diameter to more than four inches. On
the 13th of August, 1851, about 1 P.M., hailstones fell in New
Hampshire weighing 18 ounces. A sphere of solid ice weighing
18 ounces has a diameter of four inches, and a circumference of
12-j inches. In the present case the stones were somewhat po-
rous and of irregular shape, and their largest circumference ex-
T
130 METEOKOLOGY.
ceeded 15 inches. A few years since, hailstones weighing sixteen
ounces fell in the city of Pittsburg, and hailstones weighing over
half a pound have fallen in several places of the United States.
On the 7th of May, 1822, there fell at Bonn, in Germany, hail-
stones weighing from twelve to thirteen ounces, and stones weigh-
ing half a pound have repeatedly fallen in France and Italy.
Large hail is of common occurrence in India. On the llth of
May, 1855, about 6 P.M., near the Himalaya Mountains, in lati-
tude 29, hailstones fell weighing from eight to ten ounces, and
one or two weighed more than a pound.
On the 22d of May, 1851, in latitude 13 north, in the southern
part of India, many hailstones fell about the size of oranges. The
next morning, in a dry well, there was found a block of ice meas-
uring 4-|- feet long, 3 feet broad, and 18 inches thick. It is not
supposed that this ice fell from the sky in a single block, but
after their fall the separate hailstones became cemented together
so firmly by ice as to form one solid block. Similar masses of
ice derived from hail have been repeatedly seen in India, and
also in the United States.
244. Quantity of Hail. The quantity of hail which falls from
the sky in a single shower is sometimes enormous. In the New
Hampshire storm of 1851 the average depth of the hail was/owr
inches. In a storm which passed over the Orkneys, on the north
of Scotland, July 24th, 1818, the depth of the hail was nine inches.
On the 17th of August, 1830, in the streets of Mexico, hail fell to
the depth of sixteen inches.
245. Form of Hailstones. Hailstones are ordinarily of a sphe-
roidal form; .sometimes they are oval, sometimes flattened, and
sometimes of a very irregular shape. Very large hailstones often
present remarkable protuberances. They often consist of an ir-
regular assemblage of angular pieces of ice, which individually
do not exceed the size of walnuts, but cemented firmly together,
forming a; mass as large as an orange, and sometimes as large as
a turkey's egg. These small portions generally indicate a tend-
ency to crystallization. Sometimes hailstones are studded with
crystals in the form of hexagonal prisms, and when the angles melt
away the prisms become nearly cylindrical. The following fig-
ure .represents a hailstone which probably consisted originally
PRECIPITATION OF THE VAPOR OF THE AIR.
131
of numerous prisms ce-
mented together, but it
became so modified by
melting during its fall as
nearly to obliterate the
crystalline structure.
Sometimes hailstones
have the form of pyra-
mids, whose angles are
rounded by a partial
melting, and whose base
is a portion of an irregular spherical surface.
246. Structure of Hailstones. The centre of large hailstones
usually consists of hardened snow, and this is surrounded by a
coat of transparent ice. Sometimes we find alternate layers of
opaque snow and transparent ice. Often
hailstones exhibit a radiated structure, re-
sulting apparently from rows of air-bubbles
disposed in radii from the centre. Some-
times large hailstones consist of very trans- *
parent and solid ice with numerous air-bub-
bles. Fig. 56 represents a section of a hail-
stone whose external appearance is repre-
sented in Fig. 55. Hailstones with a radi-
ated structure, when broken, incline to di-
vide into spherical pyramids, with layers
parallel to their base, and this is probably
the origin of pyramidal hailstones. The
rupture of the spherical hailstone may be
due to the sudden expansion experienced
in passing from an exceedingly cold to a comparatively warm
atmosphere.
247. Geographical Distribution of Hail. Within the tropics hail
is of rare occurrence at the level of the sea, but when it does oc-
cur the stones are generally of very great size. It becomes more
common at the height of 1500 feet. In India, hail is very com-
mon on the mountains, and occurs occasionally at the level of the
sea, even south of latitude 20.
132
METEOROLOGY.
Hail is most common in the middle latitudes. In Europe hail
occurs most frequently near the Atlantic coast, and diminishes in
frequency as we proceed eastward. In France hail falls, on an
average, fifteen times in a year ; in Germany, five times ; and in
Russia only three times. H&il falls in every part of the United
States, but cases of very large hail occur but seldom. Hail falls
occasionally, but not often, in the West India Islands.
248. Track of Hail-storms. Hail-storms usually travel rapidly
over the country in straight bands of small breadth, but consid-
erable length. The track of the New Hampshire storm was sev-
eral miles in length, but only two miles in breadth. The track
of the Orkney storm was twenty miles long and a mile and
a half wide, and the storm traveled at the rate of forty miles per
hour.
On the 13th of July, 1788, a hail-storm traveled from the S. W.
part of France to the shores of Holland at the rate of 46 miles
per hour. There were two distinct bands of hail, the breadth of
that in the west being eleven miles, and that in the east six miles,
with a space of fourteen miles between them. The fall of hail
' upon these two bands was not exactly contemporaneous, but one
preceded the other about fifteen minutes. Rain fell on the outside
Fig. 57.
Ecau.va.is.
I&aqny.
of these bands of hail,
as well as on the space
between them. Each
band of hail extended a
distance of about 500
miles. Figure 57 repre-
sents a portion of the
track of this storm in
the neighborhood of
Paris. The dotted bands
represent the track of
the hail, while the three
shaded bands represent
the area of the rain.
249. Height at which Hail is formed. Observations made in
mountainous countries have enabled us to determine nearly the
elevation at which hail is formed. Small hail is of common oc-
PRECIPITATION OF THE VAPOR OF THE AIR. 133
currence on the summit of Mount Blanc, 15,744 feet above the
level of the sea, but large hail has never been seen there. In In-
dia, at the height of 8000 feet, hailstones have fallen of sufficient
size to do considerable damage.
In 1835, hailstones weighing eight ounces fell at the base of a
mountain in the southern part of France, while only small hail
fell at the height of 4000 to 5000 feet. From these and similar
observations, it is inferred that, in the middle latitudes, hail often
begins to form at an elevation exceeding 16,000 feet, but attains
its greatest size below the height of 5000 feet.
250. Origin of the Cold which causes Hail. The cold which
congeals such large masses of ice in summer is mainly due to ele-
vation. The temperature of hailstones at the instant of their fall
has often been found below 32, and sometimes as low as 25 F.
They must, then, have been subjected to a temperature considera-
bly below that of melting ice, probably to a temperature as low
as 20 F. In the neighborhood of New York, at the height of
18,000 feet, the average summer temperature is 20, and it is be-
lieved that during the formation of hail the temperature of the
upper air is considerably below the mean.
251. Noise preceding the Fall of Hail. Some seconds before the
fall of hail, and occasionally several minutes, a peculiar crackling
noise is often heard in the air. It has been compared to the
noise of walnuts violently shaken up in a bag. This noise has
been ascribed to the great velocity with which the hailstones are
driven through the air, while some have ascribed it to feeble elec-
trical discharges from one hailstone to another, for electricity al-
ways attends the progress of a hail-storm.
252. Hail attended by Two Currents of Air. The formation of
hail is invariably attended by two distinct currents of air, and one
of these currents displaces the other with great violence. The
current of air which precedes the approach of a hail-storm is ex-
tremely hot, and highly charged with moisture ; that which suc-
ceeds the fall of hail has an icy chillness. The warm and humid
air is displaced by the cold current, and is thus forced up to a
great elevation above the earth, by which means its vapor is sud-
denly condensed. Upon the front of the hail-cloud this condensed
134 METEOROLOGY.
vapor exists in the form of water, whose temperature is near 32.
In the interior of the hail-cloud the vapor is precipitated in the
form of snow, whose temperature is sometimes as low as 20.
253. Process of the formation of Hail. Observations on the
summits of mountains have shown that, on the front of the hail-
cloud, there exists a violent whirling motion about a horizontal
axis. This whirling motion causes the snow to collect in small
balls, each of which forms the nucleus of a hailstone. The snow-
ball is forced into the warm current, where it receives a layer of
water, which is congealed by the cold of the nucleus, thus render-
ing the snowy centre more compact, and adding a shell of trans-
parent ice. By means of the whirling motion, the hailstone, cov-
ered with a stratum of uncongealed water, is hurled into the snow-
cloud, where it receives a layer of snow, and again becomes thor-
oughly chilled. Thence it escapes again into the water-cloud, and
is covered with a layer of water, which is congealed by the cold
of the nucleus. Thus, by the whirling motion, it is plunged al-
ternately into the snow-cloud and the water-cloud, while each al-
ternation furnishes a layer of spongy ice and a layer of transpa-
rent ice. Thus the stone grows with immense rapidity, and in a
few minutes becomes a large ball, three or four inches in diame-
ter. This oscillatory motion of the hailstones, on the front of the
hail-cloud, was distinctly observed by M. Lecoq in 1835 on the
summit of a mountain in the southern part of France.
254. How Hail is sustained in the Air. The hailstones are sus-
tained in the air by the violent upward motion caused by the cold
current displacing the warm one. A sphere of ice two inches in
diameter, by falling through a tranquil atmosphere, soon acquires
a velocity of 90 feet per second. A hailstone of irregular shape
would experience more resistance than a sphere, and would ac-
quire a somewhat less velocity, but it would still fall from a height
of 18,000 feet in about three minutes, which time is too small to
allow the formation of masses of ice weighing one pound. An
upward current of air rising with a velocity of 90 feet per second
would sustain a sphere of ice two inches in diameter, and would
greatly reduce the velocity of stones of larger size.
255. How long -may Hailstones be sustained? The strong up-
PRECIPITATION OF THE VAPOR OF THE AIR. 135
ward movement which always attends the formation of hail is
probably sufficient to sustain hailstones of the largest size as long
as they can be kept within the influence of this vortex. A period
of ten minutes is probably sufficient for the formation of hail-
stones of the largest size. After escaping from the influence of
this vortex, small stones would fall to the earth, from an eleva-
tion of 5000 feet, in about two minutes, and very large stones in
one minute.
256. Origin of the Parallel Bands of Hail. It is not uncommon
for two or even three such vortices to form on the same day, and
nearly at the same hour, at places not very remote from each oth-
er, thus forming parallel bands of hail separated by an interval
of from 10 to 100 miles. Such was the French storm of 1788,
and similar cases have frequently occurred in the United States.
257. Origin of Sleet. The small, spongy hail of winter is prob-
ably formed in the same way as the large hail of summer ; but
since, in winter, the amount of vapor present in the air is small,
the amount of precipitation is small, and the hailstones can never
attain a large size.
258. Hail-rods. It has been proposed to preserve vineyards
and valuable farms from the ravages of hail by erecting an im-
mense number of poles, armed with iron points, communicating
with the earth, for the purpose of drawing off the electricity of
the clouds. Multitudes of these hail-rods were formerly erected
in Switzerland, but without the expected success.
It is believed that electricity performs altogether a subordinate,
if not an unimportant part in the formation of hail ; and if we
could draw off all the electricity from the hail-cloud as fast as it
was generated, it is not improbable that hail would be formed
about as large and as abundantly as at present.
But, even supposing electricity to be the sole agent in the pro-
duction of hail, hail-rods could not be expected to furnish security
against hail unless an entire continent could be studded thick
with them, for in the middle latitudes the hail-cloud advances
eastward with a velocity sometimes of 40 or more miles per hour,
and the hailstones which fall in one locality are those which were
forming when the cloud was many miles westward of that point ;
136 METEOROLOGY.
so that, to protect a small spot, the whole country for many miles
westward should be armed with rods ; and it is conceivable that
a hail-cloud arriving over a region studded with these rods might
immediately pour down a large quantity of hailstones which
would have fallen farther eastward if the rods had not discharged
the electricity of the cloud.
CHAPTEK VI.
STOKMS, TORNADOES, AND WATER-SPOUTS.
SECTION I.
THEOEY AND LAWS OF STORMS.
259. What is a Storm ? Any violent and extensive commotion
of the atmosphere is called a storm. Storms are usually attended
by a fall of rain, or snow, or hail, and frequently by thunder and
lightning ; but, although it is probable that a precipitation of va-
por always takes place over some portion of the area of every vio-
lent storm, yet the storm often extends beyond the area of rain or
snow.
260. Cause of Storms. Storms are caused by a strong and ex-
tensive upward motion of the air, by which means its vapor is
condensed by the cold of elevation.
The atmosphere receives heat from the sun, and it loses heat
by radiation. Only about one fourth of the rays of the sun are
absorbed in passing vertically through the atmosphere. The re-
maining three fourths are absorbed by the earth's surface, by
which means its temperature is raised, and heat is thence commu-
nicated to the air which rests upon the earth. The atmosphere
thus receives its heat chiefly at the bottom, and, in consequence
of radiation, loses it most rapidly at the top.
Since the density of the air is diminished by an increase of
heat, the atmosphere is in a state of unstable equilibrium, and the
lower strata tend continually to rise and take the place of the up-
per. Such ascending currents are formed on every tranquil day.
As the air ascends it comes under diminished pressure and ex-
pands, and as it expands it cools at the rate of about 38 degrees
STORMS, TORNADOES, AND WATER-SPOUTS. 137
for two miles of ascent. This ascending air carries with it the
vapor which it contained at the earth's surface, and, if it rises
high enough, the cold produced by expansion will condense a
portion of this vapor into cloud. The height to which the air
must ascend before it will become cold enough to form cloud de-
pends upon the difference between the dew-point and the tem-
perature of the air. If the dew-point be ten degrees below the
temperature of the air, cloud will begin to form when the ascend-
ing current has risen about 1500 feet.
261. Latent Heat liberated. As soon as a cloud begins to form,
the latent heat of the vapor is liberated. To convert water into
vapor requires a great amount of heat, and this heat is not appre-
ciable to a thermometer; hence it is called latent heat. When
the vapor returns to the condition of water, this heat is liberated,
and becomes sensible heat ; and when a cubic foot of water is
precipitated from the air, as much heat is liberated as would be
required to convert that amount of water into vapor. When one
inch of rain falls from the sky, the amount of water precipitated
exceeds two millions of cubic feet for each square mile of surface ;
and over each square mile of surface as much heat would be lib-
erated as would be required to evaporate two millions of cubic
feet of water.
By the heat thus liberated in the production of a cloud, the air
in the cloud is warmed ; 'it expands in volume, and the cloud con-
tinues to ascend as long as its temperature is greater than that of
the surrounding air. As the cloud ascends, more vapor is con-
densed, while the latent heat evolved raises still farther the tem-
perature of the air in the cloud.
262. Shape of the Cloud thus formed. When, in consequence
of an ascending column of air, a cloud begins to form, it is seen to
swell out at the top, but its base continues at the same level ;
that is, the base is flat, even after the cloud has acquired great
vertical height. The motion of the air here described is illus-
trated by Fig. 58, on the following page. During a warm and
tranquil day many of these ascending columns are formed, and
two or more adjacent columns often unite to form a single col-
umn.
The clouds thus formed during the day often subside and
138
METEOROLOGY.
Fisr. 58.
dissolve at evening, when tjie
surface of the earth becomes
cooled by radiation, and thus a
cloudy day is often followed
by a cloudless evening; but
when the atmosphere is unu-
sually heated, and contains a
large amount of vapor, the as-
cending columns generally go
on increasing until rain de-
scends.
263. Why the Barometer falls under the Cloud. The expansion
of the air in the forming cloud, particularly after rain begins to
fall, causes the air to spread out in all directions above, causing a
barometer under the middle of the cloud to fall below its mean
height, and beyond the limits of the cloud to rise above its mean
height. Near the limits of the cloud, the air, in consequence of
its greater weight, sinks downward, and a portion of it flows along
the earth's surface toward the centre of the ascending column,
while beyond the limits of the cloud there is a gentle wind out-
ward from the cloud. Since the air spreads out more rapidly
above the cloud than it runs in below, storms tend to increase in
diameter, and they often extend with great rapidity until they
cover an area more than a thousand miles in diameter.
264. Observations of the Barometer represented upon a Map.
For the purpose of discovering the laws of storms, observations
have been extended over a large portion of the earth's surface.
In order to have a summary of all the barometric observations
presented conveniently to the eye, we spread before us a map of
the country, and draw upon it a line connecting all those places
where the barometer at a given instant stands at its mean height.
We draw another line connecting all those places where the ba-
rometer stands half an inch below the mean, another where the
barometer stands half an inch above the mean, etc. Such lines
show at a glance where there is an excess and where there is a
deficiency of pressure, and what is the amount of this excess or
deficiency.
STORMS, TORNADOES, AND WATER-SPOUTS. 139
265. Amount of the Barometric Depression. Storms are often
experienced simultaneously over large portions of the earth's sur-
face. The storms of winter are particularly severe and extensive.
The following remarks are restricted to winter storms, because
their laws are best understood, although it is probable that \^in-
ter storms do not differ materially from summer storms, except
in their extent and severity.
Storms are generally accompanied by a considerable depression
of the barometer below its mean height, and are succeeded by a
rise of the barometer above its mean height. During the passage
of a winter storm over the middle latitudes of North America, the
barometer frequently sinks below its mean height over an area
more than a thousand miles in diameter.
This area of low barometer is sometimes nearly circular ; more
frequently its form is very much elongated, its length being two
or three times its breadth ; and in the United States, the longer
axis of this oval is uniformly turned in a north and south direc-
tion. Sometimes the barometer sinks below its mean height over
an area extending 3000 miles in a north and south direction, and
1000 miles in an east and west direction. The area over which
the barometer sinks half an inch below the mean sometimes ex-
tends 800 miles from north to south, and 400 miles from east to
west. At the centre of the storm the barometer sometimes sinks
an inch below its mean height.
Beyond the area of low barometer, the barometer rises above its
mean height frequently to the amount of half an inch, sometimes
to the amount of an entire inch, and occasionally still higher.
266. Atmospheric Waves and Ocean Waves compared. If these
inequalities of pressure were due, not to a change in the elastic
force of the atmosphere, but simply to a change of height, and the
atmosphere were a visible substance, then an observer sufficient-
ly elevated above the earth might see vast depressions and eleva-
tions in the atmospheric envelope of the earth bearing some anal-
ogy to the waves of the ocean during a storm, but having vastly
greater dimensions. The waves of the ocean have a breadth of
a few rods and a length of a few miles, while the waves of the
atmosphere sometimes have a breadth of one or two thousand
miles, and a length of several thousand miles.
140 METEOROLOGY.
267. Gradual Rise and Decline of Storms. Winter storms com-
mence gradually, and generally attain their greatest violence only
after a lapse of several days. After a certain period their vio-
lence gradually diminishes, and at length they disappear entirely.
This succession of changes requires a period of several days, some-
times one or two weeks, and possibly even longer.
Sometimes all these changes are experienced over the same
country ; that is, the storm makes no progress from place to place.
More commonly, however, the storm travels along the earth's sur-
face, and although the same storm may continue for one or two
weeks, or even longer, its duration at any one place may not ex-
ceed one or two days.
268. Direction and Rate of Progress. Throughout the middle
latitudes of this continent, when violent storms advance with con-
siderable rapidity, the direction of progress is always from west
to east. This direction is not absolutely uniform, but has been
observed to vary from about due east to north 54 east.
The rate of progress of storms has been observed to vary from
zero to forty-four miles per hour. They generally travel from
St. Louis to New York in about twenty -four hours, and from New
York to Newfoundland in another twenty -four hours.
Generally, when the barometer is unusually low at New York,
it is unusually high at St. Louis, and also high in Newfoundland.
When a storm is about stationary, the form of the area of low
barometer is nearly circular ; but when the storm travels rapidly,
this area is generally compressed in an east and west direction.
The winter storms of the United States are therefore said to move
side foremost.
269. Fall of Rain or Snow. Great and sudden depressions of
the barometer are almost invariably accompanied by a fall of rain
or snow, and the area of greatest rain or snow corresponds nearly
to the region of greatest barometric depression. Eain and snow
are produced under circumstances exactly alike, with the excep-
tion of temperature ; and the same storm frequently furnishes
snow in the northern part of the United States, and rain in the
southern part.
270. Direction of Wind on different sides of a Storm. Since the
STORMS, TORNADOES, AND WATER-SPOUTS. 141
tendency of the wind is always from a region of high barometer
toward a region of low barometer, the wind must every where
tend inward toward the centre of a violent storm ; in the same
manner as when a quantity of water is dipped from a tranquil
la'ke, the surrounding water immediately flows in to restore the
level surface.
But the currents of air thus set in motion toward the centre
of a storm can not proceed directly toward that centre. On the
north side of that centre the air is moving from a parallel which
has a less velocity of rotation eastward toward a parallel which
has a greater rotary velocity. It therefore has a relative motion
toward the west, and that which would have been a north wind
if the earth did not rotate upon its axis, becomes a northeast
wind. So, also, on the south side- of the centre, the air is moving
from a parallel which has a greater rotary velocity toward a par-
allel which has a less velocity. It therefore has a relative mo-
tion toward the east, and that which would have been a south
wind if the earth did not rotate upon its axis, becomes a south-
west wind.
The wind, therefore, instead of blowing from every point of the
compass directly toward the centre of the vortex, moves spirally
inward, making a great circuit round the centre; and in the United
States this rotation is in a direction contrary to that of the hands
g9 of a watch, or, as it is call-
ed ; from right to left. _
The force of the wind
is generally proportional
to the amount and rapid-
ity of the depression of
the barometer.
271. A European Storm.
-The direction of the
wind at the earth's sur-
face is greatly influenced
by the irregularites of the
earth, as well as by local
differences of temperature
and moisture ; neverthe-
SOinch * n , . ' . .
less, observations of vio-
142
METEOROLOGY.
lent storms show that the prevalent direction of the wind corre-
sponds to the preceding principles. Figure 59, on the preceding
page, represents the winds as actually observed near the centre
of a violent storm of rain and snow which was experienced in
Central Europe on the 25th of December, 1836. The smaller oval
shows the area within which the barometer was depressed three
fourths of an inch below the mean, and the larger oval shows
the area of one half inch barometric depression. The arrows
show the observed directions of the wind over an area about
900 miles in diameter, and this storm was nearly stationary for
four days.
272. American Storms. Figure 60 represents the winds as ob-
served near the centre of a violent storm of rain and snow, which
Fig. GO. was experienced in
"" f the neighborhood of
New York, on the
16th of February,
1842. The small
oval line shows the
area within which
the barometer sunk
eight tenths of an
inch below the mean,
and the larger oval
shows the area of
seven tenths inch
barometric depres-
sion. The long ar-
row represents the
direction in which
the storm advanced.
The short arrows
show the observed directions of the wind over an area about 500
miles in diameter.
The principles already stated are more fully illustrated by Plate
III., which represents the principal phenomena of a storm which
passed over the United States in December, 1836. The upper
map represents the phenomena for 8 P.M., December 20, and the
lower map represents the same for 8 A.M., December 21. A
\
'-.Winch.
STORMS, TORNADOES, AND WATER-SPOUTS. 143
comparison of the two maps shows the progress of the storm in
twelve hours. The area of the rain or snow is represented by
the dark shade near the middle of each map, and the lighter
sha'de on the margin of the rain represents the region where
clouds prevailed without rain. Throughout the remaining por-
tion of the United States, as far as the maps extend, clear sky pre-
vailed.
The dotted curve lines represent the state of the barometer.
On map first the inner curve shows the area where the barometer
was depressed four tenths of an inch below the mean ; the next
curve shows where the barometer was two tenths of an inch be-
low the mean ; the next curve shows the barometer at its mean
height, while farther eastward the barometer stood two tenths of
an inch and four tenths of an inch above the mean. On map
second these curves are seen to have been somewhat modified in
form, and to have been carried eastward a distance of about 450
miles.
The arrows show the directions of the wind as actually observed
at a large number of stations, and these directions will be seen to
conform generally to the principle stated in Art. 270, with a few
exceptions, which may, perhaps, be ascribed to the influence of
local causes.
273. Distinction between the Direction of the Wind and that of the
Storm's Progress. It will thus be seen that the direction of the
wind at any place is entirely distinct from that of the storm's
progress over the earth's surface. While the storm advances
steadily eastward, the wind has every possible direction at differ-
ent places within the limits of the storm.
At places on the north side of the centre of a great storm the
wind generally sets in from the north of east as the storm ap-
proaches, and as the storm passes by the wind changes to the
northwest, veering round by the north point. At places on the
south side of the centre of the storm the wind generally sets in
from the south of east as the storm approaches, and as the storm
passes by the wind changes to the southwest, veering round by
the south point.
Frequently the centre of a great winter storm is situated be-
yond the limits of the United States on the north, and then,
throughout the entire United States, as far as observations have
144 METEOROLOGY.
extended, the wind blows from the E. or S.E. on the front of the
storm, and from the W. or S.W. on the rear of the storm.
274. Lull at the Centre of a Storm. Near the centre of a great
storm there is generally a lull of the wind, and sometimes a calm.
Sometimes the clouds open, exhibiting considerable clear sky, and
occasionally the "clouds disappear entirely for several hours, ex-
hibiting a clear sky, with little wind and a mild temperature.
Soon after the centre of the storm has passed eastward of the ob-
server the wind generally changes to the west, and the barometer
begins to rise. The rain or snow, which may have been tempo-
rarily suspended, is renewed, generally with considerable violence,
which, however, in such cases, is not usually of long continuance.
275. Wind on the Extreme Borders of a Storm: Near the line
of maximum pressure which surrounds a violent storm there is
generally but little wind, and on each side of that line the winds
are irregular in their direction, but generally tend outward from
the line of greatest pressure. Hence it happens that near the ex-
treme borders of a storm the winds are found blowing in nearly
opposite directions, on one side inward toward the storm, and on
the other side outward from the storm.
276. How Winds are Propagated from Place to Place. Since on
the opposite sides of a storm the wind blows in nearly opposite
directions, while the entire storm makes progress toward the east,
it is evident that some winds must be propagated from place to
place nearly in the same direction as that in which they blow,
while others are propagated in a direction opposite to that in which
they blow. When a great storm springs up near the Mississippi,
the wind at St. Louis is generally easterly, while throughout New
York and Ohio the wind is from the west. Subsequently this
easterly wind is felt at Cincinnati, then at Pittsburg, and after-
ward at New York, while the entire storm is traveling steadily
eastward ; that is, the easterly wind is propagated from St. Louis
to New York in a direction opposite to that in which the wind
blows.
After the centre of the storm has passed, a west wind springs
up at St. Louis, and this west wind is felt successively at Cincin-
nati, Pittsburg, and finally at New York, having been propa-
STORMS, TORNADOES, AND WATER-SPOUTS. 145
gated in the same direction as that in which the wind blows.
The former wind is said to be propagated by aspiration, the latter
by impulsion, as stated in Art. 142.
277. Temperature near the Centre of a Storm. During an exten-
sive rain-storm the temperature of the air generally rises above
its mean height for that season of the year. This increase of tem-
perature frequently amounts to 10 or 20, and sometimes even
30. This is caused by the latent heat which is liberated from the
vapor when it is condensed into water. The centre of the area
of high thermometer frequently does not coincide with that of the
area of low barometer, or with the centre of the area of rain and
snow. In the United States, on the northeast side of a storm, at
a distance of over 500 miles from the area of rain and snow, the
thermometer sometimes rises even 20 above its mean height. It
seems probable that the heat which is liberated in the condensa-
tion of the vapor expands the upper portion of 'the atmosphere,
and is drifted eastward far in advance of the storm.
278. Low Temperature succeeding a Storm. As the heated air
rises, the cold upper air descends to take its place, and the storm
is suddenly succeeded by a temperature 10 or 20 degrees below
the mean. Thus, when a storm is prevailing in the middle of the
United States, the lowest temperature of the month may occur at
St. Louis on the same day that the highest temperature occurs at
New-York.
279. Course of Storms modified ~by Local Causes. Local causes
which tend to produce an upward current of the air exert an in-
fluence upon the course of storms. High mountain peaks are of
this description. The storms of Europe are very much modified,
and sometimes in a great measure controlled, by the Alps of Switz-
erland. By the interposition of these mountains, the air which
sweeps over them is forced up to a great height, where it is sud-
denly cooled ; its vapor is condensed ; heat is thereby liberated,
by which the surrounding air is expanded, and rises above the
usual limit of the atmosphere. It thence flows off laterally, leav-
ing a diminished pressure beneath the cloud ; that is, the barome-
ter shows a diminished pressure in the neighborhood of the
mountain.
K
146 METEOROLOGY.
The mountain thus becomes the centre of a great storm, and
the storm may continue stationary for several days, being appa-
rently held in its place by the action of {he mountain.
280. Influence of the Gulf Stream. The Gulf Stream also gives
rise to upward currents of the atmosphere. The Gulf Stream is
a hot river, which comes out of the Gulf of Mexico, and sweeps
round the southern part of Florida, whence it proceeds in a course
nearly parallel to the coast of the United States, and distant from
it about 100 miles. Its temperature in lat. 40 is generally 20
warmer than the surrounding ocean. The air over the Gulf
Stream is warm and highly charged with vapor. Over this
stream ascending currents are continually forming, and storms
are more frequent in its neighborhood than in other parts of the
ocean.
Moreover, if a storm commences any where in the vicinity of
the Gulf Stream, it naturally tends toward this stream, because
here is the greatest amount of vapor to be precipitated ; and
when a storm has once encountered the Gulf Stream, it continues
to follow that stream in its progress eastward, so that most of the
storms which prevail on the coast of the United States have their
centre over the Gulf Stream, and follow the path of this stream in
its progress eastward.
281. Theories of Redfield and Espy. In recent times the study
of Meteorology has been greatly promoted by the labors of Messrs.
Eedfield and Espy. Mr. Kedfield maintained that in great storms
the air moves in circles round the centre, while Mr. Espy main-
tained that the tendency of the wind is in the direction of radii to-
ward the centre, and that the actual motion of the wind is inward
toward the centre.
Observations have shown that an exactly circular motion of the
wind rarely, if ever, occurs, and also that the air never moves ex-
actly in the direction of radii toward the centre of the storm ; but
in almost all violent storms we find a combination of these two
movements, viz., a pressure of the air inward, and a tendency to
circulate round the centre, so that the actual motion of the wind
seems to be spirally inward toward the centre; and here we must
suppose the air to escape by rising upward from the earth's sur-
face, and spreading out in the upper regions of the atmosphere.
STORMS, TORNADOES, AND WATER-SPOUTS. 147
282. Cause of the Low Barometer near the Equator. The same
principles which are developed in the action of storms are ex-
emplified on a grand scale in the general circulation of the atmos-
phere. ' The N.E. and S.E. trade winds, encountering each other
near the equator, are forced up to a great height, where their vapor
is condensed ; copious rain follows; by the liberated heat the air is
expanded, and flows off laterally from above. This causes the
barometer to fall at the equator, and to rise at some distance on .
each side of the equator.
283. Low Barometer near Lat. 64. In like manner, near the
parallel of 64 northerly and southerly winds encounter each oth-
er, producing, also, an abundant precipitation, with a low barome-
ter near this parallel, and a high barometer at some distance on
each side of this parallel.
Abundant rains, then, near the equator and the parallel of 64,
are the cause of the low barometer near those parallels, and they
are also, in part, the cause of the high barometer near the poles
and the parallel of 32.
284. Cause of the Uniformity of the Monsoons. The uniformity
and strength of the S.W. monsoon in India, described in Art. 152, is
due to the vast amount of vapor precipitated on the Himalaya
Mountains. The heat which is liberated in this condensation
causes the air over the mountains to expand and flow off in the
higher regions of the atmosphere, causing a greatly diminished
pressure in the lower atmosphere, and this cause converts the S.W.
wind of India, which otherwise might be a feeble and variable
wind, into a strong and permanent wind throughout the warmer
months of the year.
SECTION II.
CYCLONES.
285. Cyclones defined. Tke inequalities of the earth's surface,
especially in hilly countries, greatly modify the direction of the
wind, so that in great storms the movements of the atmosphere
often seem very complex and anomalous. Over the ocean these
disturbing causes do not exist, and here we find that in violent
storms the movements of the air are much more regular and uni-
148 METEOROLOGY.
form. This motion of the wind has generally been found to be
in great circuits, spirally inward toward the centre of the storm,
and such storms are now commonly designated by the term
cyclone. These storms prevail in the neighborhood of the West
India Islands, where they have long been known by the name of
hurricanes. They are also common in the China Sea and in the
Indian Ocean, on both sides of the equator.
286. Season of Cyclones. In the West Indies, cyclones are al-
most exclusively confined to the months from July to October,
being most common in the month of August; also, in the China
Sea and the Bay of Bengal they are most prevalent at about the
same period of the year. In southern latitudes they are most
common from January to March.
287. Where do Cyclones originate? There is no instance on
record of a hurricane having been encountered on the equator,
nor of any one having crossed that line, although two have been
known to rage at the same time on the same meridian, but on op-
posite sides of the equator, and 10 or 12 apart. They originate
near the equatorial limit of the trade winds, where these winds are
irregular. The West India hurricanes generally originate between
lat. 10 and 20K, and long. 50 and 60 W., on the borders of the
zone of calms and variable^ winds, which corresponds with the
zone of constant precipitation of rain.
288. Paths of Cyclones. In the northern hemisphere, during
the early part of their course within the region of the trade winds,
cyclones travel toward the west, inclining somewhat toward the
north. Near lat. 20 the motion from the equator is more de-
cided, and in lat. 25 their motion is about N. W. Near the par-
allel of 30 their course is almost exactly north, and soon they be-
gin to veer toward the east, after which their motion is nearly
parallel to the coast of the United States. Several storms have
been traced from lat. 10 or 15 up to lat. 45 or 50, and the
path of the centre of greatest violence bears some resemblance to
a parabola, of which the most westerly point lies near the parallel
of 30. This path is represented by the line ABC, Fig. 61.
In the southern hemisphere cyclones pursue a similar course.
Commencing near the equator, they advance at first only a little
STORMS, TORNADOES, AND WATER-SPOUTS.
149
south, of west. This
southerly motion in-
creases until near lat.
26, when the motion
is exactly toward the
south, after which they
gradually veer toward
the southeast, the en-
E tire path, DEG, form-
ing a curve which is
almost perfectly sym-
metrical with that of
cyclones in the north-
ern hemisphere. The
latitude where the
path of the cyclone
changes from west to 'east coincides nearly with the polar limit
of the trade winds.
289. Gyratory Movement of Cyclones. The air in cyclones has
not merely a movement of translation, but also a gyratory motion
about the centre of the storm. The motion of the air is spirally
inward, as has been already shown in the storms of the United
States, but over the ocean the whirling motion is usually more
decided than it is over the land. North of the equator this gyra-
tory motion is from right to left, or in a direction contrary to that
of the hands of a watch. South of the equator the motion is from
left to right, or in the same direction as that of the hands of a
watch.
Near the centre of the hurricane there is generally a great fall
of rain, which- is usually accompanied by the most magnificent
displays of thunder and lightning.
290. Bate of Motion. The rate at which cyclones travel is very
variable. In the West India cyclones the highest rate which has
been observed is 43 miles per hour, and the least 10 miles per
hour; the mean being 26 miles. In the Bay of Bengal the ob-
served rate varies from 2 to 39 miles per hour, and in the China
Sea from 7 to 24 miles per hour. In the South Indian Ocean the
observed rate varies from 1 to 10 miles per hour. Some cyclones
150 METEOROLOGY.
travel so very slowly that they may almost be considered sta-
tionary.
The direction and velocity of the wind are, however, entirely
distinct from those of the storm's progress. While the storm
sometimes advances at the rate of less than 10 miles per hour,
the velocity of the wind may exceed 100 miles per hour.
291. Diameter of Cyclones. Cyclones extend over a circle from
100 to 500 miles in diameter, and sometimes 1000 miles. In the
West Indies they are sometimes as small as 100 miles in diame-
ter, but on reaching the Atlantic they dilate to 600 or 1000 miles.
Sometimes; on the contrary, they contract in their progress, and
while contracting they augment fearfully in violence. The vio-
lence of the wind increases from the margin to the centre, with
the exception of a limited space exactly at the centre, where the
atmosphere is frequently quite calm.
292. Premonitions of a Cyclone. Previous to the commence-
ment of a cyclone the air is observed to be close, sultry, and op-
pressive, and the wind is moderate or calm. A fresh breeze sets
in from the east, and rises and falls with a moaning sound ; after
a few hours it is succeeded by a lull, which may last for an hour
or more, after which the wind changes to the west, often with
great suddenness, and blows with increased violence, and this is
usually the time of greatest danger to vessels.
The approach of a cyclone is often announced by a swell of the
ocean, resulting from the action of the wind upon a neighboring sea,
while the waves thus excited advance more rapidly than the storm.
During the passage of the cyclone the barometer oscillates in a
remarkable manner, rising and falling rapidly, so that a great
barometric oscillation almost always announces the approach of
a tempest. The most rapid fall begins from three to six hours
before the passage of the centre. The barometer is lowest near
the middle of the storm area, and begins to rise before the strength
of the cyclone is over.
The fall of the barometer during the passage of the cyclone
varies according to the intensity of the storm. It frequently
amounts to one inch, and has been known to exceed two inches.
The rise of the barometer after the storm is usually as rapid as
was its fall on the approach of the storm. See Table XXXIII.
STOKMS, TORNADOES, AND 'WATER-SPOUTS. 151
293. Duration at any Place. The duration of the storm at any
place depends upon the extent of the storm, and the velocity with
which it advances. If the storm be only 100 miles in diameter," and
advances 20 miles per hour, its duration at any place can not ex-
ceed five hours. If the diameter of the storm be greater, or its
progress less rapid, its duration at a given place will be in-
creased.
294. Cause of the Parabolic Course of Storms. The parabolic
course of storms from near the equator toward the poles results
from the rotary motion of the earth. When a large mass of air
in the northern hemisphere is put in rotation about avertical axis,
the particles on the east side of the centre, crossing successively
parallels of latitude whose easterly motion is less than their own,
are deflected toward the east ; that is, toward the right. So, also,
the particles oh the west side of the centre, crossing successively
parallels of latitude whose easterly motion is greater than their
own, are deflected toward the west, which is also toward the right.
Particles on the north or south side of the centre are deflected in
a similar manner ; that is, the particles of the revolving mass of
air, in every portion of their circuit, are deflected toward the right.
Hence on the equatorial side of the revolving mass of air there
is a tendency toward the equator, while on the polar side there
is a tendency toward the pole. Now this deflecting force in-
creases from the equator toward the pole, being proportional to
the sine of the latitude. Hence the pressure on the polar side
toward the pole is greater than on the opposite side toward the
equator, and the revolving mass accordingly moves in the direc-
tion of greatest pressure ; that is, toward the pole.
Within the limit of the trade winds the revolving mass is car-
ried westward by the general westward motion of the atmosphere,
while it is crowded northward by the force just described, so that
the actual progress of the storm is toward the north of west. Aft-
er escaping from the trade winds, the general motion of the at-
mosphere carries the storm eastward, while the force just described
urges it northward ; that is, the actual progress of the storm is to-
ward the north of east.
By a similar course of reasoning, the parabolic path of cyclones
in the southern hemisphere may be explained.
152 METEOROLOGY.
SECTION III.
TORNADOES.
295. Sometimes near the centre of a great storm the general
inward tendency of the air causes a violent whirlwind, or tornado,
where the wind revolves with such violence as to prostrate the
largest trees, demolish buildings, and transport heavy bodies to a
great distance. Such a whirlwind occurred in Northern Ohio
February 4, 1842, near the centre of an uncommonly severe storm
of rain. In this tornado large buildings were lifted entire from
their foundations, carried a distance of several rods, and then
dashed to pieces. The fragments were strewed all along the
track, and some were carried a distance of seven or eight miles.
Large oak-trees, two feet in diameter, were snapped off like reeds,
and others were so twisted as to be reduced to a mass of splinters
not much thicker than a man's finger. The breadth of the track
did not much exceed half a mile, and the most destructive portion
was still more limited. The duration of the tornado at one place
did not much exceed one minute. The tornado advanced over
the earth, in a direction N. 33 E., with a velocity of 34 miles per
hour.
296. Tropical Tornadoes. Similar tornadoes occur within the
tropics, and here exhibit even greater violence than they do in
the United States. In the great tornado which passed over Bar-
badoes in 1780, the strongest buildings were entirely demolished;
the largest trees were torn up by the roots ; a 12-pounder gun
was moved a distance of 140 yards ; a multitude of ships were
wrecked, and over 4000 persons perished.
In a hurricane which occurred in June, 1822, near the mouth
of the Ganges, a vast amount of property was destroyed, and up-
ward of 50,000 persons perished, chiefly from the inundation of
the rivers.
297. Effects of Tornadoes. The motion of the air in tornadoes
is spirally inward and upward, so that from each side of the
track objects are drawn inward toward the centre of the track,
and very heavy bodies are carried up in the centre. Light ob-
jects are elevated high into the air, and are sometimes carried
STOEMS, TORNADOES, AND WATER-SPOUTS.
153
Fig. 62. many miles before they are
./* thrown out of the vortex.
Fig. 62 represents a por-
tion of the track of a tornado
which passed over New Ha-
ven in 1839. The tornado
advanced in a direction N.
50 E. On the right-hand
side of the track the pros-
trate trees were uniformly
inclined toward the north,
while on the left-hand side
many of them were inclined
toward the south.
Tornadoes are uniformly
preceded by an unusual heat;
they are invariably accompanied by lightning and rain, and fre-
quently by hail.
When a tornado passes over a hilly country, it sometimes rages
with destructive violence on the hill- tops, while objects in the in-
termediate valleys are entirely uninjured, showing that a violent
whirlwind may prevail at a moderate elevation, but without reach-
ing the earth's surface.
298. Appearance of Explosion. When a violent tornado passes
over a building where the doors and windows are closed, the
walls are sometimes thrown outward with great force, the house
presenting* the appearance of an explosion, indicating that the
pressure of the air on the outside of the building was suddenly
diminished, and the house was burst open by the expansion of
the air within.
SECTION IV.
PILLARS OF SAND, AND WATER-SPOUTS.
299. Tornadoes are probably similar to the small whirls which
are often seen in the streets, especially on dry and calm days of
spring or summer, and which raise up a dense column of dust,
even to the tops of the houses. In these whirls the motion of the
air is spirally inward and upward, so that light objects in their
154 METEOROLOGY.
vicinity are sucked into the vortex, and carried up to the top of
the whirl, where they escape laterally, and descend at some dis-
tance on either side. These small whirls sometimes revolve from
left to right, and sometimes from right to left, while in the north-
ern hemisphere large whirlwinds, several miles in diameter, al-
ways revolve from right to left. The whirls seen in our streets
are sometimes only a few inches in diameter, but sometimes in the
open fields they occur several feet in diameter, and carry up leaves
of trees and light objects of considerable size.
On the deserts of Africa similar whirls often raise vast pillars
of sand, which sometimes prove fatal to entire caravans. Bruce
states that in Abyssinia he beheld eleven vast columns of sand
moving over the plain at the same time. Similar whirls are of
common occurrence in India.
300. Whirlwinds caused by Fires. These whirls may be set in
motion by whatever causes a strong upward motion of the air.
An extensive fire frequently produces this effect. When large
fires are burning on the Western prairies, violent whirls are fre-
quently formed, having a force sufficient to lift a man from the
ground and transport him to a considerable distance. At such
times the flame is sometimes collected into a fiery column, rising
to the height of 200 feet or more.
Some years since, during the burning of a canebrake in Ala-
bama, several whirls were formed in the midst of the flames, some
of which rose to the height of 200 feet, and in form resembled the
upper cone of an hour-glass.
Similar effects were produced by the conflagration of Moscow,
September 14-20, 1812.
301. Water-spouts. When a violent whirl is formed over water,
considerable spray is raised from the surface of the water, and
this spray is carried up in the centre of the whirl, presenting the
appearance of a dense solid column. This phenomenon is called
a water-spout. Water-spouts are of variable dimensions, but
sometimes they attain a diameter of several rods, and a height of
half a mile.
These whirls generally form, in the first instance, at a consider-
able height in the air, and do not reach down to the surface of
.the sea. If there is a low cloud over it, the under surface of the
STORMS, TORNADOES, AND WATER-SPOUTS.
155
cloud is rolled into a conical form. This inverted cone seems at-
tached to the cloud, and sometimes becomes rapidly elongated.
Sometimes it swings backward and forward, coils up, and disap-
pears, and the spout is not completed ; but at other times it grad-
ually extends so as to reach down to the surface of the water.
As the column approaches the surface of the sea, the latter be-
comes violently agitated, and the spray is whirled round with a
rapid motion. The spout now forms a continuous column, ex-
tending from the water to the cloud, and often resembles a large
elephant's trunk dangling from the clouds. Its color is generally
of a sombre gray, like that t)f the clouds, but sometimes it appears
black, like a dense smoke.
This spout has both a rotary and a progressive motion. The
whirling motion extends to but a moderate distance around the
column, and beyond this there prevails a calm. The phenomenon
lasts but a short time. After a few minutes the trunk contracts
so as no longer to reach the surface of the sea, the black cloud
draws itself up ; and the trunk gradually disappears. Sometimes
the spout commences with the rising of spray from the surface of
the water, which gradually ascends until the column is complete
from the water to the clouds. When the spout is complete, there
is heard a roaring noise like that of a great waterfall. '
. 63.
156
METEOKOLOGY.
Subsequently the cloud sometimes discharges itself in a heavy
rain, and this rain is never salt, even in the open ocean, showing
that this water was precipitated from the clouds, as in ordinary
rains. Fig. 63, on the preceding page, shows a water -spout in
three stages of its progress. First, the column is incomplete ;
next, the column is entire ; and, finally, the smoky aspect of the
column disappears, and the column begins to break up.
Water-spouts generally form during a period of great heat, and
are most frequent in the calm regions between the tropics. Two
or three of these spouts are sometimes formed simultaneously,
proceeding from the same cloud. In May, 1820, on the edge of
the Gulf Stream, seven water-spouts were seen in the course of
Fig. 64.
half an hour. Fig. 64 represents a water-spout seen in 1858 on
the Eiver Khine.
302. Showers of Toads, Fishes, etc. During violent storms show-
ers of small animals sometimes descend from the sky. M. Peltier,
of France, states that he once saw a multitude of small toads de-
scend to the earth. They fell upon his hat, upon his hands, and
the ground about him was covered with them. Several observ-
ers in France, in India, and elsewhere, have seen showers of small
STORMS, TORNADOES, AND WATER-SPOUTS. 157
fish descend from the sky. Others have observed showers of
sand, of straws, etc.
These phenomena are explained by supposing that the objects
mentioned were elevated from the earth in a violent whirl, which
transported them to a considerable distance, and then dropped
them upon the earth.
In 1833, near Naples, a whirlwind passed over an orange-grove,
and a multitude of oranges were carried up in the Vhirl. Some
minutes afterward a shower of oranges fell upon a roof at a con-
siderable distance.
In 1835, in France, the water of a small pond containing a large
quantity of fish was drawn off by a whirlwind. . These animals
may have been transported a distance of many rods, perhaps
several miles, but they must ultimately- have fallen to the earth,
furnishing a shower of fishes.
SECTION V,
PREDICTIONS OF THE WEATHER.
303. The character of the weather at any place is affected by
so many circumstances 'which may transpire at distant parts of
the world, and which can be but very irRperfectly known to us,
that it is impossible to predict, except very imperfectly, what
weather may.be expected at a given time and place. To a limit-
ed extent, however, such predictions are possible.
304. Predictions founded upon the Constancy of Climate. Rely-
ing upon the constancy of climate, which has been established by
observation, we may predict the probable general character of
any month of the year.
The climate of a country remains permanently the same from
age to age. Observations continued for an entire century at va-
rious places in the United States and Europe indicate no change
in the mean temperature of the year, or that of the separate
months ; no change in the range of the thermometer ; no change
in the time of the last frost of spring or the first frost of autumn ;
in the annual amount of rain or snow, or in the mean direction of
the wind. It is not certain that the climate of any country, in
either of these respects, has changed appreciably in 2000 years.
By the destruction of forests, the earth is more directly exposed
158 METEOROLOGY.
to the rays of the sun ; the moisture of the ground is more readi-
ly evaporated ; streams more frequently dry up in summer, and
droughts become more frequent and severe. But these changes
do not seem to affect in a sensible manner the mean temperature
of any place, or the annual amount of rain.
Assuming, then, the established constancy of climate, we can
predict beforehand the probable character of any month of the
year. Thus, tit New Haven, the probable mean temperature of
any future January will be 26. We may be tolerably sure that
it will not be higher than 36, nor lower than 17. The ther-
mometer in January will never rise above 64, nor sink below
24. The entire annual amount of rain at New Haven will not
exceed 55 inches, and will not be less than 34 inches.
305. Conclusions drawn from anomalous Months. Moreover, if
several months in succession have been unusually warm or unu-
sually cold, instead of concluding that the climate has permanently
changed, and that the succeeding months will be sin^lar in .char-
acter, we should rather anticipate months of the opposite descrip-
tion, since the mean temperature of the year fluctuates within
very narrow limits, and the longer a period of unusually warm
weather continues, the greater is the probability that the succeed-
ing months will be unusually cold. Predictions of this kind are
legitimate deductions from scientific data.
306. Predictions founded upon the established Laws of Storms.
Since great storms have been found to observe pretty well defined
laws, both as respects the motion of the wind and the direction of
their progress, we may often recognize such a storm in its prog-
ress, and anticipate changes which may succeed during the next
few hours. When it is possible to obtain telegraphic reports of
the weather from several places in the Valley of the Mississippi
and its tributaries, we may often predict with confidence the ap-
proach of a great storm twenty-four hours before its violence is
felt at New York.
307. Observations of the Meteorological Instruments at a single
Place. When we are restricted to observations at one locality,
our predictions of the weather must needs be more uncertain, and
the conclusions to be derived from a motion of meteorological
159
instruments are not the same for all parts of the world. Along
the Atlantic coast of the United States the approach of a violent
N.E. storm is generally indicated by the barometer rising above
its mean height; at the same time the wind veers to the N.E.,
and the atmosphere grows hazy. After the rain or snow com-
mences, the barometer begins to fall ; when the barometer reaches
its lowest point, the wind changes to N. or N.W., after which the
barometer begins to rise.
If a gale sets in from the E. or S.E., and the wind veers by the
S., the barometer will continue falling until the wind becomes
S.W., when a comparative lull may occur, after which the gale
will be renewed, and the change of the wind toward the N.W. will
be accompanied by a fall of the thermometer, as well as a rise of
the barometer.
A considerable and rapid depression of the barometer for in-
stance, a fall of three fourths of an inch in twenty -four hours in-
dicates an approaching storm, with rain or snow. The wind will
be from the northward if the thermometer is low for the season,
from the southward if the thermometer is high. If the barometer
falls with a rising thermometer and increased dampness, wind and
rain may be expected from the southward.
A rapid rise of the barometer indicate^ unsettled weather; a
slow rise indicates fair weather. The result of all rapid changes
in the weather, or in any of the instrumental indications, is brief
in duration, while that of a gradual change is more durable.
308. Prognostics from the Clouds, Face of the Sky, etc. When
the upper clouds move in a direction different from that of the
lower clouds, or that of the wind then blowing, they foretell a
change of wind.
When the outlines of cumulus clouds are sharp, it indicates
a dry atmosphere, and therefore presages fine weather. Small
inky-looking clouds foretell rain. A light scud driving across
hazy clouds indicates wind and rain.
Eemarkable clearness of the atmosphere near the horizon, and
an unusual twinkling of the stars, indicate unusual humidity in
the upper regions of the atmosphere, and are therefore indications
of approaching rain.
Hal os, corona, etc., presage approaching rain or snow.
Dew and fog are indications of fine weather.
160
METEOROLOGY.
CHAPTER VII.
ELECTRICAL PHENOMENA.
SECTION I.
ATMOSPHERIC ELECTRICITY.
309. Means of observing the Electricity of the Atmosphere. The
atmosphere is almost always charged with electricity, and this
electricity exerts an important influence upon various meteoro-
logical phenomena.
In order to observe this electricity, an insulated conductor
should be elevated to a considerable height above the earth. At
the Observatory of Kew, near London, a tube of thin copper, 16
feet high, and surmounted by platinum points, is supported by a
cylinder of glass placed under the dome at the top of the Ob-
servatory. The copper tube passes through the top of the dome
without touching it, and the rain is excluded from this opening
by an inverted copper dish fitted to the tube. This copper tube
may be made to communicate at pleasure with the electrometers.
310. Electrometers. The most common electrometer is Volta's.
Fig. 65.
This consists of two straws, D, Fig. 65, two
inches in length, suspended by hooks of fine
copper wire, and at a distance of one twen-
tieth of an inch from each other, and cover-
ed by a glass jar, A. When the two straws
are similarly electrified they recede from
each other, and the intensity of the charge
is indicated by the amount of the divergence.
This divergence is measured by an ivory
scale graduated to twentieths of an inch.
B is a metallic dish to protect the electrom-
eter from the rain, and C is a pointed con-
ductor for collecting the electricity.
It is desirable to have a series of electrom-
eters for measuring electricity of different
degrees of intensity. For the feeblest elec-
ELECTRICAL PHENOMENA. 161
tricity the gold - leaf electrometer may sometimes be employed ;
and when the electricity is very intense it is important to have
an instrument for measuring the length of the spark. This may
consist of a sliding rod terminated by a brass ball, which can be
set at any distance from the insulated conductor.
311. Electricity at considerable Elevations. The electrical con-
dition of the higher strata of the air has been ascertained by
means of kites and balloons. When a kite is used for this pur-
pose, the string should be wound with fine wire in order to make
it a conductor of electricity, and the kite must be insulated by
attaching the lower end of the string to some non-conductor such
as silk or glass.
Small balloons are sometimes employed for the same purpose,
and a conducting cord connects the balloon with an electrometer
near the earth's surface.
By instruments like these it is found that the air is generally
charged with positive electricity, but it is subject to great varia-
tions of intensity, and clouds are frequently charged with nega-
tive electricity.
312. Diurnal Variation of Electricity. The intensity of atmos-
pheric electricity is found to vary with the hour of the day.
From the mean of three years' observations made at Kew, it ap-
pears that at 4 A.M. the electric tension is represented by 20 on
Yolta's electrometer; from this hour the electricity increases to
10 A.M., when it is represented by 88; from that time it de-
creases to 4 P.M., when it is represented by 69 ; it then increases
to 10 P.M., when it is represented by 104 ; from which time it
decreases till 4 A.M. ; that is, there are two daily maxima of in-
tensity and two daily minima.
313. Monthly Variation of Electricity. The intensity of atmos-
pheric electricity also varies with the season of the year. At
Kew, the mean electric tension is least in June, remaining nearly
the same through the summer months, after which the electricity
increases steadily till January, continuing nearly the same through
Eebruary, after which it decreases till the next June; that is, there
is one annual maximum of intensity and one minimum.
At Brussels, also, the maximum occurs in January and the
L
162 METEOROLOGY.
minimum in June, while at Munich the maximum occurs in De-
cember and the minimum in May.
At Brussels the electric tension in winter is nine times as great
as in summer ; at Kew it is six times as great ; and at Munich it
is only twice as great in winter as in summer.
314. Variations with the Altitude. The intensity of atmospher-
ic electricity increases with the altitude above the surface of the
earth. This law has not been fully verified for elevations ex-
ceeding 100 feet. Experiments with electric kites have obtained
signs of electricity the more powerful as the kite rose to a great-
er elevation. Experiments of this kind have been carried to the
height of 810 feet.
Similar results have been obtained by means of an arrow
projected into the air, the arrow being provided with a con-
ducting wire whose extremity communicated with a straw elec-
trometer.
Gay-Lussac, during his aerial voyage in 1804, suspended from
his balloon a wire 170 feet long, and connected the upper end
with an electrometer. This experiment indicated that the elec-
tricity of the air was positive, and increased with the altitude.
In a balloon ascent in 1862, Mr. Glaisher found that the air
was charged with positive electricity, but becoming less and less
in amount with increasing elevation, till at the height of 23,000
feet the amount was too small to measure.
315. Electricity in cloudy Weather. When the sky is covered
with clouds, the electricity is subject to frequent changes of kind
as well as intensity, being sometimes positive and sometimes
negative. The electricity is seldom negative except when rain
is falling. During a snow-storm the lower strata of the air ex-
hibit electricity of great intensity.
During the passage of a thunder -shower, the electricity fre-
quently changes in two or three minutes from positive to nega-
tive, and then back again to positive, sometimes half a dozen of
these changes occurring in a single shower. The electricity also
at such times has great intensity, and sparks are sometimes ob-
tained from the conductor more than an inch in length, giving* a
severe shock when passed through the human system.
ELECTKICAL PHENOMENA. 163
316. Is Atmospheric Electricity the result of Friction? Philoso-
phers are by no means agreed as to the origin of atmospheric
electricity. Friction is one of the most common sources of elec-
tricity. Dry air rubbing against dry air, or any other substance,
develops little if any electricity ; but moist air rubbing against
the surface of the earth acquires positive electricity.
In violent tornadoes we uniformly observe electricity of great
intensity. This may be due in part to the friction of the air upon
the earth. But we can not consider friction to be the principal
source of atmospheric electricity, because there is no uniform
relation between the force of the wind and the intensity of the
electricity.
317. Is it the result -of Combustion? Combustion is another
source of electricity. When coal is burning, the carbonic acid
gas which escapes is positively electrified, while the coal has
negative electricity. The atmosphere, therefore, must receive
some electricity from the combustion which takes place on the
surface of the earth ; but this cause must be entirely inadequate
to account for the enormous quantities- of electricity exhibited in
thunder-showers.
318. Is it the result of Vegetation? Vegetation is a source of
electricity. During the day, plants give out oxygen which is
charged with negative electricity ; and during the night they
give out carbonic acid gas, which is charged with positive elec-
tricity. These two processes in a measure neutralize each other.
319. Is it the result of unequal Temperature? The unequal tem-
perature of the different parts of the earth has been supposed to
be a source of atmospheric electricity. There are several metals
which develop electricity when brought in contact and unequal-
ly heated. In some of the mines of England, currents of elec-
tricity have been detected within the earth, and these currents
have been ascribed to a varying temperature acting upon the
heterogeneous materials of the earth.
This cause may explain permanent currents existing in the
earth, but does not seem adequate to account for the enormous
quantity of free electricity which often exhibits itself in thunder-
showers.
164 METEOROLOGY.
320. Is it the result of sudden Condensation of Vapor? Since
atmospheric electricity is feeble before the formation of a storm,
and rapidly attains its maximum during a thunder-storm, it has
been supposed that electricity is liberated in the act of condensa-
tion of the vapor of the air.
When the steam issuing from the boiler of a steam-engine is
suddenly condensed, a great amount of electricity is liberated.
But it is claimed that this electricity is not due to simple con-
densation, but to the friction of the condensed particles against
the sides of the orifice through which the steam escapes.
321. Is Atmospheric Electricity due to Evaporation? Evapora-
tion is probably the principal source of atmospheric electricity.
The following experiment shows the production of electricity by
evaporation. If upon the top of a gold-leaf electrometer we place
a metallic vessel containing salt water, and drop into the water a
heated pebble, the leaves of the electrometer will diverge. The
vapor which rises from the water is charged with positive elec-
tricity, while the water retains negative electricity.
The water used in this experiment must not be perfectly pure,
but must contain a little salt, or some foreign matter. The evap-
oration of the water of the ocean must therefore furnish a large
amount of electricity ; and fresh water must also furnish some
electricity, for the water of the earth is never entirely pure.
322. Diurnal change of Electricity explained. The diurnal va-
riation in the intensity of atmospheric electricity is to be ascribed
partly to real changes in the amount of electricity present in the
air, and partly to variations in the conducting power of the air.
Just before sunrise the electricity has a feeble intensity, be-
cause the moisture of the preceding night has transmitted to the
earth a portion of the electricity which was previously present in
the air. After the sun rises, new vapor ascends and carries with
it positive electricity, so that the amount of electricity in the air
increases. Toward noon the air becomes dry, and transmits less
readily the electricity accumulated in the upper regions of the
atmosphere ; so that, although the amount of electricity in the air
is continually increasing, an electrometer near the earth's surface
indicates an apparent diminution. Toward evening the air g'rows
cool, again becomes humid, and transmits more readily to the
ELECTKICAL PHENOMENA. 165
earth the electricity accumulated in the upper regions of the at-
mosphere. The effect produced upon an electrometer therefore
increases until some hours after sunset; but since during the
night there is a constant discharge of electricity from the air to
the earth, the electrometer soon indicates a diminished intensity,
which continues until toward morning.
323. Monthly change of Electricity explained.- The same princi-
ple explains why the electricity of the air appears less intense in
summer than in winter. In summer the air is warm and dry,
and opposes more resistance to the flow of electricity from the
higher regions of the atmosphere, while in winter the moist air
produces a contrary effect ; so that, although the atmosphere
doubtless contains more electricity in summer than in winter, it
generally produces a less effect upon an electrometer placed near
the earth's surface.
324. Electricity developed in dry Houses. During the cold
weather of a Northern, winter, in houses which are kept quite
warm and dry, and whose floors are covered with heavy woolen
carpets, electricity is abundantly excited by simply walking to
and fro upon the carpet. Sometimes in this manner there is de-
veloped electricity sufficient to give an unpleasant shock, and ^o
ignite ether, gas, or other combustible substances. This electrici-
ty results from the friction of dry leather upon the woolen carpet,
and it is prevented from escaping by the insulating power of the
dry carpet, and the extremely dry floor of the building.
SECTION II
THUNDER-STOKMS.
325. How clouds become Electrified. We have found that the
atmosphere ordinarily contains a large quantity of electricity.
Since dry air is a non-conductor, the electrified particles in clear
weather are in a measure insulated, and the electricity can not
acquire much intensity ; but when the vapor of the air is precip-
itated and a cloud is formed, the electricity, which was previously
confined to the separate particles of the air, now finds a conduct-
ing medium more or less perfect, and it spreads itself over the
surface of the cloud, thereby acquiring considerable intensity. It
166 METEOROLOGY.
is generally admitted that tlie same quantity of electricity which
exists in the cloud, existed in the air before the formation of the
cloud, and that the cloud performs no other office than that of a
conductor.
326. Clouds negatively Electrified. A cloud thus electrified
must necessarily have positive electricity, since in clear weather
the electricity of the atmosphere is always positive. Such a
cloud, when it approaches near another cloud having less elec-
tricity, or none at all, acts by induction upon the latter, decom-
posing its natural electricity, attracting the negative electricity
and repelling the positive. The positive electricity thus repelled
may sometimes be drawn off by near approach to another cloud,
or to the earth, leaving only negative electricity upon the cloud.
Hence probably result the frequent alternations of positive and
negative electricity observed during a thunder-shower.
; 327. Lightning. Two clouds having opposite electricities at-
tract each other, and when the clouds come sufficiently near, the
two electricities rush toward each other with great violence. This
phenomenon is called lightning, and is accompanied by an explo-
sive noise called thunder.
Since clouds are very imperfect conductors, when the electricity
of one part of a cloud is discharged, the electricity of a distant
part of the cloud is but slightly changed. Thus a single dis-
charge does not establish a complete electrical equilibrium ; but
there is a change in the distribution of the electricities upon the
surrounding clouds, and there must be a succession of discharges
before the electricity is entirely neutralized. Hence results a suc-
cession of flashes of lightning and peals of thunder.
328. Discharge of Electricity to the Earth. A cloud charged
with electricity exerts an inductive influence upon the earth's
surface immediately beneath it, decomposing its natural electrici-
ties, repelling electricity of the same kind, and attracting the op-
posite kind. Accordingly there will sometimes be a discharge of
electricity from the cloud to the earth. This charge is usually
received by the most elevated objects, such as mountains, hills,
trees, spires, high buildings, etc. Trees are particularly exposed
to strokes of lightning on account of their elevation, as well as of
ELECTRICAL PHENOMENA. 167
the moisture which they contain, and which renders them partial
conductors of the electric fluid.
329. Different forms of Lightning. Lightning exhibits a variety
of forms, which have been designated by the terms zigzag, ball,
sheet, and heat lightning.
Zigzag -lightning presents a long, irregular, jagged line of light,
like the ordinary spark drawn from an electric machine. This
zigzag path is sometimes four or five miles, and perhaps even ten
miles in length.
The irregularity of the path is ascribed to the compression of
the air before the electricity, thereby opposing greater resistance,
and turning the fluid aside to seek some path upon which the re-
sistance is less.
330. Ball Lightning.-*- Ball lightning appears like a ball of fire,
and is usually accompanied by a terrific explosion. It probably
results from a charge of electricity unusually intense, which forces
a direct instead of a circuitous passage through the air.
Some have supposed that ball lightning was the agglomera-
tion of ponderable substances in a state of great tenuity, strongly
charged with electricity.
331. Sheet Lightning. Sheet lightning is a diffuse glare of light,
sometimes illuminating only the edges of a -cloud, and sometimes
spreading over its entire surface.
This may sometimes be due to distant lightning which illu-
mines a cloud, while the direct flash is hidden from the observer
by intervening clouds. Sometimes it may result from a move-
ment of electricity in the interior of a cloud which is a very im-
perfect conductor, producing an illumination analogous to that
observed on a plate of moist glass employed in discharging an
electrical machine.
332. Heat Lightning. During the evenings of summer, the
horizon is sometimes illumined for hours in succession by flashes
of light unattended by thunder, this is called heat lightning.
This illumination is sometimes due to the reflection from the at-
mosphere of the lightning of clouds so distant that the thunder
can not be heard.
168 METEOROLOGY.
Sometimes, however, this light overspreads the entire heavens,
showing that the electricity of the clouds escapes in flashes so
feeble that they produce no audible sound. Such cases may oc-
cur when the air is very moist, the air being then a tolerable con-
ductor, and offering just sufficient resistance to the passage of the
electricity to develop a feeble light.
333. Color of Lightning. The color of lightning varies from
white to a rose color and violet. Zigzag lightning is generally
white, sometimes of a purplish violet or bluish tinge. Diffuse
flashes of lightning are often of an intense red, sometimes mixed
with blue or violet.
These differences depend upon the density and moisture of the
strata of air in which the clouds are formed, and also upon its con-
ducting power. When the density of the medium is slight, the
light becomes diffuse and reddish ; when the density is considera-
ble, the light is concentrated and brilliant.
Similar variations in the color of the electricity are perceived
when the fluid is passed through a glass receiver in which the air
has been rarefied by means of an air-pump.
334. Duration of Lightning. The duration of ordinary flashes
of lightning is less than the thousandth part of a second. This is
proved by receiving the light of an electric discharge upon a
white disc marked with black rays, when the disc is made to re-
volve with great rapidity. However great the velocity of rota-
tion may be, the disc, when illumined by lightning, always appears
stationary, showing that during the continuance of the illumina-
tion the disc had not revolved through any appreciable angle.
If the disc were illumined for an instant by means of a lamp, by
lifting and dropping a screen as suddenly as possible, the disc
would appear of a uniform tint, and no separate rays would 'be
seen.
335. Cause of Thunder. Thunder is generally regarded as the
result of the sudden re-entrance of the air into a void space, as in
the experiment of a bladder tied over an open-mouthed receiver,
and burst by the pressure of the external air. This vacuum is
supposed to be generated by the lightning in its passage through
the air. Electricity communicates a powerful repulsive force to
ELECTRICAL PHENOMENA.
169
the particles of air along the path of its discharge, producing thus
a momentary void, into which immediately afterward the sur-
rounding air rushes with a violence proportioned to the intensity
of the electricity.
336. Interval between the Flash and Report. Since the transmis-
sion of light is nearly instantaneous, while sound moves only 1100
feet per second, the sound will not reach the ear until some inter-
val after the flash. 'By observing the interval between the flash
and the report, the distance of the point where the discharge takes
place can be computed. The longest interval mentioned by any
observer is 72 seconds, indicating a distance of 15 miles. With
the exception of this single instance, the longest interval recorded
is 50 seconds, indicating a distance of 10 miles. This fact is very
remarkable, since the noise of cannon may be heard to a much
greater distance.
The average interval between the flash and the report is 12
seconds, and the shortest interval recorded is one second.
If we measure the Angular height of the flash whose distance
from the observer has been determined, we may compute the vert-
ical elevation of the cloud above the earth.
*
337. Duration of Thunder. Since a separate sound is produced
at each point alpng the entire
line of the flash, and these
points are generally at une-
qual distances from the ob-
server, the sounds produced at
different points of the line of
discharge, though in fact si-
multaneous, reach the ear in
slow succession. Thus an ob-
server at A, Fig. 66, will first
hear the sound resulting from
the concussion at a, next at c, and finally at b. If b were 11,000
feet more remote than a, the first sound would be heard ten sec-
onds before the last, and the thunder would be continuous for
ten seconds.
The average duration of peals of thunder is 22 seconds, and the
longest duration recorded is 56 seconds.
170 METEOROLOGY.
The prolonged duration of some peals of thunder is in part the
effect of echoes. ' In mountainous countries thunder peals are
much longer continued, and the sound is more intense than in
plane countries. This is due to the reflection of the thunder
from the sides of the mountains in the same manner as the sound
of a cannon is reflected. These echoes may also be produced by
reflection of sound from clouds, as has been proved by the firing
of cannon over the ocean.
338. Rolling of Thunder. The variable intensity or rolling of
thunder is due partly to the zigzag form of the discharge, in con-
sequence of which there are frequently several different points of
the flash which are equally distant from the observer; and the
sounds produced at these points reach the ear simultaneously,
producing the effect of a double or triple sound.
It is due in part to the "unequal distance of different parts of
the flash, the loudness of sound varying inversely as the "square
of the distance.
It may also be due in part to the fact that the electricity, in its
long zigzag course, may pass through strata of air differing ma-
terially in density, which may result either from difference of ele-
vation or difference in amount of moisture.
The rolling of thunder is also without doubt in a considerable
degree the effect of echoes.
339. Remarkable succession of Phenomena in Thunder. There is
a certain succession of phenomena in thunder which occurs so
frequently as to indicate that it is the result of a combination of
circumstances of common, if not habitual occurrence. These phe-
nomena occur in the following order :
1st. The flash of lightning.
2d. After an interval, generally of 10 or 12 seconds, the thunder
begins with a rattling or rumbling noise, which increases, some-
times regularly, sometimes with vibrations, up to its maximum.
3d. Five or ten seconds after the first rumbling we hear a loud
crashing sound, which sometimes continues for 5, 10, or even 20
seconds, and this again is succeeded by a rumbling noise, which
gradually dies away.
Sometimes several maxima and minima succeed each other
with great rapidity.
ELECTRICAL PHENOMENA. 171
This circumstance of a crashing sound succeeding by a consid-
erable interval the first rumbling of the thunder may perhaps be
explained by the imperfect conducting power of the cloud. If we
coat a Leyden jar with brass filings instead of tin foil, and charge
it with electricity, upon discharging the jar in a dark room we
find the light exhibits numerous ramifications, spreading out like
branches from the trunk of a tree. A similar effect may be pro-
duced when electricity is discharged from a cloud. Let A B, Fig.
67, represent the zigzag discharge from one cloud to another, and
Fig. 67.
suppose the discharge of electricity from the interior of one cloud
takes place by the branches A C, A C', etc., and from the interior
of the other cloud by the branches B D, B D', etc. Then an ob-
server at E would first hear the rattling sound resulting from the
motion of the electricity along the paths A C, A C', etc., and this
noise would not be of very great intensity. After a few seconds
the sound of the concentrated discharge through A B will reach
him, and he will hear a crashing noise, which will continue for
several seconds with variable intensity. This will be succeeded
by a low, rumbling noise, resulting from the partial discharge
along B D, B D', etc., and this noise will be faint on account of the
great distance.
340. Height of Thunder Clouds. Thunder clouds are some-
times limited to a height of less than a quarter of a mile, and
sometimes they rise to the height of at least three or four miles.
Observers on the summit of hills less than a quarter of a mile in
height, have seen thunder-showers below them, while they were
172 METEOROLOGY.
enjoying a cloudless sky. On the other hand, La Condamine en-
countered a violent thunder-storm on a peak of the Cordilleras at
the height of 15,970 feet.
341. Lightning Tubes. When lightning descends into a sandy
soil, the sand is sometimes melted by the discharge, and the path
of the lightning is marked by a tube of vitrefied sand. Such, a
tube is called a, fulgurite. These tubes are sometimes three inches
in external diameter, with sides nearly an inch in thickness, and
they sometimes extend to a depth of thirty feet. The inside part
of lightning tubes is smooth and very bright. It scratches glass,
and strikes fire as a flint. By passing a powerful electrical dis-
charge through a mixture of sand and salt, similar tubes have
been produced artificially.
342. Geographical distribution of Thunder-storms. Thunder-
storms occur most frequently in the equatorial regions, and dimin-
ish as we proceed toward the poles. From the equator to latitude
30 the average number of thunder-showers annually is 52 ; from
latitude 30 to latitude 50 it is 20 ; from latitude 50 to 60 it is
15 ; and from latitude 60 to 70 it is only 4. Beyond latitude
70 lightning is of very rare occurrence ; and beyond the parallel
of 75 it is believed to be entirely unknown.
Within the tropics, where the trade winds prevail, thunder-
storms are rare ; but in those calm regions where there is no
steady prevalent wind they are of frequent occurrence. They
are produced by ascending columns of air in the form of torna-
does ; they cover but a small area, commence suddenly, and rare-
ly last over half an hour.
In Lower Peru, where it never rains, thunder is never heard.
Thunder-storms are most frequent in warm climates, because
here evaporation supplies electricity in the greatest abundance,
and the vapor of the air is precipitated most copiously. In the
middle latitudes thunder occurs chiefly in the summer months,
and it is most frequent about the middle of the afternoon.
343. Lightning caused by Volcanoes. The eruptions of volca-
noes are frequently accompanied by vivid flashes of zigzag light-
ning. This electricity is probably developed in the same way as
the electricity of common thunder-storms. The volcano shoots
ELECTRICAL PHENOMENA. 173
up to a great height vast volumes of heated air. This air is cooled
by elevation, its vapor is condensed, and a cloud is formed. This
cloud serves as a conductor for the electricity previously existing
in the air, by which means it becomes highly charged, and the elec-
tricity thus collected is discharged upon the peak of the volcano.
For the same reason, violent whirlwinds and water-spouts are
generally attended by thunder and lightning.
344. Telegraph Wires affected ~by Thunder-storms. The wires of
the electric telegraph present conductors of electricity of vast ex-
tent, and they are powerfully affected during the passage of a
thunder-storm. The electricity of a distant cloud is sufficient to
charge a telegraph wire, and when the electricity of the cloud is
discharged, a spark is perceived wherever there is a small interrup-
tion in the telegraph wire. This effect is produced at a distance
of several miles, and during summer these sparks are often seen in
telegraph offices, being sometimes caused by a thunder-storm so
remote that no lightning is perceived at the place of observation.
345. Pointed Objects tipped with Light. If in a dark room we
hold a pointed conductor near to an electrified body, we may ob-
serve the point to be tipped with light. Similar phenomena often
occur in nature upon a grand scale. When the lower atmosphere
is highly electrified, pointed objects are sometimes seen tipped
with light. The tops of the masts, and the ends of the spars of
ships, the lances of soldiers, the tips of horses' ears, the point of
an umbrella, and similar pointed objects, are frequently luminous
at night. Sometimes the hair of the head stands erect, and ap-
pears tipped with flame.
All these phenomena are due to a moderate charge of electrici-
ty, not sufficient to force its way explosively, but escaping by a
gentle current.
SECTION III.
AURORA POLARIS.
346. The aurora polaris is a luminous appearance frequently
seen near the horizon as a diffuse light like the morning twilight,
whence it has received the name of aurora. In the northern hem-
isphere it is usually termed aurora borealis, because it is chiefly
seen in the north. A similar phenomenon is seen in the south-
174
METEOROLOGY.
era hemisphere, where it is called the aurora australis. Each of
them may with greater propriety be called aurora polaris, or po-
lar light.
347. Varieties of Aurora. Auroras exhibit an infinite variety
of appearances, but they may generally be referred to one of the
following classes:
First. A horizontal light like the morning aurora or break of
day. The polar light may generally be distinguished from the
true dawn by its position in the heavens, since in the United
States it always appears in the northern quarter. This is the
most common form of aurora, but it is not an essentially distinct
variety, being due. to a blending of the othep varieties in the dis-
tance. The upper limit of the light is an arc of a small circle,
which, though indefinite, is better defined than the twilight.
348. Second. An Arch of Light somewhat in the form of a Rain-
bow. This arch frequently extends entirely across the heavens
from east to west, and cuts the magnetic meridian nearly at right
angles. This arch does not long remain stationary, but frequent-
ly rises and falls ; and when the aurora exhibits great splendor,
several parallel arches are often seen at the same time, appearing
as broad belts of light, stretching from the eastern to the western
horizon. In the polar regions, five, six, and even seven such
arches have been seen at once ; and on two occasions have been
seen nine parallel arches separated by distinct intervals. Fig. 68
represents auroral arches seen a few years since in Canada.
Fig. 68.
ELECTRICAL PHENOMENA.
175
349. Third. Slender, luminous learns or columns, well-defined
and often of a bright light. These beams rise to various heights
in the heavens from 2 or 3 up to 90 or more ; sometimes, though
rarely, passing the zenith, Fig. 69. Their breadth varies from a
Fig. 69.
quarter of a degree up to two or three degrees. Frequently they
last but a few minutes, sometimes they continue a quarter of an
hour, a half hour, or even a whole hour. Sometimes they remain
at rest, and sometimes they have a quick lateral motion. This
light is commonly of a pale yellow, sometimes reddish, occasion-
ally crimson, or even of blood color. Sometimes the luminous
beams are interspersed with dark ra}^s resembling dense smoke.
Sometimes the tops of the beam's are pointed, and, having a wav-
ing motion, they resemble the lambent flames of half-extinguish-
ed alcohol burning upon a broad flat surface, Fig. 70, page 176.
Faint stars are visible through the substance of the beams.
350. Fourth. The Corona. Luminous beams sometimes shoot
up simultaneously from nearly every part of the horizon, and con-
verge to a point a little south of the zenith, forming a quivering
canopy of flame, which is called the corona. The sky now resem-
bles a fiery dome, and the crown appears to rest upon variegated
fiery pillars, which are frequently traversed by waves or flashes of
light. This may be called a complete aurora, and comprehends
most of the peculiarities of the other varieties. See Fig. 74.
176
METEOROLOGY.
Fig. 70.
The corona seldom continues complete longer than one hour.
The streamers then become fewer and less intensely colored ; the
luminous arches break up, while a dark segment is still visible
near the northern horizon, and at last nothing remains but masses
of delicate cirro-cumulus clouds.
During the exhibition of brilliant auroras, delicate fibrous
clouds are commonly seen floating in the upper regions of the
atmosphere ; and on the morning after a great nocturnal display
we sometimes recognize the same streaks of cloud which had
been luminous during the preceding night. Sometimes during
the day these clouds arrange thetnselves in forms similar to the
beams of the aurora, constituting what has been called a day
aurora.
351. Fifth. Waves or Flashes of Light. The luminous beams
sometimes appear to shake with a tremulous motion ; flashes like
waves of light roll up toward the zenith, and sometimes travel
along the line of an auroral arch. Sometimes the beams have a
slow lateral motion from east to west, and sometimes from west
to east. These sudden flashes of auroral light are known by the
name of merry dancers, and form an important feature of nearly
every splendid aurora.
ELECTRICAL PHENOMENA. 177
352. Duration of Auroras. The duration of auroras is very
variable. Some last only an hour or two ; others last all night ;
and occasionally they appear on two successive nights under cir-
cumstances which lead us to believe that, were it not for the light
of the sun, an aurora might be seen uninterruptedly for 36 or 48
hours. For more than a week, commencing August 28th, 1859,
in the northern part of the United States, the aurora was seen al-
most uninterruptedly every clear night. In the neighborhood of
Hudson's Bay, the aurora is seen for months almost without ces-
sation.
353. Recurring Fits. Auroras are characterized by recurring
fits of brilliancy. After a brilliant aurora has faded away, and
almost wholly disappeared, it is common for it to revive, so as to
rival and often to surpass its first magnificence. Two such fits
are common features of brilliant auroras, and sometimes three or
four occur on the same night.
354. Colors of the Aurora. The color of the aurora is very
variable. If the aurora be faint, its light is usually white or a
pale yellow. When the aurora is brilliant, the sky exhibits at
the same time a great variety of tints ; some portions of the sky
are nearly white, but with a tinge of emerald green ; other por-
tions are of a pale yellow, or straw color ; others are tinged with
a rosy hue ; while others have a crimson hue, which sometimes
deepens to a blood red. These colors are ever varying in posi-
tion and intensity.
355. Geographical Extent of Auroras. Auroras are sometimes
observed simultaneously over large portions of the globe. The
aurora of August 28, 1859, was seen over more than 140 degrees
of longitude, from California to Eastern Europe, and from Ja-
maica, on the south, to an unknown distance in British America,
on the north. The aurora of September 2, 1859, was seen at the
Sandwich Islands ; it was seen throughout the whole of North
America and Europe; and the magnetic disturbances indicated
its presence throughout all Northern Asia, although the sky
was overcast, so that at many places it could not be seen. An
aurora was seen at the same time in South America and New
Holland.
M
178 METEOKOLOGY.
The auroras of September 25, 1841, and November 17, 1848,
were almost equally extensive.
356. Dark Segment. In the United States an aurora is uni-
formly preceded by a hazy or slaty appearance of the sky, partic-
ularly in the neighborhood of the northern horizon. When the
auroral display commences, this hazy portion of the sky assumes
the form of a dark bank or segment of a circle in the north, rising
ordinarily to the height of from five to ten degrees, Fig. 71. This
dark segment is not a cloud, for the stars are seen through it, as
through a smoky atmosphere with little diminution of brilliancy.
This dark bank is simply a dense haze, and it appears darker
from the contrast with the luminous arc which rests upon it. In
high northern latitudes, when the aurora covers the entire heav-
ens, the whole sky seems filled with a dense haze ; and still nearer
the pole, where the aurora is sometimes seen in the south, this
dark segment is observed resting on the southern horizon, and
bordered by the auroral light. This phenomenon was visible in
the United States in the aurora of August, 1859.
The highest point of this dark segment is generally found in the
magnetic meridian. Exceptions, however, frequently occur, and
in some places there is a constant deviation often degrees or more.
ELECTKICAL PHENOMENA. 179
357. Position of Auroral Arches. The dark segment is bounded
by a luminous arc, whose breadth varies from half a degree to
one or two degrees. The lower edge is well defined ; but, unless
the breadth be very small, the upper edge is ill defined, and blends
with a general brightness of the sky. If the aurora becomes bril-
liant, other arcs usually form at greater elevations, sometimes
passing through the zenith.
The summit of these arcs is situated nearly in the magnetic
meridian, and the arc sometimes extends symmetrically on each
side toward the horizon. Frequently, however, the summit of
the arc deviates ten degrees or more from the magnetic merid-
ian, and in some places this deviation appears to be tolerably con-
stant.
Sometimes the arch is incomplete, extending only part of the
way from one horizon to the other.
358. Breadth of Auroral Arches. The apparent breadth of
auroral arches varies with their elevation above the horizon.
The result of a large number of observations made in Scandina-
via gave seven degrees as the average breadth of arches seen in
the north at altitudes less than 60 ; for arches seen in the south
at altitudes less than 60, the average breadth was eight degrees ;
while for arches between the limits of 30 zenith distance either
north or south, the average breadth was twenty-five degrees.
When an arch appears to move across the sky from north to
south, or the reverse, its angular breadth exhibits corresponding
changes.
If the distance of an arch from the earth remained constant
during its movement of translation, and the arch was of the form
of a ring whose section was a circle, its breadth when in the ze-
nith should be double that at an elevation of 30. But its actual
breadth in the former case is three or four times as great as in the
latter, showing that the greatest breadth of a section of the ring
is parallel to the earth.
359. Form of Auroral Arches. Auroral arches are not arcs of
great circles; that is, they do not cut the horizon at points 180
from each other. Careful measurements made at several points
of some of the most remarkable arcs have shown that, except
near the horizon, they may be regarded as portions of small cir-
180
METEOKOLOGY.
cles parallel to the earth's surface. Such a circle seen obliquely
would Lave the appearance of an ellipse. Near the horizon the
elliptic form of the auroral arch has sometimes been quite notice-
able, the extremities of the arch being bent inward. Occasion-
ally an ellipse has been seen almost entire, and there is one in-
stance on record in which the ellipse appeared complete, the di-
ameters of the ellipse being as two to one, and the centre of the
ellipse being elevated about 15 above the horizon.
360. Anomalous forms of Arches. Sometimes an auroral arch
consists of rays arranged in irregular and sinuous bands of vari-
ous and variable curvatures, presenting the appearance of the
undulations of a ribbon or flag waving in the breeze. Some-
times the appearance is that of a brilliant curtain whose folds are
agitated by the wind, Fig. 72. These folds sometimes become
Fig. 72.
very numerous and complex, and the arch assumes the form of
a long sheet of rays returning into itself, the folds enveloping
each other, and presenting an immense variety of the most grace-
ful curves. Sometimes these curves are continually changing,
and develop themselves like the folds of a serpent.
361. Movements of Auroral Arches. An auroral arch does not
maintain invariably a fixed position. It is frequently displaced,
and is transported parallel to itself from north to south, or from
ELECTKICAL PHENOMENA. 181
south to north. An arch which first appears near the northern
horizon sometimes rises gradually, attains the zenith, descends to-
ward the southern horizon, remains there for a time stationary,
and then perhaps retraces its course. A series of observations- in
Scandinavia presented sixty cases in which auroral arches moved
from north to south, and thirty-nine cases from south to north.
In the United States the motion from north to south is about ten
times as frequent as the motion from south to north.
Sometimes there is a movement of the arch from west to 'east,
or from east to west.
The rate of motion of arches is very variable. The angular
motion of translation sometimes amounts to 17 per minute, and
frequently amounts to 5 per minute. With a vertical elevation
of 125 miles above the earth, the last rate of motion would imply
an actual velocity of 1000 feet per second.
362. Structure of* Auroral Arches. Auroral arches generally
tend to divide into short rays running in the direction of the
breadth of the arch, and converging toward the magnetic zenith.
They frequently seem to be formed of transverse fibres termin-
ating abruptly in a regular curve, which forms the lower edge of
the arch, Fig. 73. Arches entirely nebulous are not the most fre-
Fig. 73.
quent; striated arcs are very common, and auroral arches present
every intermediate variety between these two extremes. Fre-
quently a nebulous arc resolves itself into a striated arc without
changing its general form. Sometimes the rays are distinct and
isolated. In such a case the arch generally increases in breadth,
extending on the side of the zenith. Sometimes auroral beams
arrange themselves in the form of an arch, which is subsequently
replaced by an arch of nebulous matter. When the light of the
182 METEOROLOGY.
rays is uniform, the dark intervening spaces sometimes present
the appearance of dark rays or black strice perpendicular to the
arch. Sometimes an auroral arch is formed of short streams par-
allel to each other, presenting the appearance of a row of comet's
tails.
363. Motion of Auroral Seams. This motion is either longitu-
dinal, the beam extending toward the zenith or the horizon, or it
is a lateral movement which displaces the beam parallel to itself.
Frequently a beam extends suddenly either upward or down-
ward. This motion is most common downward, and sometimes
with very great velocity. It sometimes takes place simultane-
ously in a large number of neighboring beams. When a beam
rises and falls alternately without any considerable change of
length, it is said to dance. This is a common occurrence in high
latitudes, where it is known by the name of the merry dancers.
Beams sometimes move laterally from east to west, and some-
times from west to east ; but in the United States the former mo-
tion is the most common. Beams advance either from north to
south, or from south to north, but the former motion is the most
common.
364. The Corona. When the sky is filled with a large number
of separate beams all parallel to each other and to the direction
of the dipping needle, according to the rules of perspective, these
beams will seem to converge to one point, viz., the magnetic ze-
nith, or the point toward which the dipping needle is directed, Fig.
74. Hence results the appearance of a corona, or crown of rays,
whose centre is generally, but not always dark.
Numerous measurements have been made of the position of
the corona, and they show that the centre of the corona is always
very near the magnetic zenith, but not always exactly coincident
with it.
The corona is sometimes incomplete, sectors of greater or less
extent being deficient. The passage of a striated arch over the
magnetic zenith frequently presents the appearance of a corona.
If the arch advances from north to south, before reaching the
magnetic zenith it forms a half crown on the northern side ; at
the instant of passing the magnetic zenith we have a complete
corona of an elliptic form, whose rays descend nearly to the hori-
ELECTRICAL PHENOMENA.
Fig. 74.
183
zon on the eastern and western sides ; and after the arch has
passed the magnetic zenith there is formed a half crown on the
southern side.
365. Auroral Clouds. When an aurora becomes less active its
beams become less luminous, their edges become more diffuse,
they increase in breadth while they diminish in length, and as-
sume the appearance of luminous clouds. Sometimes they ex-
hibit a fibrous "structure, and present a strong resemblance to cir-
rus clouds. These auroral clouds generally make their appear-
ance later in the evening than arches or beams.
366. Auroral Vapor. During the exhibition of a brilliant au-
rora there is frequently an appearance of general nebulosity or
luminous vapor covering large portions of the heavens, and some-
184 METEOROLOGY.
times almost the entire celestial vault. Its light is generally faint,
especially in the upper part of the sky, sometimes but little ex-
ceeding that of the milky way ; but sometimes, near the horizon,
the light is intense, resembling a vast conflagration. This seems
to indicate that the vertical thickness of the auroral vapor is small
in comparison with its horizontal dimensions.
This auroral vapor may appear during any phase of a grand
aurora, and is frequently seen during the intervals between the
disappearance and reappearance of arches and beams.
367. Height of the Aurora. The great auroral exhibition of
August and September, 1859, was very carefully observed at a
large number of stations, and these observations afford the ma-
terials for determining the height of the aurora above the earth's
surface.
At the most southern stations where these auroras were ob-
served, the light rose only a few degrees above the northern
horizon ; at more northern stations the aurora rose higher in the
heavens; at certain stations it just attained the zenith ; at stations
farther north the aurora covered the entire northern heavens,
as well as a portion of the southern ; and at places farther north
nearly the entire visible heavens from the northern to the south-
ern horizon were overspread with the auroral light.
In Fig. 75, AB represents a portion of the earth's surface, and
beneath are given the names of some of the places where observ-
Fig. 75.
ations were made upon the- aurora of August 28, 1859, all at the
same hour of the evening. The dotted lines drawn from the
five most southern stations represent the elevations of the upper
ELECTKICAL PHENOMENA. 185
boundary of the auroral light above the northern horizon. The
point D thus determined is then the upper edge of the auroral
light, near its southern margin, and this point is found to be 634
miles above the earth's surface.
The dotted lines from the five most northern stations show the
elevation of the lower limit of the auroral light above the south
horizon. The point C thus determined is the lower edge of the
auroral light, near its southern margin, and this point is found to
be 46 miles above the earth's surface. The line CD represents,
therefore, the southern boundary of the auroral illumination.
These results, combined with a vast number of other observ r
ations, show that the aurora of August 28th, 1859, formed a stra-
tum of light encircling the northern hemisphere, extending south-
ward in North America to latitude 38, and reaching to an un-
known distance on. the north ; and it pervaded more or less the
entire interval between the elevations of 46 miles and 500 miles
above the earth's surface. This illumination consisted chiefly of
luminous beams or columns every where nearly parallel to the di-'
rection of a magnetic needle when freely suspended ; that is, in
the United States the upper extremities of these beams inclined
southward at angles varying from 15 to 30. These beams were
therefore about 500 miles in length, and their diameters varied
from 5 to 50 miles, and perhaps sometimes they were still greater.
The height of a large number of auroras has been computed
by similar methods, and the average result for the upper limit of
the streamers is 450 miles.
From a multitude of observations, it is concluded that the au-
rora seldom appears at an elevation less than about 45 miles
above the earth's surface, and that it frequently extends upward
to an elevation of 500 miles. Auroral arcs having a well-defined
border are generally less than 100 miles in height.
368. Conflicting Estimates of the Height. Some persons contend
that the aurora is sometimes seen at elevations of less than one
mile above the earth's surface. It is claimed that the aurora is
sometimes seen between the observer and a cloud, but this ap-
pearance is believed to result from, a cloud of very small density
strongly illumined by auroral light, which shines through the
cloud so as to produce the same appearance as if the aurora pre-
vailed on the under side of the cloud.
186 METEOROLOGY.
Sometimes the lower extremity of an auroral streamer appears
to be prolonged below the summit of a neighboring mountain or
hill. This appearance is probably an illusion. The same phe-
nomenon has been noticed by careful observers, who ascribed the
result to the reflection of the auroral light from the snow which
covered the mountains.
Although it is possible that the aurora may sometimes descend
nearly to the earth's surface, there is no sufficient evidence to
prove that the true polar light has ever descended so low as the
region of ordinary clouds.
369. Noise of the Aurora. There is no satisfactory evidence
that the aurora ever emits any audible sound. It is a common
impression, at least in high latitudes, that the aurora sometimes
emits sound. This sound has been called a rustling, hissing,
crackling noise. But the most competent observers, who have
spent several winters in the Arctic regions, where auroras are seen
in their greatest brilliancy, have been convinced that this sup-
posed rustling is a mere illusion. It is therefore inferred that the
sounds which have been ascribed to the aurora must have been
due to other causes, such as the motion of the wind, or the crack-
ing of the snow and ice in consequence of their low temperature.
If the aurora emitted an audible sound, this sound ought to
follow the auroral movements after a long interval. Sound re-
quires four minutes to travel fifty miles. But the observers who
report auroral noises make no mention of any interval. It is
therefore inferred that the "sounds which have been heard during
auroral exhibitions are to be ascribed to other causes than the
aurora.
370. Geographical Distribution of Auroras. Auroras are very
unequally distributed over the earth's surface. They occur most
frequently in the higher latitudes, and are almost unknown with-
in the tropics. At Havana, latitude 23, but six auroras have been
recorded within a hundred years, and south of Havana auroras
are still more unfrequent. As we travel northward from Cuba,
auroras increase in frequency and brilliancy ; they rise higher in
the heavens, and oftener attain the zenith. Near the parallel of
40, we find, on an average, only ten auroras annually. Near the
parallel of 42, the average number is twenty annually ; near 45,
ELECTKICAL PHENOMENA.
187
the number is forty ; and near the parallel of 50, it amounts to
eighty annually. Between this point and the parallel of 62,
auroras are seen almost every night. They appear high in the
heavens, and as often to the south as the north. Farther north
they are seldom seen except in the south, and from this point
they diminish in frequency and brilliancy as we advance toward
the pole. Beyond latitude 62 the average number of auroras is
reduced to forty annually. Beyond latitude 67 it is reduced to
twenty, and near latitude 78 to ten annually.
Fiff. 76.
188 METEOROLOGY.
If we make a like comparison for the meridian of St. Peters-
burg, we shall find a similar result, except that the auroral region
is situated farther northward than it is in America, the region of
eighty auroras annually being found between the parallels of 66
and 75.
Upon Fig. 76, the dark shade indicates the region where the
average number of auroras annually amounts to at least eighty,
and the lighter shade indicates the region where the average
number of auroras annually amounts to at least forty.
We thus see that the region of greatest auroral action is a zone
of an .oval form surrounding the north pole, and whose central
line crosses the meridian of Washington in latitude 56, and the
meridian of St. Petersburg in latitude 71. Accordingly, auroras
are much more frequent in the United States than they are in the
same latitudes of Europe.
The form of this auroral zone bears considerable resemblance
to a magnetic parallel, or line every where perpendicular to a
magnetic meridian, and it is probable that there is a real connec-
tion between the two phenomena.
371. Auroras in the Southern Hemisphere. Auroras in the south-
ern hemisphere are nearly, if not quite as frequent as they are in
the corresponding magnetic latitudes of the northern hemisphere,
and it is probable that the geographical distribution of auroras in
the two hemispheres is somewhat similar.
372. Simultaneous Auroras in loth Hemispheres. By comparing
the records of auroras in the two hemispheres we find a remark-
able coincidence of dates, which seems to justify the conclusion
that an unusual auroral display in the southern hemisphere is
always accompanied by an unusual display in the northern hem-
isphere; that is, a great exhibition of auroral light about one
magnetic pole of the earth, is uniformly attended by a great ex-
hibition of auroral light about the opposite magnetic pole.
373. Diurnal Periodicity of Auroras. Auroras appear at all
hours of the night, but not with equal frequency. The average
number increases uninterruptedly from sunset till about mid-
night, from which time the number diminishes uninterruptedly
till morning. In Canada the maximum occurs an hour before
ELECTKICAL PHENOMENA. 189
midnight ; farther north, in latitude 52, the maximum occurs at
midnight ; and still farther north to the Arctic Ocean, the maxi-
mum occurs an hour after midnight.
374. Annual Periodicity of Auroras. Auroras occur in each
month of the year, but not with equal frequency. In New En-
gland and New York the least number of auroras is recorded in
winter, and the greatest number in the autumn ; but if we make
allowance for the diminished length of the nights in summer, we
must conclude that auroras are about as frequent in summer as
in autumn. There is a decided diminution in the frequency of
auroras in winter, and a period of maximum frequency from
April to September, with perhaps a slight diminution during the
intervening month of June.
Observations in Canada lead to similar conclusions, except that
the unequal length of the days has a somewhat greater influence
upon the number of auroras recorded in summer.
375. Secular Periodicity of Auroras. The number of auroras
seen in different years is extremely variable. Sometimes for
several years auroras are remarkable for their number and mag-
nificence, and then succeeds a barren interval during which au-
roras are almost entirely forgotten.
If we compare the observations made at any one station for a
long period of years, we shall discover that the inequality in the
number of auroras upon successive years bears a strong resem-
blance to a secular periodicity.
From a continued series of observations of the aurora made at
Boston and New Haven from 1742 to the present time, it appears
that auroras were unusually frequent from 1780 to 1791, and
again from 1837 to 1854 ; but from 1742 to 1768, and again from
1792 to 1837, they were much less frequent and brilliant. These
observations indicate two periods of maximum abundance, the
culminating points of which were about 1787 and 1845; that is,
during the past century the frequency of auroras in New En-
gland has been subject to an inequality bearing considerable re-
semblance to an astronomical periodicity, the period being about
fifty -eight years.
A comparison of European observations for the past two cen-
turies leads to similar conclusions. We find periods of unusual
190 METEOROLOGY.
abundance culminating about 1728, 1780, and 1842 ; and between
these periods we find long intervals of great barrenness. These re-
sults show a considerable degree of uniformity approximating to-
ward a period of fifty-nine years from one maximum to another.
At the same time, the observations indicate important excep-
tions, which seem to point to a subordinate period of ten years.
In both the American and European observations there is a fre-
quent alternation of meagre and abundant years, and the inter-
vals do not differ much from ten years. The observations then
seem to indicate a maximum every ten years, and a maximum of
extraordinary brilliancy every fifty-nine or perhaps sixty years.
376. Disturbance of the Magnetic Needle. The aurora is ordina-
rily accompanied by a considerable disturbance of the magnetic
needle, and the effect increases with the brilliancy and extent of
the aurora. Auroral beams cause a disturbance of the needle,
particularly when the beams themselves are in active motion.
Auroral waves or flashes, especially if they extend to the zenith,
cause a violent agitation of the needle, consisting of an irregular
oscillation on each side of its mean position.
These extraordinary deflections of the needle prevail almost
simultaneously over large portions of the globe, even where the
aurora itself is not visible. During the great auroral display of
September 2, 1859, the disturbances of the magnetic needle were
very remarkable throughout North America, Europe, and North-
ern Asia, as well as in New Holland. At Toronto the declina-
tion of the needle changed 3 45' in half an hour. The inclina-
tion was observed to change 2 49' when the needle passed be-
yond the limits of the scale, so that the entire range of the needle
could not be determined. The horizontal force was observed to
change to the extent of one ninth of its whole value when the
needle passed beyond the limits of the scale, so that its entire
range was unknown. At several observatories in Europe still
more remarkable disturbances were recorded.
377. Progress of Magnetic Disturbances. These irregular de-
flections of the magnetic needle are not quite simultaneous at dis-
tant stations. Over the surface of Europe they appear to be
propagated in a direction from N. 28 E. to S. 28 W. at the rate
of about 100 miles per minute. Over the surface of North Amer-
ELECTRICAL PHENOMENA. 191
ica they are propagated at about the same velocity in a direction
fromK68E.toS.68W.
378. Influence of the Aurora upon the Telegraph Wires. Auroras
exert a remarkable influence upon the wires of the electric tele-
graph. During the prevalence of brilliant auroras the telegraph
lines generally become unmanageable. The aurora develops elec-
tric currents upon the wires, and hence results a motion of the
telegraph instruments similar to that which is employed in tele-
graphing, and this movement, being frequent and irregular, ordi-
narily renders it impossible to transmit intelligible signals. Dur-
ing several remarkable auroras, however, the currents of electrici-
ty on the telegraph wires have been so steady and powerful that
they have been used for telegraph purposes as a substitute for a
voltaic battery ; that is, telegraph messages have been transmitted
from the auroral influence alone. This result proves that the au-
rora develops on the telegraph wires an electric current similar
to that of a voltaic battery, and differing only in its variable in-
tensity.
THEOEY OF THE POLAE LIGHT.
379. Is the Aurora caused ly Nebulous Matter falling into our At-
mosphere? Some have ascribed the polar light to a rare nebulous
matter occupying the interplanetary spaces, and revolving round
the sun at such a distance that a portion of this matter occasion-
ally falls into the upper regions of the atmosphere with a velocity
sufficient to render it luminous from the condensation of the air
before it. But we can see no reason why matter, reaching the
earth from such a source, should always be confined to certain
districts of the earth, and be wholly unknown in other portions.
This hypothesis, therefore, can not be reconciled with the known
geographical distribution of auroras.
380. Auroral Exhibitions are Terrestrial Phenomena. Auroral
exhibitions take place in the upper regions of the atmosphere,
and partake of the earth's rotation. All the celestial bodies have
an apparent motion from east to west, arising from the rotation of
the earth ; but bodies belonging to the earth, including the atmos-
phere and the clouds which float in it, partake of this rotation, so
that their relative position is not affected by it. The same is true
192 METEOROLOGY.
of the aurora. Whenever a corona is formed, it maintains sensi-
bly the same position in the heavens during the whole period of
its continuance, although the stars meanwhile revolve at the rate
of 15 per hour.
381. The Auroral Light is Electric Light. This is proved by
the effect of an aurora upon the telegraph wires. The electric
telegraph is worked by a current of electricity generated by a
voltaic battery r and flowing along the conducting wire which
unites the distant stations. This current, flowing round an elec-
tro-magnet, renders it temporarily magnetic, so that its armature
is attracted, and a mark is made upon a roll of paper. During a
thunder-storm the electricity of the atmosphere affects the con-
ducting wire in a similar manner, and a great auroral display
produces a like effect. During the auroras of August and Sep-
tember, 1859, there were remarked all those classes of effects
which are considered as characteristic of electricity.
A. In passing from one conductor to another, electricity exhib-
its a spark of light. During the auroras of 1859, at numerous
stations both in America and Europe, brilliant sparks were drawn
from the telegraph wires when no battery was attached.
B. In passing through poor conductors, electricity develops
heat. During the auroras of 1859, both in America and Europe,
paper and even wood were set on fire by the auroral influence
alone.
C. When passed through the animal system, electricity com-
municates a well-known characteristic shock. During the auroras
of 1859 several telegraph operators received severe shocks when
they touched the telegraph wires.
D. A current of electricity develops magnetism, in ferruginous
bodies. The auroras of 1859 developed magnetism so abundant-
ly and so steadily that it was more than sufficient for the ordi-
nary business of telegraphing.
E. A current of electricity deflects a magnetic needle from its
normal position. In England the usual telegraph signal is made
by a magnetic needle surrounded by a coil of copper wire, so
that the needle is deflected by an electric current flowing through
the wire. Similar deflections were caused by the auroras of 1859,
and these deflections were greater than those produced by the
telegraph batteries.
ELECTKICAL PHENOMENA. 193
F. A current of electricity produces chemical decompositions.
The auroras of 1859 produced the same marks upon chemical pa-
per as are produced by an ordinary voltaic battery ; that is, they
decomposed a chemical compound.
G. Certain bodies, such as a solution of sulphate of quinine,
when illumined by an electric spark, present a very peculiar ap-
pearance, as if they were self-luminous. This appearance is
termed fluorescence. The same effect is produced upon these sub-
stances by the auroral light.
These facts demonstrate that the fluid developed by the aurora
on telegraph wires is indeed electricity. This electricity may be
derived from the a*urora either by transfer or by induction. If
we adopt the former supposition, then the auroral light is certain-
ly electric light. If we adopt the latter supposition, then, since
we know of but two agents, magnetism and electricity, capable of
inducing electricity in a distant conductor, and since the auroral
fluid is luminous while magnetism is not luminous, we must ad-
mit that the auroral light is electric light.
382. Colors of the Aurora. The colors of the aurora are the
same as those of ordinary electricity passed through rarefied air.
When a spark is drawn from an ordinary electrical machine in
air of the usual density, the light is intense and nearly white. If
the electricity be passed through a glass vessel in which the air
has been partially rarefied, the light is more diffuse, and inclines
to a delicate rosy hue. If the air be still farther rarefied, the light
becomes very diffuse, and its color becomes a deep rose or purple.
The same variety of colors is observed during the aurora. The
transition from a white or pale straw color to a rosy hue, and
finally to a deep, red, probably depends upon the height above the
earth, and upon the amount of condensed vapor present in the air.
The emerald green light which is seen in some auroras is as-
cribed to the projection of the yellow light of the aurora upon
the blue sky, since a combination of yellow and blue light pro-
duces green. A similar effect is often produced in the evening
twilight by a combination of the yellow light of the sun with the
blue of the celestial vault.
383. The Auroral Corona. The formation of an auroral corona
near the magnetic zenith is the effect of perspective, resulting
N
194 METEOKOLOGY.
from a great number of luminous beams all parallel to each other.
A collection of beams parallel to the direction of the dipping
needle would all appear to converge toward the pole of the nee-
dle, and no other supposition will explain all the appearances
which we observe. The auroral crown, therefore, every where
appears in the magnetic zenith, and it is not the same crown
which is seen at different places any more than it is the same
rainbow which is seen by different observers.
384. What are Auroral Seams f The auroral beams are sim-
ply illumined spaces caused by the flow of electricity through the
upper regions of the atmosphere. During the auroras of 1859
these beams were nearly 500 miles in length, and their lower ex-
tremities were elevated about 45 miles above the earth's surface.
Their tops inclined toward the south about 17 in the neighbor-
hood of New York.
It was formerly supposed that the electric current necessarily
moved in the direction of the axis of the auroral beams ; that is,
that the electric discharge was from the upper regions of the at-
mosphere to the earth, or the reverse. Eecent discoveries throw
some doubt upon this conclusion. When a stream of electricity
flows through a vessel from which the air is almost wholly ex-
hausted, under certain circumstances the light becomes stratified,
exhibiting alternately bright and dark bands crossing the electric
current at right angles, from which it might be inferred that elec-
tricity flowing horizontally through the upper regions of the at-
mosphere might exhibit alternately bright and dark bands like
the auroral beams. But this stratification of the electric light is
due to intermittences in the intensity of the electric discharge, and
it is not probable that such intermittences can take place in na-
ture with sufficient rapidity to produce a similar effect. It is there-
fore more probable that auroral beams are the result of a current
of electricity traveling in the direction of the axis of the beams.
385. Cause of the Dark Segment. The slaty appearance of the
sky which is remarked in all great auroral exhibitions arises from
the condensation of the vapor of the air, and this condensed va-
por probably exists in the form of minute spiculae of ice or flakes
of snow. Fine flakes of snow have been repeatedly observed to
fall during the exhibition of auroras, and this snow only slightly
ELECTKICAL PHENOMENA. 195
impairs the transparency of the atmosphere, without presenting
the appearance of clouds. It produces a turbid appearance of the
atmosphere, and causes that dark bank which in the United States
rests on the northern horizon. This turbidness is more noticea-
ble near the horizon than it is at great elevations, because near
the horizon the line of vision traverses a greater depth of this
hazy atmosphere. When the aurora covers the whole heavens,
the entire atmosphere is filled with this haze, and a dark seg-
ment may be observed resting on the southern horizon.
386. Circulation of Electricity about the Earth. The vapor which
rises from the ocean in all latitudes, but most abundantly in the
equatorial regions of the earth, carries into the upper regions of
the atmosphere a considerable quantity of positive electricity,
while the negative electricity remains in the earth. This posi-
tive electricity, after rising nearly vertically with the ascending
currents of the atmosphere, would be conveyed toward either
pole by the upper currents of the atmosphere.
The earth and the rarefied air of the upper atmosphere may be
regarded as forming the two conducting plates of a condenser,
which are separated by an insulating stratum, viz., the lower por-
tion of the atmosphere. The two opposite electricities must then
be condensed by their mutual influence, especially in the- polar
regions, where they approach nearest together, and whenever their
tension reaches a certain limit, there will be discharges from one
conductor to the other. When the air is humid it becomes a
partial conductor, and conveys a portion of the electricity of the
atmosphere to the earth. On account of the low conducting pow-
er of the air, the neutralization of the opposite electricities would
not be effected instantaneously, but
by successive discharges, more or
less continuous, and variable in in-
tensity. These discharges shpuld
frequently occur simultaneously at
the two poles, since the electric ten-
sion of the earth should be nearly N |
the same at each pole.
Fig. 77 represents the system of
circulation here supposed, the north
and south poles of the earth being
denoted by the letters N". and S.
196 METEOKOLOGY.
387. Cause of the Auroral Beams. When electricity from the
upper regions of the atmosphere discharges itself to the earth
through an imperfectly conducting medium, the flow can not be
every where uniform, but must take place chiefly along certain
lines where the resistance is least ; and this current must develop
light, forming thus an auroral beam. It might be expected that
these beams would have a vertical position, but their position is
controlled by the earth's magnetism. It is found that when mag-
netic forces act upon a perfectly flexible conductor through which
an electric current passes, the conductor "must assume the form of
a magnetic curve. Now at each point of the earth's surface the
dipping needle shows the direction of the magnetic curve passing
through that point. Hence the axis of an auroral streamer must
lie in the magnetic curve which passes through its base; and since
adjacent streamers are sensibly parallel, the beams appear to con-
verge toward the magnetic zenith.
388. Position of Auroral Arches explained. When electricity
escapes from a metallic conductor under a receiver from which
the air has been exhausted, and this conductor is the pole of a
powerful magnet, the electric light forms a complete luminous
ring around this conductor.
In like manner, the auroral arch is a part of a luminous ring
nearly parallel to the earth's surface, having the magnetic pole for
its centre, and cutting all the magnetic meridians at right angles ;
and this position results from the influence of the earth's mag-
netism.
389. Anomalous Position of Auroral Arches. Auroral arches are
not always exactly perpendicular to the magnetic meridian, and
in some places this deviation is uniform, and may amount to ten
degrees. Such a deviation may be explained as follows :
The direction of the magnetic needle at any place is determ-
ined mainly by its position with respect to the magnetic poles of
the earth, but partly by local causes, such as the conformation of
the land and sea, etc. In consequence of these local causes, the
direction of the magnetic needle at some places probably differs
several degrees from what it would be if it were controlled en-
tirely by the magnetic poles. This local influence probably di-
minishes as we rise above the earth's surface, so that at the height
ELECTKICAL PHENOMENA. 197
of the auroral streamers the direction of the magnetic needle may
differ several degrees from that at the surface of the earth.
390. Cause of the Auroral Flashes. The flashes of light ob-
served in great auroral displays are 'due to inequalities in the mo-
tion of the electric currents. On account of the imperfect con-
ducting power of the air, the flow of electricity is not perfectly
uniform, but escapes by paroxysms. The flashes of the aurora
are therefore feeble flashes of lightning.
391. Cause of the Magnetic Disturbances. The disturbance of
the magnetic needle during auroras is due to currents of electric-
ity flowing through the atmosphere or through the earth. A
magnetic needle is deflected from its mean position by an electric,
current flowing near it through a good conductor like a copper
wire. A stream of electricity flowing through the earth or the
atmosphere must produce a similar effect.
It is probable that the directive power ol the magnetic needle
is due to electric currents circulating around the globe from east
to west. Such currents would cause the magnetic needle every
where to assume a position corresponding with what is actually
observed ; and the existence of such currents has been proved by
direct observation.
According to Art. 386, positive electricity circulates from the
equator toward either pole through the upper regions of the at-
mosphere, and thence through the earth toward the equator, to re-
store the equilibrium which is continually disturbed by evapora-
tion from the waters of the equatorial seas. This current from
the polar regions must modify the regular current which is sup-
posed to be constantly circulating from east to west, resulting in
a current from northeast to southwest, in conformity with observ-
ations.
This current does not, however, flow uninterruptedly from KE.
to S.W., but alternates at short intervals with a current in the
contrary direction. Such currents of electricity must produce a
continual disturbance of the magnetic needle, and they are suffi-
cient to account for the disturbances actually observed.
392. Effect of the Aurora upon Telegraph Wires. The effect of
the aurora upon the telegraph wires is similar to that of electric-
198 METEOROLOGY.
ity in thunder-storms, except in the intensity and steadiness of its
action. During thunder-storms the electricity of the wires is dis-
charged instantly with a flash of lightning, while during auroras
there is sometimes a strong and steady flow continuing for several
minutes.
393. Cause of the Diurnal Inequality of Auroras. The diurnal
inequality in the frequency of auroras is due to the same cause as
the diurnal variation in the intensity of atmospheric electricity.
The same causes which favor the escape of electricity from the
upper atmosphere to the earth will produce an aurora whenever
the electricity of the upper air is sufficiently intense, and the Son-
ducting power of the air is favorable for the slow transmission of
an electric current.
394. Cause of the Annual Inequality of Auroras. The unequal
frequency of auroras in the different months of the year depends
partly upon the amount of electricity present in the upper air,
and partly upon the humidity of the air by which this electricity
may be discharged. The supply of electricity must be greatest
when the evaporation is most rapid, that is, in summer, and this is
probably the reason why in North America auroras are more fre-
quent in summer than in winter. In Europe auroras are seldom
seen in midsummer, because in those latitudes where auroras are
most frequent, twilight in midsummer continues all night.
395. Cause of the Secular Inequality of Auroras. The secular
inequality in the frequency of auroras indicates the influence of
distant celestial bodies upon the electricity of our globe. The
periods of auroras observe laws which.are similar to those of two
other phenomena, viz., the mean diurnal variation of the magnetic
needle, and the frequency of black spots upon the sun's surface.
The magnetic needle has a small diurnal variation, the north
end moving a little to the east in the morning, and toward the
west about the middle of the day. The mean daily change of the
needle not only varies with the locality, but also varies from one
year to another at the same locality, and these variations present
a decided appearance of periodicity. At Prague the mean daily
change of the needle in 1838 was 12', from which time the range
diminished steadily to 1844, when it was only 6', from which time
ELECTRICAL PHENOMENA. 199
it increased to 1848, when it amounted to 11', the interval from
one maximum to another being a little more than ten years.
Observations made at other places, and extending back nearly
a century, indicate a maximum in the range of the magnetic nee-
dle every ten or eleven years, but the successive maxima are not
equal to each other. They exhibit variations which indicate a
periodicity, the greatest values occurring at intervals of from fifty
to sixty years. See Table XXXIV.
The relative frequency of the solar spots exhibits a similar pe-
riodicity, and the maximum number of spots corresponds with
the maximum value of the magnetic variation.
*These three phenomena, the solar spots, the mean daily range
of the magnetic needle, and the frequency of auroras, exhibit two
distinct periods ; one a period of from ten to twelve years, the
other a period of from fifty-eight to sixty years. The first of
these periods corresponds to one revolution of Jupiter, and the
other period corresponds to five revolutions of Jupiter, or two of
Saturn, and we can scarcely doubt that the above-mentioned phe-
nomena depend upon the movements of these planets. Observa-
tions have also indicated subordinate fluctuations which are prob-
ably due to the action of Yenus.
We do not know how the planets exert an influence upon the
sun's surface ; but we may suppose that there are circulating
round the sun powerful electric currents, which may possibly be
the source of the sun's light ; these currents may act upon the
planets, developing in them electric currents ; and the currents
circulating round the planets may react upon the solar currents
with a force varying with their distances and relative positions,
exhibiting periods corresponding to the times of revolution of
the planets. These disturbances of the solar currents may be
one cause of the solar spots, and an unusual disturbance of the
solar currents may cause a disturbance of the currents of the
earth's surface, giving rise to unusual displays of the aurora.
396. Geographical Distribution of Auroras. The geographical
distribution of auroras depends chiefly upon the relative intensity
of the earth's magnetism in different latitudes. According tQ ex-
periments with artificial magnets, the electric light tends to form
a' ring around the pole, and at some distance from it. The elec-
tric light should therefore be most noticeable in the neighbor-
200 METEOROLOGY.
hood of the earth's magnetic pole, but not directly over it. ' Au-
roras are, accordingly, most abundant along a certain zone which
follows nearly a magnetic parallel, being every where nearly at
right angles to the magnetic meridian of the place.
397. Why Auroras do not occur within the Tropics. The elec-
tricity of the tropical regions has great intensity, and moves with
explosive violence in thunder - showers, while the magnetic in-
tensity in those regions is very feeble, and is insufficient to con-
trol the movements of the electricity. In the higher latitudes
thunder-showers become infrequent, the electricity of the atmos-
phere passes to the earth in a slow and quiet manner, and thse
discharges are controlled by the magnetism of the earth.
398. Cause of the simultaneous Displays in both Hemispheres.
We can not explain the great auroral displays in the northern
hemisphere by supposing that the electricity of the atmosphere
is temporarily -diverted from one hemisphere to the other, for the
mean range of the magnetic needle exhibits its maxima simul-
taneously in both hemispheres ; neither can we suppose that the
absolute amount of electricity for the entire globe, as developed
by evaporation from the water of the ocean, should undergo great
periodical variations, for the mean temperature of the earth's sur-
face does not change sensibly from one year to another. We
seem, therefore, compelled to ascribe these great auroral displays
in no small degree to the direct action of the sun through the
agency perhaps of its magnetism, or of the electric currents cir-
culating around it. Such an effect should take place simultane-
ously in both hemispheres.
399. Possible System of Electrical Circulation. Hence it appears
probable that great auroral displays are not exclusively atmos-
pheric phenomena, but are to some extent the result of the in-
fluence of extra terrestrial forces. But, if these extraordinary
electrical currents are mainly determined by celestial forces, then,
since the earth exhibits many of the properties of a permanent
magnet, the two magnetic poles of the earth ought to exert op-
posite influences, and we should expect that, the currents in the
neighborhood of the two poles would move in contrary direc-
tions. Hence we naturally infer a system of circulation similar
OPTICAL METEOROLOGY.
201
Fig. 78.
to that which is represented
by Fig. 78, where 1ST and S
are supposed to represent the
north and south magnetic
poles of the earth, n and s
the poles of an imaginary
magnet representing the
magnetism of the earth. The
east and west bands repre-
sent auroral arches upon
which stand auroral stream-
ers. The dotted lines repre-
sent magnetic curves passing
from auroral streamers in
the southern hemisphere to
streamers in the northern
hemisphere, showing the
path pursued by the currents
of electricity in passing from
one hemisphere to the other,
above the atmosphere.
Tnis hypothesis agrees sub-
stantially with that stated in
Art. 386, so far as the phe-
nomena can be observed in the northern hemisphere, but they
lead to different results in the southern hemisphere. We have
not the requisite observations from the southern hemisphere to
enable us to decide between these two hypotheses.
CHAPTER YIII
OPTICAL METEOROLOGY.
SECTION I.
MIRAGE.
400. Mirage is an atmospheric phenomenon which produces an
apparent displacement of distant objects, sometimes elevating and
sometimes depressing them ; sometimes leaving the image erect,
and sometimes inverting it, as when objects are seen reflected
202 METEOROLOGY.
from a lake of tranquil water. It is frequently observed on
sandy plains intensely heated by the sun, especially in Egypt and
Arabia.
Lower Egypt is a vast sand,plain, with occasional villages situ-
ated upon small eminences. In the middle of the day, these vil-
lages, seen from a distance, appear as if situated in the rnidst of a
lake, in which are seen the inverted images of houses and trees.
The outline of these images is a little indistinct, often exhibiting
an undulatory motion, as if reflected from agitated water. As
the spectator approaches the boundary of the apparent lake, the
waters seem to retire, and the same illusion appears around the
next village. Similar phenomena are common in some parts of
California, and are occasionally seen in all parts of the United
States. Fig. 79, p. 203, represents the mirage as seen in Abyssinia.
Fig. so. Sometimes at sea, when a ship is barely visible
in the distant horizon, we perceive above the
ship, A, Fig. 80, its inverted image, B, and perhaps
above that again a second erect image, C. Some-
times of the two upper images only the invert-
ed one is seen, and sometimes only the erect
one.
All these phenomena are due to unusual varia-
tions in the refractive power of the air, arising
from extreme changes of temperature. The mi-
rage is chiefly seen over a large sandy plain, or
over water.
401. Mirage upon a Desert. Imagine a sandy plain nearly hor-
izontal, and intensely heated by the rays of the sun. The stratum
of air which rests upon the sand becomes heated by it ; this heat
is partially communicated to the superincumbent strata, so that
the density of the air increases rapidly as we rise above the earth
up to a moderate height.
Let AB, Fig. 81, represent a tree which may be viewed from C
in its true position through air of nearly uniform density ; and
suppose the air beneath it to consist of strata of variable density,
decreasing from A to the surface of the ground. The rays of
light, AD, BE, which proceed from the top and bottom of the
tree, passing successively through strata of less density, will be
deviated more and more from a vertical direction, until at last
OPTICAL METEOKOLOGY.
Fig. 79.
203
they meet a stratum at such an angle that they are unable to
enter it, and they are totally reflected from this stratum at D and
Fig. 81.
E. After reflection, these rays, traversing strata more and more
dense, will be refracted upward, and at C reach the eye of the ob-
server, who perceives the tree in the direction of the last refracted
rays. An image, A'B', will therefore be seen below the real ob-
ject, and it will appear inverted, because the rays have suffered
reflection. The effect is similar to that produced by the reflec-
tion of a tree from the surface of a tranquil lake, and the ob-
server is thus led to imagine himself to be surrounded entirely
by water.
Since the difference of refraction of the successive strata of air
204 METEOROLOGY.
is necessarily small, the ray AD must be very oblique ; that is,
the object must be elevated but little above the ground, and the
observer must be at a considerable distance.
402. Experimental Illustration. The mirage may be imitated
artificially by superposing in the same vessel two liquids of differ-
ent densities, such as water and alcohol, or water and sirup of
sugar, or simply cold and warm water. These liquids, by partial
mixture, produce a medium whose refractive power decreases
from the alcohol to the water, so that by looking through this
mixture at an object held behind the vessel, an inverted image of
it may be seen.
When a sandy plain is intensely heated by the sun, and the air
is very calm, if we place the eye near the ground we may gen-
erally see the inverted image of grass and other objects at a
distance.
403. Mirage at Sea. Mirage is produced at sea when the at-
mosphere is perfectly calm, and the air in contact with the water
is colder and consequently denser than the stratum of air imme-
diately above it ; this second stratum is denser than the one next
above it, and so on. In such a case, an inverted image of a dis-
tant object, as a ship, may be seen with a distinctness almost equal
to that of the object itself, and this image will be formed above
the object.
Let AB, Fig. 82, represent a ship near the horizon seen in
Fig. 82. its true position by
direct rays coming
to the eye at E,
through strata of
air of nearly uni-
form density. Sup-
pose the air consists
of strata of variable
density, the density
from below upward. The rays of light, AD, BC, which proceed
from the top and bottom of the ship, passing from a denser to a
rarer medium, will be deviated more and more from a vertical,
until at last they meet a stratum so obliquely that they are unable
OPTICAL METEOROLOGY. 205
to enter it, and are totally reflected from this stratum at D and C.
These rays, in passing from the rarer to the denser medium, are
now refracted downward, and meet the eye at E, which perceives
the ship in the direction of the last refracted rays, and the object
appears inverted because the rays have suffered reflection.
Other rays, that never could reach the eye at E in the ordinary
state of the atmosphere, may likewise be bent into curves which
do not cross before reaching the eye. In this case an erect image
of the ship may be seen, and both the direct and inverted images
may be seen simultaneously.
404. Lateral Mirage. In mountainous countries, or near a high
coast, it may happen that th,e air is divided by a nearly vertical
plane into two portions, one of which is heated by the sun, while
the -other is in the shadow of a hill or a bank. The transition
from the warm to the cold air will not be abrupt, but the density
of the vertical sections will increase gradually from the warmer to
rig. SB. the colder portion. If an ob-
server were situated at B, Fig.
83, near this bounding plane,
he might see in the warmer
part a symmetrical image, C'D',
^ of objects, CD, situated in the
colder part, as if in a vertical
mirror. This is called a later-
al mirage. It is less frequent-
ly seen than the other varieties, and its duration is more transient.
405. Displacements. Under certain circumstances, objects near
the horizon may 'appear displaced ; sometimes laterally, as in the
vicinity of mountains, but more frequently in a vertical direction,
in which case they appear elevated above their true position.
Sometimes an object appears double, certain rays reaching the
eye without sensible deviation, while others, traversing strata of
increasing density, describe a curve line. This phenomenon dif-
fers from the true mirage in this respect, that the image is not
inverted, showing that the light has not suffered reflection.
206 METEOROLOGY.
SECTION II.
ABSORPTION AND REFLECTION OF LIGHT BY THE ATMOSPHERE.
406. Absorption of Light. The atmosphere is never perfectly
transparent, but absorbs a portion of the light which traverses it.
Hence distant objects, as the summits of mountains, generally ap-
pear dim, as if enveloped in a mist or a bluish smoke. This loss
of light is due partly to the presence of minute particles of con-
densed vapor, and also small particles of dust, and partly to the
difference of density of the strata arising either from a difference
of pressure or a difference of temperature. In passing from one
stratum to another of a different density, a portion of light is re-
flected, so that the transmitted portion is continually diminished.
After a rain, when by a general mingling of the strata the tem-
perature of the air has been rendered nearly uniform, its trans-
parency is greatly increased.
407. Redness of the Evening Sky. The redness of the evening
sky is due principally to the condensed vapor of the air, a portion
of which begins to be precipitated as the temperature of the day
declines.
If we transmit the sun's light throug.h a glass prism at different
hours of the day, we shall find that the spectrum changes with
the altitude of the sun. As the sun approaches the horizon, the
violet part of the spectrum contracts, and at length disappears al-
together, while the red end of the spectrum remains entire. We
hence conclude that the violet rays, which are the most refrangi-
ble, have the least power of penetrating the dense atmosphere, in-
cluding the dust and the condensed vapor near the horizon, and
therefore, when the sun is near the horizon, his light exhibits an
excess of those rays bordering upon the red end of the spectrum,
and this color is communicated not only to the evening sky, but
also to the clouds which float in the atmosphere.
From the same cause, the sun, just before setting, sometimes as-
sumes a deep red color, as if seen through a smoked glass, and
this redness is more noticeable in the setting than in the rising
sun, because in the morning the condensed particles of vapor have
descended to the earth, or are converted again into invisible va-
por b} 7 the increasing heat of the morning.
OPTICAL METEOEOLOGY. 207
408. Reflected Light of the Sky. An observer at night, in the
neighborhood of a large city, may notice a decided illumination
of the heavens, arising from the light of the city reflected from
the sky, and during an extensive conflagration this illumination
is sometimes very brilliant. The atmosphere therefore reflects a
portion of the light which, falls upon it. It is this light of the
.sky which prevents our seeing the stars in the daytime, and its
brightness is but little inferior to that of the moon, for during the
day the moon appears like a small white cloud. It is chiefly
from this source that, during the day, apartments which are not
accessible to the direct rays of the sun derive their illumination.
The brightness of the sky is variable. It depends upon the
purity of the air, increasing ^with the number of the particles of
condensed vapor suspended in it. It depends also upon the
weight of the air above the observer, being less on the summits
of mountains than at the level of the sea.
The light of the sky is greatest in the vicinity of the sun, and
diminishes rapidly as we recede from his disc.
409. Blue Color of the SJcy. While the red rays of the sun have
a greater power of penetrating a dense atmosphere, the blue rays
are more readily reflected by it, but this difference is not sensible
until the light has traversed large masses of air. The azure color
of the sky is therefore due to the light reflected by the air, and
the purer the air the more decided is this azure tint. When
mountains covered with snow are illumined by a rising sun, they
appear of a rosy or orange tint upon the eastern side, while on
the western side they exhibit a bluish tint. The blue color of
the sky is therefore due to the reflection of light, and not to a
peculiar color belonging to the particles of air.
410. Cyanometer. The intensity of the blue color of the sky
exhibits very great variety. In order to measure it, Saussure in-
vented an instrument which he called the cyanometer. This
instrument has 27 colored surfaces, of which the first is almost
white, and the last is of the deepest cobalt blue, while the inter-
mediate surfaces present every gradation between white and blue,
and the surfaces are numbered from 1 to 27. It has also a sec-
ond series of colored surfaces, beginning with the deepest blue of
the preceding series, and ending with a jet black, while the inter-
208 METEOKOLOGY.
mediate surfaces present every gradation between blue and black,
and these surfaces are numbered from 27 to 53.
In using this instrument, the observer selects that particular
tint upon the scale which corresponds nearest to the color of the
sky, and the color of the sky is denoted by the number attached
to that tint. Other cyanometers have been invented depending
upon the properties of polarized light.
The blueness of the sky generally increases from the horizon
to the zenith. When the color of the sky near the zenith is
indicated by .20 on the cyanometer, it will generally be about 4
near the horizon.
The blueness of the sky is greatest after a rain, when the air is
most pure, and it diminishes with an increase of the particles of
condensed vapor suspended in the air. Hence a pale sky is a
sign of rain.
The blueness of the sky decreases as we recede from the equa-
tor. At Cumana,in latitude 10, the average blueness of the sky
is 24, while in Europe it is only 14. On a clear, bright day, the
average blueness of the sky at New Haven corresponds to about
18 on the cyanometer.
The blueness of the sky increases with the altitude, and at an
elevation of 16,000 feet the heavens become almost black. On
the top of Mount Blanc, Saussure found the color of the sky 39,
while at the foot of the mountain the color of the sky near the
zenith was represented by 18.
411. Twilight. If there were no atmosphere, night would com-
mence as soon as the sun descends below the horizon, and the
day would begin with equal abruptness. The astronomical limit
of twilight is generally understood to be the instant when stars
of the sixth magnitude begin to be visible in the zenith at even-
ing, or disappear in the morning. In our climate the evening
twilight generally terminates when the sun is 17 or 18 below
the horizon. The morning twilight commences at a somewhat
less depression, since the vapor of the atmosphere condensed dur-
ing the night does not rise to so great a height in the morning as
at evening. These limits, however, are variable, the duration of
twilight depending upon the state of the atmosphere. When the
sky is of a pale color, indicating the presence of an unusual amount
of condensed vapor, twilight is of longer duration. This happens
OPTICAL METEOROLOGY. 209
habitually in the polar regions. On the contrary, within the trop-
ics, where the air is pure and dry, twilight sometimes lasts only
fifteen minutes.
412. Twilight Curve. A little before sunset the western sky
grows yellow, and in the east we observe a purple tint arising
from the reflection of the sun's rays, which have traversed the
atmosphere horizontally, and which communicate their color to
whatever they illumine. After the sun has set, we perceive near
the eastern horizon a dark blue segment, above which we notice
the purple tint already mentioned. As the sun declines this seg-
ment .rises higher; it subsequently reaches the zenith, and finally
the western horizon, when the twilight entirely ceases. The out-
line of this segment sometimes appears very sharply defined, and
is called the twilight curve. This segment is a part of the coni-
cal shadow of the earth, which intercepts the sun's rays from a
portion of the atmosphere, and this portion reflects only that dif-
fuse light which comes from other parts of the sky.
413. Colors of the Morning TwilightWhen the sun is still 12
below the eastern horizon, the horizon generally appears border-
ed with a red or orange band, above which the twilight curve
rises 7. The orange zone gradually extends, becomes bordered
with yellow, and afterward with green, while the twilight curve
ascends toward the zenith. When the sun is only 2 below the
horizon, the eastern horizon becomes yellow, the green zone be-
comes more decided, and extends from 3 to 18, the twilight
curve extends to within 3 of the western horizon, and is border-
ed with a purple zone about 12 in breadth. As the sun rises,
the western horizon appears bordered with a rosy band, sur-
mounted by yellow. The red in the east disappears, and is suc-
ceeded by yellow, surmounted by green, which continues after
*he yellow has disappeared, when the sun is 2 or 4 above the
horizon.
The red and yellow zones are ascribed to the absorptiqn pro-
duced by the different thicknesses of air traversed by the rays of
light. The green color results from a combination of the yellow
rays with the blue rays of diffuse light reflected from the parti-
cles of the atmosphere, green being produced by a mixture of
yellow and blue.
210 METEOROLOGY.
414. Height of the Atmosphere deduced from Twilight. Attempts
have been made to compute the height of the atmosphere from
the position of the twilight curve at a given instant after sunset ;
but the results thus obtained are not uniform, being greatest when
the sun is lowest below the horizon. From such computations it
has been inferred that the height of the atmosphere can not ex-
ceed 36 miles; but this can only be regarded as the height of
that portion of the atmosphere which has a density sufficient to
reflect an appreciable amount of light. Other phenomena indi-
cate that an extremely rare atmosphere extends to a much great-
er height.
415. Prognostics derived from Twilight. Since the colors and
duration of twilight, especially at evening, depend upon the
amount of condensed vapor which the atmosphere contains, these
appearances should afford some indication of the weather which
may be expected to succeed. The following are some of the rules,
which are relied upon by seamen. When, after sunset, the west-
ern sky is of a whitish yellow, and this tint extends to a great
height, it is probable that it will rain during the night or the
next day. Gaudy or unusual hues, with hard, definitely outlined
clouds, foretell rain and probably wind. If the sun, before setting,
appears diffuse and of a brilliant white, it foretells a storm. If
it sets in a sky slightly purple, the atmosphere near the zenith
being of a bright blue, we may rely upon fine weather.
A red sky in the morning presages bad weather, or much wind
if not rain ; but if the sky presents simply a rosy or grayish tint,
we may expect fair weather.
SECTION III.
THE RAINBOW.
416. The rainbow consists of a series of circular bands colored
with the tints of the solar spectrum from red to violet, and is sit-
uated in that part of the sky which is opposite to the sun. It is
caused by the refraction and reflection of the sun's light from
drops of rain whose form is sensibly spherical.
It is proved in Natural Philosophy (Olmsted, p. 419) that, if
i represents the angle of incidence of a ray of light,
r " " " refraction " "
OPTICAL METEOKOLOGY. 211
D represents the angle of deviation of a ray of light,
n the index of refraction for water ;
then, for the maximum deviation after one reflection, we have
/n 2 l
cos. i= Y - ; sin. i=n sin. r; D=4r 2i.
If we assume the index of refraction for the red rays to be
1.3309, and for the violet rays 1.3442, we shall find
for the red rays, fc59 32' ; D=42 24' ;
" violet rays, ^58' 46'; D=40 28'.
For the minimum deviation after two reflections we have
cos. i= \ r 1 ~ ; sin. i=n sin. r ; D=ir+2i 6r.
V 8
Whence, by computation, we find
for the red rays, i=71 55' ; D=50 20' ;
" violet rays, i=71 29' ; D=53 46'.
The exterior radius of the primary bow should therefore be
42 24', increased by half the diameter of the sun ; and its breadth
should be 1 56', increased by the apparent diameter of the sun,
which is about 30', making 2 26'. The mean of numerous care-
ful measurements gives 41 33' for the radius of the middle part
of the primary bow.
The interior radius of the secondary bow should be 50 20', di-
minished by half the diameter of the sun, and its breadth should
be 3 26' + 30', or 3 56'.
417. Necessary Conditions of Visibility. If the altitude of the
sun be greater than the radius of the bow, then no rainbow can
be seen. For this reason, during more than six months of the
year at New Haven, the primary bow can never be seen at noon,
and near the summer solstice the primary bow can not be seen
for more than six hours near the middle of the day.
If the observer be sufficiently elevated above the earth, as in a
balloon, he may see the rainbow as a complete circle, but on the
surface of the earth we only see a semicircle when the sun is in
the horizon.
Lunar rainbows are occasionally seen, but the colors are faint,
and generally only a white or yellowish arc is distinguishable.
418. Supernumerary Bows. The Newtonian theory of the rain-
212 METEOROLOGY.
bow is incomplete, inasmuch as it only considers those rays which
experience the maximum or minimum deviation, and entirely
neglects those rays which pass a little beyond these limits. The
effect of these other rays is to extend the breadth of the primary
bow upon the inside, and also to produce secondary bands which
the Newtonian theory does not explain. When the rainbow is
brilliant, we often perceive faint bands alternately red and green
within the violet of the primary bow, or perhaps superposed upon
the violet, which then assumes a purplish tint. Near the violet
bow we frequently see an arch of rose-red, succeeded by one of
yellowish-green ; then perhaps a second arch of rose-red and a
second of yellowish -green. Two supernumerary bows are not
very uncommon ; three have repeatedly been seen, and occasion-
ally even four.
These supernumerary bows are due to the interference of rays
which traverse a drop in a direction differing but little from that
of maximum deviation. To every angle of deviation a little less
than the maximum, there correspond two rays, one whose angle
of incidence is a little greater, and the other whose angle is a lit-
tle less than that which gives the maximum deviation. These
rays, having pursued routes slightly unequal, interfere and pro-
duce alternations of light and darkness, or alternately bright and
dark bands. The bands resulting from these interferences for
each of the colors of the spectrum, being superposed upon the
skj, produce bands analogous to the colored rings of thin plates.
419. Theory explained from a Diagram. A ray of light, SA,
once reflected from the inner surface of a drop of rain at B, ex-
periences its greatest deviation, viz.,
41, when the angle of incidence, FE A,
is 59. Suppose a ray of light, S'A',
falls upon the drop at an angle great-
er than 59, it will experience a devia-
tion less than 41. So also a ray, S"
A", which falls upon the drop at an
angle less than 59, will experience a
deviation less than 41. That is, there
are always two rays which experience
an equal deviation (for example, 40),
and therefore emerge parallel, one of them making with the
OPTICAL JVtETEOKOLOGY. 213
drop an angle greater than 59, and the other a less angle. The
paths of these two rays within the drop are slightly unequal,
and there are two rays the difference of whose paths within the
drop is equal to half the breadth of a wave of light. These
waves, being in opposite phases, will interfere with each other,
and produce darkness. There are two other rays the difference
of whose paths within the drop is equal td*the breadth of a wave
of light. These waves, being in the same phase, will conspire to
produce a double illumination. There are two other rays the
difference of whose paths is equal to one and a half undulations,
and which consequently interfere with each other.
Thus we have rays the difference of whose paths is equal to 1,
2, 3, 4, etc., undulations, and which therefore conspire ; and there
are other rays the difference of whose paths is equal to -J-, 1-^, 2-J-,
3-g-, etc., undulations, and which therefore interfere.
420. Consequence of these Interferences. If, then, the sun fur-
nished red light only, we should see opposite to the sun, when
drops of rain are falling, circular arcs, alternately red and black.
If the sun's light were entirely violet, we should see circular arcs
alternately violet and black, but the diameter of the violet arcs
would be less than that of the red arcs. The other colors of the
spectrum would produce arcs of intermediate dimensions. Now,
since the sun's light contains all the colors of the spectrum, all
these colored arcs are in fact formed simultaneously and super-
posed, and, being of unequal diameters, the colors are partially
blended. But near the usual primary bow two or three of these
narrow bands of prismatic colors are often sufficiently distinct to
be visible. In consequence of this reflection of light from the
drops of rain, it results that the sky within the primary bow is
brighter than that without it.
421. Size of the Drops of Rain. The smaller the drops of rain,
the broader will be these colored bands. In order that the su-
pernumerary bows may be formed beyond the first violet bow,
the drops. must be extremely minute. It is found by computa-
tion thaft if the drops be W inch in diameter, a second red band
will be formed 2 from the outer red of the primary bow, and it is
near this point that the first supernumerary bow is usually seen.
If we consider the interval between the first and second maxi-
214 METEOROLOGY.
mum unity, the breadths of the succeeding intervals for the same
color will be expressed by the numbers,
second interval, 0.587 ; fourth interval, 0.440 ;
third interval, 0.493 ; fifth interval, 0.404.
Supernumerary bows are sometimes seen on the outside of the
secondary rainbow, and they are to be explained in a similar
manner.
422. Fog-low explained. If the drops be less than -^ inch in
diameter, the primary bow will be wider than two degrees, the
breadth of the bow depending simply upon the size of the drops.
But as the breadth of the bow increases, the colors are spread
over a greater surface, and consequently they are less vivid and
distinct. When the diameter of the drops is -r^h inch, which is
the average diameter of particles of fog, the bow becomes a very
faint arch 4 or 5 in breadth, with only a slightly rosy tint upon
the outside. Such is the bow actually observed when the sun
shines upon a dense fog.
The undulatory theory of light, therefore, explains not only
the supernumerary bows, but the variable breadth of the prima-
ry bow.
SECTION IV.
CORONA.
423. The sun and moon, when partially covered by light, flee-
cy clouds, are often seen encircled by one or more colored rings,
which are called coronas. This phenomenon is most frequently
noticed about the moon, since we are too much dazzled by the
light of the sun to distinguish faint colors surrounding his disc.
In order to examine coronas about the sun, it is best to view them
by reflection from a blackened mirror, by which means the bril-
liancy of the sun's light is very much reduced.
424. Order of the Colors. When a corona is complete, we may
observe several concentric colored circles. The one next to the
sun is blue, the second is nearly white, and the third* is red.
These form the first series of rings. In the second series the or-
der of the colors is purple, blue, green, pale yellow, and red. In
the third series the colors are pale blue and pale red. These
rings are partially represented in Fig 88.
OPTICAL METEOROLOGY. 215
The diameter of these rings is not always the same. The di-
ameter of the first red ring varies from 3 to 6, and that of the
second red ring from 5 to 10.
425. Cause of Coronce. Coronas are produced by the diffraction
of the rays of light in their passage through the small intervals
between the particles of condensed vapor in a cloud. If we look
at the moon through a very small aperture (as a pinhole in a
plate of sheet-lead) we shall see the hole surrounded by colored
rings, whose tints are the same as those observed in coronae. The
light of the moon, passing through the small interstices between
the particles of a cloud, is diffracted in a similar manner. The
particles of a cloud must not be too numerous, otherwise no rays
can pass between them ; and the smaller the intervals between the
particles, the greater will be the diameter of the rings.
426. Coronas produced artificially. If we sprinkle upon a pane
of glass a little lycopodium, or any very fine dust of nearly uni-
form fineness, and look at the moon through this glass, we shall
see it surrounded by rings of the prismatic colors, precisely like
those formed by a cloud.
If, on a cold winter evening, we breathe upon a pane of glass,
the breath will condense in small globules and freeze ; and if we
look at the moon, or even at a street lamp, through this glass, we
shall see a similar system of colored rings, having violet on the
inside.
427. Glow surrounding the Shadow of an Observer. When the
sun is near the horizon, and the shadow of the observer falls on
grass covered with. dew, one may often observe a vivid glow sur-
rounding, the shadow of his head. If the shadow falls upon a
cloud or a fog, the head will appear surrounded by a luminous
glory, exhibiting the prismatic colors. The order of the colors is
the same as in coronae, and sometimes four and even five series
of rings have been observed.
The light of the sun is reflected to the eye most powerfully by
the particles of fog near the head ; for the light reflected both
from the anterior and posterior face of such particles will reach
the eye. This explains the glow of light surrounding the shad-
ow of the observer. The color is produced by the diffraction of
the light thus reflected, precisely as in the case of a corona.
216 METEOROLOGY.
SECTION V.
HALOS AND PARHELIA.
428. Halos are circles of prismatic colors formed around the
sun or moon. They are of larger size than coronse, and present
a greater variety of appearances. The following is an enumera-
tion of those which are most frequently seen.
Halo of 22 Radius. When the sky is hazy, and presents a
dull-, milky appearance, we frequently notice around the sun or
moon a colored circle, A, Fig. 85, having a radius of 22, the sun
Fig. 85.
occupying the centre of the circle. The inner edge of the circle
is colored red, and is tolerably well defined ; the outer edge is of
a pale blue, and is not sharply defined. Such a circle is never
seen when the sky is perfectly clear. The sky within the halo
is much darker than it is for a distance of several degrees without
the halo.
The light of this halo is always polarized in the direction of a
tangent to the circumference, which proves that its light has suf-
fered refraction and not reflection.
429. Theory of this Halo. This halo is formed by the refraction
of the light of the sun or moon through crystals of ice floating in
the atmosphere. Snow. consists of crystals of ice. The simplest
form of an ice-crystal is a right prism, whose section is a regular
hexagon, and terminated by two bases perpendicular to the edges
of the prism. The alternate faces of such a prism are inclined to
OPTICAL METEOROLOGY. 217
Fig. 86. each other at angles of 60, so that we
G may consider the hexagonal prism ABC
DEF as a triangular one, GHK, with an-
gles of 60.
When a ray of light passes through a
prism, it is deviated toward the base of
the prism ; and there is a certain position
4C of the prism in which the deviation is the
least possible. This deviation for a prism
of ice may be computed in the following manner :
Let i represent the angle of incidence
of a ray of light ; r the angle of refrac-
tion ; m the index of refraction ; and A
the refracting angle of the prism. Then
sin. i=m sin. r.
But when the deviation is a minimum,
r=30; and for red light the value of m is 1.307. Hence z=40
48-5-'. And the deviation of a ray of light is
2i A, which equals 21 37'.
The minimum deviation for the violet rays, for which the val-
ue of m is 1.317, is found, in like manner, to be 22 22'.
430. How a Circle of Light is formed. If we conceive a beam
of light to be admitted through a small aperture into a dark room,
and to fall upon a large number of ice prisms having angles of
60, and occupying every possible position, all the incident rays
will be deviated from their first direction, but in no case will the
deviation be less than about 22. A large number of spectra
will be cast upon the opposite wall, but opposite to the aperture
through which the light is admitted there will be a circle of 22
radius upon which no spectrum can fall, and the red end of each
spectrum will be turned toward the centre of the circle. If the
number of the spectra be sufficiently great, they will together
form a circle of 22 radius, bordered with red upon the inside ;
but beyond the red the different colors of the spectrum will be
so superposed as to produce a light nearly white.
Whenever halos are formed about the sun, the air is filled with
fine prismatic crystals of ice, and these crystals occupy every pos-
sible position with respect to the sun's light. The halo of 22
radius is formed by the light of the sun shining through these
218 METEOROLOGY.
crystals of ice. If the sun's light furnished only red rays, we
should have an illuminated surface with a circular opening of
21-|- radius, of which the inner edge would be quite light. If
the sun furnished only violet rays, we should have a similar vio-
let surface, with a circular opening of 22-| radius, and the inter-
mediate colors would furnish circles of intermediate dimensions.
Now, since the sun's rays contain all the colors of the spectrum,
these different circles are formed simultaneously and superposed.
The red projecting on the inside is unmingled with any other
color, and is therefore pure ; all the other colors are more or less
mingled, but in unequal proportions, so that the outer portion of
the halo is nearly white.
Such a halo may be formed in midsummer, because at a mod-
erate elevation above the earth's surface the condensed vapor of
the air is frozen even in the hottest weather. The circle within
the halo is much darker than the space without it, because from
no part of this circle can a ray of the sun refracted by ice prisms
reach the eye of the observer.
The mean of eighty-three measurements of the radius of the
red circle of this halo is 21 36', which is almost identical with
the radius computed from theory.
431. Halo 0/4:6 Radius. Sometimes we notice around the sun
a second colored circle, H, Fig. 85, having a radius of 46. The
inner edge of this circle is also red, and tolerably well defined,
while the outer edge is of a pale blue color, and is poorly defined.
This halo is formed by the refraction of the sun's rays through
ice prisms having an angle of 90, this being the angle which each
side of the hexagonal prism forms with its base. The minimum
deviation of a ray of red light through a prism of ice having such
a refracting angle is found by computation to be 45 6', and for a
ray of blue light 46 50'. The average of the best observations
give 45 46' as the radius of the brightest part of this halo, a coin-
cidence as exact as can be expected in observations of this nature.
432. ffalos produced artificially. The production of halos may
be experimentally illustrated by crystallizing some salt like alum
upon a glass plate, and then looking through the plate at the sun
or a candle. A few drops of a saturated solution of alum spread
over a plate of glass will soon cover it with a layer of minute
OPTICAL METEOROLOGY.
219
crystals. If we place the eye close behind the smooth side of the
glass, and look at a candle, we shall see the candle surrounded
by three halos of different dimensions. Each crystal of alum is a
regular octaedron, with the six angles truncated, forming the out-
line of a cube. It has therefore faces inclined to each other at
angles of 70, 90, and 110, and these angles occupy every possi-
ble position with respect to the glass plate. The smallest halo is
formed by the refraction of the rays of light through a pair of
faces inclined to each other at an angle of 70 ; the second halo
is formed by a pair of faces inclined to each other 90 ; and the
third halo by faces inclined at an angle of 110.
433. Halo o/90 Radius. A. third halo of about 90 radius, H',
Fig. 88, is occasionally seen surrounding the sun. Unlike the
other two halos, this halo shows scarcely any traces of the pris-
jnatic colors. Only three observations of this halo are on record,
220 METEOROLOGY.
and its exact dimensions have not been well determined. In
two of the observations the radius was estimated at 90, and in
the third it was estimated from 85 to 90.
This halo has been ascribed to rays which, after entering one
of the sides, AB, of a triangular ice prism,
meet the face, BC, at such an angle that
they are totally reflected, and emerge
through the face AC. The angle of total
reflection, r, is determined by the equation
1
sin. r= .
F m
For violet rays in ice, m=1.317; whence r=49 24', or BFE
=40 36'. Hence FEL=10 36'.
Also KED=m sin.FEL=14 1'.
The inclination of DE to GH is equal to 120 2. KED=91
58'.
Such a reflection from an indefinite number of ice prisms would
therefore furnish an illumined surface with a circular opening of
about 92 radius, and having a tinge of violet on the side next to
the sun. The radius above computed is somewhat greater than
that indicated by the observations, and there are other objections
to this explanation, so that this hypothesis is quite doubtful ; but
no satisfactory explanation of this halo has hitherto been pro-
posed, and the observations are not sufficiently precise to enable
us to choose between conflicting hypotheses.
434. Parhelic Circle. When a halo is formed around the sun
we often notice a white circle passing through the sun and par-
allel to the horizon. See Fig. 88. This is called a parhelic cir-
cle, and is produced by the reflection of the sun's light from ice
prisms or snow crystals whose surfaces have a vertical position.
When the air is tranquil, the flakes of snow which are present in
the atmosphere descend slowly to the earth, and they tend to as-
sume that position in which they experience the least resistance
from the air. For most forms of snow-flakes, this position will
be when the principal faces of the crystal are perpendicular to
the horizon, and the light of the sun may reach the eye reflected
from such snow-flakes as are situated on a horizontal circle pass-
ing through the sun. This circle never exhibits prismatic colors
like the first-mentioned halos.
OPTICAL METEOROLOGY. 221
435. Parhelia. Near those points where halos cut the parhelic
circle there is a double cause of light, and here the illumination
is sometimes so great as to present the appearance of a mock sun,
and is called a parhelion. Parhelia are generally red on the side
which is toward the sun, and they sometimes have a prolongation
in the form of a tail several degrees in length, and whose direc-
tion coincides with that of the horizontal circle.
Parhelia of 22. The number of parhelia is very variable.
One is commonly seen near each of the points where the parhelic
circle cuts the halo of 22 radius,^p, Fig. 88, but the distance of
this parhelion from the sun increases with the elevation of the
sun above the horizon. When the atmosphere is calm, the prisms
,of ice which are present in the air, and are slowly descending to
the earth, will tend to assume a vertical position ; and if the sun
be near the horizon, the brightness of this halo will be greatest at
each extremity of a horizontal diameter. As the sun rises above
the horizon, the rays of light traverse these vertical prisms in a
direction oblique to the axis, and the minimum deviation of a
ray is increased, and the parhelion recedes from the circumfer-
ence of the halo. For an elevation of 20 this deviation amounts
to a degree and a quarter ; at an elevation of 40, it amounts to
more than five degrees ; and at an elevation of about 50, this
parhelion entirely disappears on account of the oblique angle at
which the rays meet the ice prisms.
Parhelia 0/4:6. A parhelion is sometimes seen at each of the
points PP, Fig. 88, where the parhelic circle cuts the halo of 46
radius. These parhelia have never been seen to depart much from
the circumference of the halo ; but since the breadth of the halo
is 1|- , and that of the parhelion is still greater, it is not certain
that the coincidence is exact. These parhelia can not be ascribed
to ice prisms with angles of 90, the edges of these angles being
vertical, for such a position of the base of an hexagonal prism
would be unstable. Moreover, upon such an hypothesis, as the
sun rises above the horizon, the parhelion ought to recede rapidly
from the halo of 46, which is contrary to observation.
These parhelia are probably produced by rays which have ex-
perienced the minimum deviation in the same direction in two
vertical hexagonal prisms, in which case the total deviation of
the rays would be double of that produced by a single prism.
Upon this hypothesis the parhelia should not exactly coincide
222 METEOROLOGY.
with the halo of 46, but for elevations not exceeding 30 the dif-
ference might easily escape observation. The observations are
not sufficiently precise to decide whether this explanation is ad-
missible or not.
Parhelia o/"120. Two other parhelia are sometimes seen on
the parhelic circle, about 120 distant from the sun. These may
Fig. 90. be caused by two reflections of the rays of the
4ts( sun from the vertical faces of snow crystals,
whose form is such as is represented by Fig.
90. The ray GH, after two reflections at H
and K, takes the direction KL, experiencing
a total deviation of 120. The image formed
by this reflection is white, and its size about equal to that of the
sun's disc.
Parhelia have also been observed at distances of 50 and 98
from the sun, which may result from the reflection of the sun's
rays from the faces of snow crystals of more complicated forms.
Sometimes a parhelion is seen on the parhelic circle at A, Fig.
88, directly opposite to the sun. This is more properly called an
anihelion.
Phenomena similar to parhelia are produced by the light of the
moon, in which case these bright spots are called paraselenes.
436. Contact Arches. Arcs of colored circles with variable
curvatures are sometimes seen touching the halos of 22 and 46
at their highest and lowest points, a > 6, Fig. 88. These are due to
the refraction of the sun's light through ice prisms, some of them
having their axes perpendicular to the sun's rays, and others in-
clined at various angles, but all in a horizontal position. The
sun's light, refracted by such prisms as have their axes not only
horizontal, but perpendicular to the solar rays, will produce a
bright image directly over or under the sun. But the sun's light,
passing through prisms whose* axes are inclined to the solar rays,
will experience a greater deviation, and also a deflection from a
vertical plane. Thus, if we look at a long straight bar through
a prism whose axis is parallel to the bar, the straight bar appears
curved, the deviation being greatest in the case of those rays
which are oblique to the axis of the prism.
OPTICAL METEOKOLOGY. 223
437. Variable form of Contact Arches. The form of these con-
tact arches depends upon the height of the sun above the hori-
zon. When the sun is near the horizon, we sometimes see two
brushes of light, like horns, rising from that point in the halo of
22 which is directly over the sun. As the sun rises higher,
these two horns diverge from each other, and when the sun has
an altitude of 12, they approach in form to an arc of a circle,
with its convexity toward the sun. When the sun reaches an
altitude of 30, these arcs become concave toward the sun ; they
bend downward, and partially envelop the halo.
When the sun has an altitude of 25, a contact arch is some-
times seen at the point of the halo directly beneath the sun. At
first it appears like an arc of a circle, with its convexity turned
toward the sun. As the sun rises higher, the curvature of this
arc diminishes, and at an altitude of 32 the arc becomes concave
toward the sun. At the height of 45 the curvature of the lower
contact arch is nearly the same as that of the upper arch, and
Fig. 91. both together form an elliptical figure, sur-
rounding the halo of 22, as shown in Fig. 91.
When this ellipse is greatest, the length of
its horizontal axis is about 64. As the sun
rises still higher, the major axis of this el-
lipse contracts, and when the sun's altitude
is 60, the horizontal axis of the ellipse is re-
duced to 50. At the altitude of 70, the ellipse differs so little
from the halo itself as to be scarcely distinguishable from it.
All these arcs are due to the sun's light, refracted by ice prisms
having their axes horizontal, as may be verified experimentally
by passing the sun's light through a triangular water prism held
in the proper position with respect to the sun's light.
438. Arcs touching the Halo c/46 . When the sun has an alti-
tude of 12, a brilliant arch in the form of an inverted rainbow is
sometimes seen to touch the halo of 46 at its highest point, 6,
Fig. 88. As the sun rises higher in the heavens this arc be-
comes more curved, and it disappears when the sun attains an
altitude of 31.
When the sun has an altitude of 60, a colored arch is some-
times seen touching the halo of 46 at its lowest point ; but its
light is faint, and it is concave toward the sun, so that this arc is
224 METEOROLOGY.
easily confounded with the halo itself. As the sun rises higher
in the heavens, this arch approaches still nearer to coincidence
with the halo, and it disappears entirely when the altitude of the
sun is 78.
These arcs are formed by the refraction of the sun's light
through ice prisms having angles of 90, the edges of these an-
gles being situated in a horizontal plane ; and the angles will have
this position when the axis of the hexagonal prism is vertical.
A ray of the sun can not pass through so large a refracting an-
gle except when the sun has a particular altitude above the hori-
zon. It is for this reason that the upper contact arch is never
seen except when the sun's altitude is between 12 and 31 ; and
the lower contact arch is never seen except when .the sun's alti-
tude is the complement of the preceding, viz., from 59 to 78.
439. Intersecting Arcs opposite to the Sun. Sometimes we notice
two arcs of circles nearly white, A, Fig. 88, intersecting the par-
helic circle at a point directly opposite to the sun, and inclined to
this circle at angles of about 60.
They are probably due to reflection from surfaces oblique to the
horizon ; perhaps from the slender spicula3 of snow-flakes having
Fig. 92. Fig. 93. the form and position shown in Fig.
92, or from hexagonal snow- plates
whose bases are covered with striae
arising from the superposition of oth-
er hexagonal plates in the manner
shown in Fig. 93. Flakes of snow
having such a figure have been repeatedly observed.
440. Vertical Columns passing through the Sun. Sometimes,
near sunset, we notice a luminous column, perpendicular to the
horizon, rising from the sun to a height of 10 or 15, and occa-
sionally still higher. This column is due to the reflection of the
sun's light from the under faces of ice crystals which are nearly
parallel to the horizon. Sometimes a little before sunset a sim-
ilar column of light is seen to shoot down from the sun toward
the horizon. This is formed in a similar manner by rays of the
sun reflected from the upper faces of crystals in a nearly hori-
zontal position. Sometimes columns are seen simultaneously
both above and below the sun; and if the halo -of 22 is seen at
SHOOTING-STAKS, METEOKS, AND AEROLITES. 225
the same time, this column, together with the
parhelic circle, presents the appearance of a
rectangular cross within the halo, Fig. 94.
These luminous columns are probably formed
only when the air is very tranquil, and the
reflecting surfaces may be the rectangular
r terminations of spiculse of ice which are slow-
ly falling to the earth, with their axes nearly
in a vertical position.
When we remember the immense variety in the forms of snow-
flakes, a few of which are represented in Fig. 52, we should antici-
pate a very great variety in the figures which might be produced
from the refraction or reflection by them of the sun's light. In
addition to the figures which have been described in this section,
many others have occasionally been seen, but the descriptions
which have been furnished of them are not, in general, sufficiently
precise to enable us to decide respecting their proper explanation.
CHAPTER IX.
SHOOTING-STARS, DETONATING METEORS, AND AEROLITES.
SECTION I.
SHOOTING-STAKS.
441. Shooting-stars described. The term shooting-star, or fall-
ing-star, is employed to designate that luminous stream which at
night is frequently seen to shoot rapidly across the sky, and pres-
ently vanishes, appearing as a star which is shot away from its
place in the firmament to a distant region of the heavens. Shoot-
ing-stars may be seen on every clear night, and at times follow
each other so rapidly that it is quite impossible to count them.
442. Number seen at different Hours. Shooting-stars are not
seen with equal frequency at all hours of the night. They gen-
erally increase in frequency from the evening twilight through-
out the night until the morning twilight ; and when the light of
day does not interfere, they are generally most numerous about
6 A.M. The following table shows the average number of shoot-
P
226
METEOROLOGY.
ing-stars which may be seen by a single observer at each hour of
a clear night, in the absence of the moon :
-From
6
to
7 P.M.,
3.8
From
12
to
1
AM.,
7.2
"
7
11
8 "
3.8
u
1
a
2
a
7.8
8
a
9 "
4.0
u
2
it
3
u
8.7
u
9
u
10 "
4.7
3
4
"
10.3
li
10
n
11 "
5.3
u
4
u
5
a
11.2
it
11
tt
midnight,
6.0
u
5
u
6
a
11.2
Observations show that the whole number of shooting -stars
visible at one place must be at least six times the number which
can be seen by one observer. Hence the average number of
meteors that traverse the atmosphere, and that are large enough
to be visible to the naked eye, if the sun, moon, and clouds would
permit, is 42 in an hour, or 1000 daily.
443. Number seen in the different Months. Shooting -stars, are
not seen with equal frequency at all seasons of the year. The
following table shows the average hourly number which may be
seen by a single observer near midnight, during each month, on
clear nights, in the absence of the moon:
January .
. 5.1
May .
February
. 5.0
June .
March
. 4.8
July .
April
. 4.6
August
4.0
4.9
September .
October . .
7.4
7.7
10.0
November .
7.4
12.8
December .
5.4
We thus see that many more shooting-stars appear from July
to December than during the other six months of the year ; and
they are ordinarily most abundant in the month of August.
444. Altitude of Shooting-stars. If two observers, at a suitable
.distance from each other, note the apparent altitude and azimuth
of a shooting-star at the commencement of its flight, and do the
same also for its termination, we have the data for computing the
absolute height of beginning and end above the surface of the
earth. The earliest observations of this kind were made in 1798
by Benzenberg and Brandes in Germany, and since that time sim-
ilar observations have been made in many parts of Europe, and
algjo in the United States. It is found that when the base-line
employed is only three or four miles in length, a shooting-star
SHOOTING-STARS, METEORS, AND AEROLITES. 227
is seen in nearly the same direction at both stations, showing that
its altitude is much greater than the length of that base. When
the base-line is 30 or 40 miles, the average change of position of
the star is about 15. The base-line should not be less than 40
or 50 miles in length, and one of 75 or 100 miles would not be too
great. Observers at distances of over 150 miles see for the most
part different shooting-stars.
The heights of over 500 meteor paths have been computed,
and it is thus found that shooting-stars begin to be visible at ele-
vations of from 40 to 120 miles, and perhaps sometimes 150 miles,
or an average height of 74 English statute miles. They dis-
appear at elevations of from 30 to 80 miles, and perhaps some-
times 100 miles or more, giving an average height at disappear-
ance of 52 English statute miles.
445. Length of Path and Velocity. The length of the visible
path of shooting-stars varies from 10 to 100 miles,_ though some-
times they are even 300 and 400 miles long ; the average length
being 28 miles. The time of describing the visible path varies
from less than one second to five seconds, and in some rare cases
amounts to ten seconds ; but their average duration is less than
one second. The average duration of meteors whose brightness
exceeds that of stars of the first magnitude is estimated at one
and a half seconds.
Their velocity relative to the earth's surface varies from 10 to
45 miles per second, and the average velocity of the brighter class
of shooting-stars amounts to about 30 miles per second.
446. Direction of their Motions. Shooting-stars are seen to
move in all directions through the heavens. Their apparent
paths are, however, generally inclined downward, though some-
times they move upward, and after midnight they come in the
greatest numbers from that quarter of the heavens toward which
the earth is moving in its annual course around the sun.
447. Magnitude of Shooting-stars. The magnitude of shooting-
stars is very variable. Some of them have been computed to
have a diameter of 100 or 200 feet, and others 1000 up to 5000
or 6000 feet. We must, however, regard this as the diameter of
the blaze of light which surrounds the meteor, while the meteor
228 METEOROLOGY.
itself, before it takes fire, may have a diameter of only a few feet,
or perhaps only a fraction of an inch. The apparent size of me-
teors is greatly magnified by irradiation.
448. Visible Train. Occasionally shooting-stars appear in great
splendor, flashing with a brightness nearly equal to that of the
full moon, and leaving behind them a train of dazzling light,
which lasts for several seconds, and even for whole minutes.
Their color is usually white, with a reddish tinge; but occasion-
ally they exhibit a green light, and sometimes a mixture of green
and blue, or purple. Even quite faint shooting-stars sometimes
leave trains.
The path of shooting-stars is frequently curved sometimes the
path consists of two portions inclined to each other at a consider-
able angle and at the end the meteor sometimes bursts like a
rocket into numerous fragments. In such cases the place of ex-
plosion is usually indicated by a smoky cloud, which sometimes
continues visible for ten minutes.
449. Are Shooting -stars accompanied by any Sound? Observers
frequently imagine that they hear a whizzing noise accompany-
ing the passage of a brilliant meteor. It is easily proved that
such impressions are an illusion. When we compute the path of
the meteor, from which the sound was supposed to proceed, we
always find that it was quite distant from the observer, 20, or 50,
and perhaps 100 miles. Now sound moves with a velocity of
1120 feet per second, or 50 miles in about four minutes. If, then,
any noise was caused by the motion of the meteor, the sound
could not possibly be heard until considerable time after the me-
teor disappeared, viz., 2, 5, or even 10 minutes, according to its
distance.
450. Cause of the Light of Shooting-stars. This light is proba-
bly due to the high temperature resulting from the resistance of
the atmosphere to the rapid motion of the meteor. Since, at the
ordinary elevation of shooting-stars, the air is exceedingly rare, it
might be supposed that the resistance would not develop sufficient
heat to give meteors their brilliant appearance. The researches
of philosophers have enabled us to compute the quantity of heat
that may be developed by the stoppage of a meteor in the atmos-
SHOOTING-STARS, METEORS, AND AEROLITES. 229
phere. A portion of the living force of the body is expended in
setting the air in motion, and a portion in heating the meteor and
the air. This living force and the consequent heat that may be
developed is proportioned to the mass of the body and to the
square of its velocity. The arresting the motion of a meteor
whose velocity is thirty miles per second, and whose specific heat
is 0.12, would, if the whole living force were changed into heat,
be sufficient to raise the temperature of the meteoric body more
than four million degrees of Fahrenheit's scale. If even the
larger part of this force was expended in giving motion to the
air, there would remain enough to furnish a brilliant light, and to
melt or disintegrate the meteor.
451. Daily number of Shooting-stars for the whole Globe. The
mean distance of shooting-stars from the observer is found to be
about 105 miles. The average height above the earth of the
middle points of their paths is 63 miles. Hence the mean hori-
zontal distance of the paths may be regarded as about 90 miles.
It is reasonable to suppose that the number of shooting-stars ac-
tually falling within a circle of 90 miles radius is at least equal
to the number seen at one place. In fact, careful computations
show that it is about one fourth greater. The area of this circle is
25,447 miles, while the entire surface of the globe is 196,662,000
square miles. The ratio of these numbers is 7728, whence we
may safely conclude that the number of shooting-stars over the
whole earth is more than eight thousand times the number visi-
ble at one place.
The average daily number of shooting-stars visible to the naked
eye at one place has been estimated at 1000, Art. 442. Hence
the average number of meteors that traverse the atmosphere
daily, and that are large enough to be visible to the naked eye,
if the sun, moon, and clouds would permit, must be more than
1000 X 8000, or more than eight millions.
452. Number of telescopic Shooting-stars. The observations of
Pape and Winnecke indicate that the number of meteors visible
through the comet-seeker employed by the latter is about 40 times
the number visible to the naked eye. A further increase of op-
tical power would doubtless reveal a still larger number of these
small bodies. Hence we must conclude that the source from
230 METEOROLOGY.
which these meteors come is of immense extent, otherwise it
would long since have been exhausted.
The mass of these bodies is, however, so small, and their dis-
tance from each other so great, that they exert no appreciable in-
fluence upon the motion of the planets. It is computed that the
average distance from each other of shooting-stars, such as under
favorable circumstances would be visible to the naked eye, is
about three hundred miles.
453. Meteoric Orbits. Having determined the velocity and di-
rection of a meteor's path with reference to the earth, and knowr
ing, also, the direction and velocity of the earth's motion about
the sun, we can compute the direction and velocity of the motion
with reference to the sun. This computation has been made for
several different meteors, and has shown that these bodies, before
they approached the earth, were revolving about the sun in el-
lipses of considerable eccentricity. In some instances the veloci-
ty has been found to be so great as to indicate that the path dif-
fered little from a parabola.
It is thus demonstrated that ordinary shooting-stars are small
meteoric bodies, moving through space in paths similar to the
comets, and it is probable that they do not differ materially from
the comets except in their dimensions, and perhaps, also, in their
density.
454. Periodic Meteors of November. "We have seen, Art. 443,
that the average number of shooting-stars for the different months
of the year is quite unequal, and occasionally the display of me-
teors is very extraordinary. The most remarkable exhibitions
of this kind during the last two centuries have occurred in No-
vember. On the morning of November 13, 1833, throughout
most of North America, shooting-stars appeared in such numbers
that it was found impossible to count them. At Boston it was
estimated that the meteors fell at the rate of 575 per minute.
Most of these meteors moved in paths which, if traced backward,
would meet in a single point, or small area, situated near y Le-
onis.
On the 13th of November, in 1832, shooting-stars appeared in
very unusual numbers, and there was a moderate display on the
same day of 1834, 1835, and 1836.
SHOOTING-STARS, METEORS, AND AEROLITES.
231
On the morning of November 12th, 1799, an extraordinary fall
of shooting-stars was witnessed in South America by Humboldt,
and it was also seen throughout a considerable part of North
America. The examination of old historical records has led to
the discovery of at least ten other similar appearances at about
the same season, of the year. These occurred in the years 902,
931, 934, 1002, 1101, 1202, 1366, 1533, 1602, and 1698.
455. Meteoric Shower of November 14$, 1866. These remarka-
ble displays having occurred at intervals of 33 or 34 years, or
some multiple of that period, led to a general expectation of a
brilliant shower in 1866. At New Haven, on the night of No-
vember 13th-14th, 881 meteors were counted in five hours, which
is six times the average number for November ; but a far more
brilliant display was witnessed in Europe. On the morning of
November 14th, at Greenwich, from midnight to 1 o'clock, there
were observed 2032 meteors ; from 1 to 2 o'clock, 4860 meteors ;
and from 2 to 3, 832 meteors, the maximum occurring about a
quarter past one, when the number amounted to 120 per minute.
The curve line, Fig. 95, shows the number of meteors observed
IO.P.M.
MIDNIGHT
Pig. 95.
UW. 2
80
40
20
each minute from 10 P.M., November 13th, to 5 A.M., November
14th, the number visible at each instant being indicated by the
numerals to 120 on the left of the diagram. Nearly all of these
meteors proceeded from a point in the constellation Leo; this
232 METEOROLOGY.
point being in latitude 10 N, and its longitude being about 90
less than that of the sun.
A similar display was noticed throughout Europe ; also in Asia
as far eastward as Calcutta, and in corresponding longitudes in
the southern hemisphere. Throughout all this region the max-
imum display occurred at about the same instant.of absolute time.
456. Meteoric Shower of November 14^, 1867. An equally re-
markable display of meteors occurred in the United States on the
morning of November 14th, 1867. Until 3 A.M. the number of
shooting-stars was not remarkable, but from that hour the num-
ber rapidly increased, and at New Haven attained its maximum
about 4-J- A.M., after which the number declined, and before six
o'clock had ceased to be specially noticeable. Near the time of
maximum the number visible to a single person was 43 per min-
ute, making about 240 per minute for the entire heavens, and this
in the presence of a full moon, which probably eclipsed two thirds
of those which would otherwise have been visible. These mete-
ors almost without exception moved in paths which, if produced
backward, would intersect, not all precisely in a single point, but
within a small area situated in Leo. This area was of an oval
form, having a diameter of about 5 in longitude and 1 in lati-
tude. Its centre was in longitude 143, and latitude 10 10' N.,
and most of the meteors appeared to diverge pretty accurately
from this centre. Many of them left trains which were distinct-
ly visible for several seconds, notwithstanding the light of the
moon.
457. Procession of the Node along the Ecliptic. The day of the
year upon which the great displays of the November meteors oc-
cur becomes gradually later and later. In 1866 and 1867 the
great display was November 14th ; in 1832 and 1833 it was No-
vember 13th ; in 1799 it was November 12th ; in 1698 it was No-
vember 9th ; and the earliest recorded corresponding displays oc-
curred in October. If we suppose that these meteors, before they
encounter the earth, form a ring, or a portion of a ring, about the
sun, then we must conclude that the node of this ring has a direct
motion along the ecliptic amounting to 102 seconds annually
with respect to a fixed equinox.
SHOOTING-STARS, METEORS, AND AEROLITES. 233
458. Period of the November Meteors. A comparison of the dates
mentioned in Art. 454 shows that the grand displays recur after a
cycle of about one third of a century, and that a grand display
may occur on two consecutive years. A number greater than
usual may be observed also for three or four consecutive years.
Hence we must conclude that these meteors belong to a system
of small bodies describing an elliptic orbit about the sun, and ex-
tending in the form of a stream along a considerable arc of that
orbit. It is evident that the meteors can not make more than
two complete revolutions in a year, for the major axis of an orbit
which should be completed in one third of a year would not reach
from the sun to the earth. Hence we conclude that in one year
the group of meteors must describe either 2=fc-^-j or li-sV, or -g- 1 ^
revolutions ; that is, the periodic time must be either 180, 185,
354 or 376 days, or 33J years.
The motion of the node of a group of meteors describing an
orbit about the sun in each of the preceding periods has been
computed, and it is found that the motion corresponding to either
of the first four mentioned periods would be entirety incompatible
with the motion actually observed ; but if the period be assumed
33J years, the computed motion of the node due to the action of
the planets agrees almost exactly with the observed motion.
This coincidence is regarded as demonstrating that the true pe-
riod of the November meteors is 33 J years.
458. Elements of the November Meteors. Assuming the period
as thus determined, and also the position of the radiant point
shown by the observations, it is possible to compute the elements
of the orbit. These elements are given in the first part of the fol-
lowing table :
November Meteors. Comet of 1866.
Period ...... 33.25 years. 33.18 years.
Semi-axis major . . . 10.3402 10.3248
Eccentricity .... 0.9047 0.9054
Perihelion distance . . 0.9855 0.9765
Inclination 16 46' 17 18'
Longitude of node . . 51 28' 51 26'
Longitude of perihelion . 58 19' 60 28'
Motion Ketrograde. Retrograde.
Figure 96, p. 234, shows the form and dimensions of this orbit.
234
METEOROLOGY.
Fijr. 96.
459. First Comet 0/1866. The elements of the first comet of
1866 bear a remarkable resemblance to those of the November
meteors. These elements are given in the last column of the pre-
ceding table. It is very improbable that so close a coincidence
should be accidental, and hence we seem authorized to conclude
that the comet of 1866 is a very large meteor belonging to the No-
vember stream.
460. Dimensions of the November Stream. The November
stream of meteors is several years in passing its node. The
length of the period during which extraordinary displays of me-
teors may occur is more than one year, and an unusual number
of shooting-stars, sufficient to attract attention, may be seen
through a period of at least 5 or 6 years. Hence we conclude
that the length of the denser portion of the group, when at peri-
helion, is at least one fourth of the circumference of the orbit,
SHOOTING-STARS, METEORS, AND AEROLITES. 235
or one thousand millions of miles ; while a large number of me-
teors extend still farther along the orbit.
Since the shower of 1833 lasted two or three hours, the thick-
ness of the ring at that point must have been the distance passed
over by the earth in that time, multiplied by the sine of the in-
clination of the orbit, or about 50,000 miles. The comet of 1866
passed the path of the earth at a distance of six hundred thou-
sand miles, which seems to imply that the breadth of the ring is
much greater than its thickness.
461. Conclusions. It thus appears to be pretty well established
that the meteors of November are derived from a cosmical cloud,
composed of very minute elements, each of which, before it en-
countered the earth, was moving in an elliptic path about the sun
with a period of 33J years. This cloud has the form of an ellip-
tic arc, the denser portion of which is at least 600 millions of
miles in extent when near perihelion, and the rarer portion ex-
tends very much farther along the ellipse, while its thickness,
where greatest, is over 50,000 miles. This cloud, although of
immense extent, has very small density. It is computed that
the mean distance of the individual elements of the group from
each other when near perihelion is 30 or 40 miles ; and although
some of the meteors may have considerable size, their weight is
doubtless very small. Hence the planets pass freely through
the densest portion of this cloud without any sensible loss of
motion.
462. Division of Bields Comet. Admitting that the November
meteors have a period of 33J years, we find by computation that
Biela's comet passed extremely near, and probably through the
meteoric stream near the close of December, 1845. It has been
conjectured that this collision may have produced the separation
of this comet into two parts a separation which was first no-
ticed December 29th.
It is probable, however, that the density of the stream of mete-
ors at this point was extremely small, so that this cause would
seem inadequate to account for the division of Biela's comet.
463. The Periodical Meteors of August. Another season at
which meteors appear each year in unusual numbers occurs about
236 METEOROLOGY.
the 10th of August. The periodicity of this display was estab-
lished in 1837, since which time an extraordinary number of me-
teors has been uniformly observed each year, both in Europe
and America, from the 6th to the 13th of the month, the greatest
number being generally seen on the morning of the 10th. At
the time of the maximum, the number of meteors visible is about
three times as great as for the average of the entire month, and
five times as great as for the average of the entire year.
The meteors of August, like those of November, seem also
to emanate chiefly from a fixed point in the heavens. This
point is in the constellation Perseus, being in R A. 44, and
Dec. 56 N.
464. Elements of the Orbit of the August Meteors. Assuming
that the meteors radiated from the point just stated; that the or-
bit is a parabola, and that the earth crossed the centre of the
group in 1866, Aug. 10.75, the elements given in the first part of
the following table have been computed:
August Meteors. Third Comet of 1862.
Longitude of perihelion . 343 28' 344 41' .
Longitude of node . . 138 16' 137 27'
Inclination 64 3' 66 26'
Perihelion distance . . 0.9643 0.9626
Period 121.5 years.
Motion Retrograde. Retrograde.
The elements of the third comet of 1862, given in the last col-
umn of the preceding table, bear a remarkable resemblance to
those of the August meteors. The difference is no greater than
can be accounted for by the want of precision in the data for com-
puting the paths of the meteors. Hence we conclude that the
great comet of 1862 was one of the August meteors, and probably
one of the largest of that group.
465. Dimensions of the August Stream. It is considered, then,
highly probable that the August meteors describe a very large
elliptic orbit about the sun, extending considerably beyond the
orbit of Neptune. It is probable that the meteors are spread
over the entire circumference of this orbit, but not in equal num-
bers. There are on record 63 remarkable displays of meteors
which are considered to belong to this group, the earliest having
SHOOTING-STARS, METEORS, AND AEROLITES. 237
occurred A.D. 811. A comparison of these dates affords some in-
dication of a maximum of brilliancy recurring at intervals of 108
years.
The earth, moving at the rate of 68,000 miles per hour, is at
least seven days in passing entirely through the ring, which indi-
cates that the thickness of the ring is more than eleven millions
of miles.
The density of this stream of meteors is quite small, the mean
distance of the individuals of the group from each other being
computed to be more than a hundred miles.
466. Origin of Meteoric Streams. Streams of meteors moving
about the sun in orbits of vast extent may be supposed to have
resulted from a nebulous mass, or cosmical cloud, acted upon by
the attraction of the sun. Let us suppose a cosmical cloud, con-
sisting of very small meteors, to be drawn from stellar space by
the attraction of the sun. The individual particles of the cloud
will move in elliptic orbits about the sun, but these ellipses will not
be exactly equal to each other. If the form of the cloud were at
first spherical, its shape would be gradually changed, and it would
ultimately be drawn out into a parabolic or elliptic arc, of which
the sun is the focus. If the orbit were an ellipse, the original
form of the cloud would never be regained. At each perihelion
passage the length of the stream would be increased, and after a
certain number of revolutions the cloud would become a con-
tinuous ring. The stream would be at first periodic, but finally
the flow would be constant. If the primitive form of the group
was not spherical, similar results would follow. The meteors of
November are supposed to belong to such a group, in which the
ring is only partially formed, while the August meteors repre-
sent a group- which has been transformed into a continuous ring.
Hence it is inferred that the November group is of compara-
tively recent formation.
467. Other Periods of Shooting-stars. Besides the months of
August and November, there are several other periods at which,
either annually or occasionally, shooting-stars have been observed
in unusual numbers. Of these, the best established periods are
shown in the following table, which also gives the radiant point
from which the meteors seem chiefly to emanate. These meteors
238 METEOROLOGY.
are generally found to have a pretty definite radiant point, like
the meteors of November and August.
Date of Display. Kadiant Point of the Meteors.
Jan. 2 . .A. K. 234 ; N. Dec. 51, near Cor. Borealis.
April 20 . " 277; " 35, " a Lyrse.
July 28-29 " 304; " 40, " 7 Cygni.
Oct. 24 . " 83; " 12, " a Orionis.
Dec. 8-13 " 105; " 30, " T Geminorum.
The meteors which are seen on ordinary nights, and which do
not show any marked uniformity of direction, have been called
sporadic. It is, however, not improbable that meteors which at
present are regarded as sporadic, may hereafter be proved to be
periodical. It seems probable that shooting-stars, before they
encounter the earth, form in the planetary spaces a multitude of
currents or continuous rings, differing greatly in size and density,
situated at various distances from the sun, and having all possible
inclinations to the ecliptic. The unequal number of shooting-
stars witnessed on different days of the year is the consequence
of the unequal distribution of these meteoric streams throughout
the planetary spaces.
SECTION II.
DETONATING METEORS.
468. Detonating Meteors defined. Ordinary shooting-stars are
not accompanied by any audible sound, although they are some-
times seen to break into pieces. Occasionally meteors of extra-
ordinary brilliancy are succeeded by a loud detonation, or explo-
sion, followed by a noise like that of musketry, or the discharge
of cannon. These have been called detonating meteors.
469. The New Jersey Meteor of November 15th, 1859. On the
morning of November 15th, 1859, about 9f o'clock, a remarkable
meteor appeared in the heavens over the southern part of New
Jersey. It was so brilliant that, although the sun was unclouded,
and had an elevation of about 20 above the horizon, the flash
attracted the attention of multitudes of persons as far north as
Albany and Boston, and as far south as Fredericksburg, Virginia.
Its apparent path was downward, inclined a few degrees to the
west, and it left behind it a cloud of a rounded form like a puff
SHOOTING-STARS, METEORS, AND AEROLITES. 239
of smoke. Soon after the flash there was heard a series of terrific
explosions, which were compared to the discharge of a thousand
cannon. These explosions were heard throughout Delaware and
most of New Jersey. From a comparison of numerous observa-
tions, it was computed that the height of this meteor, when first
seen, was over 60 miles, and when it exploded its height was 20
miles. The length of its visible path was more than 40 miles.
It described this path in two seconds, so that its velocity relative
to the earth was at least 20 miles per second. The column of
smoke resulting from the explosions was a thousand feet in di-
ameter, and several miles in length.
Comparing the motion of this meteor with that of the earth in
its orbit, we find that its velocity relative to the sun was about 28
miles per second, which is the velocity belonging to a parabolic
orbit. The lowest admissible estimate of its velocity would indi-
cate that this meteor was moving about the sun in a very eccen-
tric ellipse; the most probable velocity would indicate that its
path was either a parabola or an hyperbola.
470. The Tennessee Meteor of August 2d, I860. On the 2d of
August, 1860, about 10 P.M., a magnificent fire-ball was seen
throughout the whole region from Pittsburg to New Orleans, and
from Charleston to St. Louis, an area of nine hundred miles in di-
ameter. It was described as equal in size to the full moon, and
before its disappearance it broke into several fragments. A few
minutes after its disappearance, there was heard throughout sev-
eral counties of Kentucky and Tennessee a tremendous explosion
like the sound of distant cannon.
From a comparison of a large number of observations, it has
been computed that this meteor, when first seen, was about 82
miles above the earth's surface, and it exploded at an elevation
of 28 miles. The length of its visible path was about 240 miles,
and time of flight 8 seconds, showing a velocity relative to the
earth of 30 miles per second. It is hence computed that its ve-
locity relative to the sun was 24 miles per second.
471. Number , Velocity, etc. Examples of detonating meteors
similar to the preceding are of yearly occurrence, and if every
case was duly reported, they would probably be found to be of
daily and perhaps hourly occurrence. The number of detonating
240 METEOROLOGY.
meteors found recorded in scientific journals is over 800. Their
average height at the first instant of apparition is 92 miles, and at
the instant of vanishing is 32 miles. Their average velocity rel-
ative to the earth is estimated at 19 miles per second.
472. Multiple Nuclei, etc. Sometimes the head of a meteor ap-
pears divided, consisting of two or more brilliant bodies in the
form of elongated drops, each followed by a tail of fiery appear-
ance. In a few- cases as many as a dozen heads have been count-
ed, but generally these secondary heads follow the principal body
of light so closely that they give to the meteor an elongated ap-
pearance, which has been sometimes compared to a child's kite, a
pear, a fish, etc.
The track of the meteor is often marked by a permanent streak,
which sometimes continues visible for many minutes. This streak
gradually changes its shape and position, like a cloud moved by
the wind, sometimes assuming a serpentine form, sometimes bend-
ing up like a crescent or a horse-shoe, and drifting with a velocity
of more than 100 miles per hour.
473. Periodicity of Detonating Meteors.. An unusual number of
detonating meteors has been seen about the time of the grand
meteoric display of November 13th ; also about the time of the
grand display of August 10th; and also December 8th -13th.
Moreover, several detonating meteors have been recorded Janua-
ry 2d and April 20th. This coincidence in the times of unus-
ual display of detonating meteors and of ordinary shooting-stars,
taken in connection with the results obtained respecting their
paths and velocities, leads us to infer that both belong to the same
class of bodies, and that they do not probably differ much from
each other except in size and density. We conclude, then, that
detonating meteors are small bodies which revolve about the sun
in orbits which are generally ellipses of considerable eccentricity,
but perhaps sometimes parabolas or even hyperbolas. They are
bodies of considerable density, and the noise which succeeds their
appearance is probably in great part due to the collapse of the
air rushing into the vacuum which is left behind the advancing
meteor. No audible sound proceeds from ordinary shooting-
stars, because they are bodies of small size or of feeble density,
and are generally dissipated or consumed while yet at an eleva-
tion of 50 miles above the earth's surface.
SHOOTING-STABS, METEORS, AND AEROLITES. 241
SECTION III.
AEROLITES.
474. Aerolites described. There is no evidence that any deposit
from ordinary shooting-stars ever reaches the earth's surface, but
occasionally solid substances descend to the earth from beyond
the earth's atmosphere. The fragments generally penetrate a foot
or more into the earth, and if picked up soon after their fall are
found to be warm, and sometimes even hot. These small bodies
are called aerolites. They are called meteoric stones when they
present a stony appearance, or meteoric iron when they are almost
entirely metallic.
Although numerous instances of the fall of aerolites had been
recorded from the earliest historic times, and especially during
the last century, these accounts were received by many scientific
men with incredulity. But during the present century these
cases have been so numerous, and they have been witnessed by
BO many observers, that we can no longer doubt that stones have
fallen to the earth from beyond the earth's atmosphere.
475. The Weston, Connecticut, Aerolite. On the morning of De-
cember 14th, 1807, a meteor of great brilliancy was seen moving
through the atmosphere over the town of Weston, Connecticut.
Its apparent diameter was about one half that of the full moon ;
and soon after its disappearance there were heard by those near-
ly under the place of disappearance three loud explosions like
those of a cannon, followed by a quick succession of smaller re-
ports. Immediately after the explosions, one observer heard a
sound like that occasioned by the fall of a heavy body, and, upon
examination, found that a stone had fallen upon a rock near his
house, and was broken into small fragments. The fragments were
still warm, and together were estimated to weigh about twenty
pounds.
In another place, about five miles from the former, a fresh hole
was found in the turf, and at the bottom of the hole, at the depth
of two feet, was found a stone weighing thirty -five pounds. In
the neighborhood was found a third stone weighing about ten
pounds, a fourth weighing thirteen pounds, a fifth weighing twen-
ty pounds, and a sixth weighing thirty-six pounds. At a spot
Q
242 METEOKOLOGY.
about four miles distant from the preceding, a large mass of
stones, estimated to weigh 200 pounds, fell upon a rock, and was
broken into minute fragments. It was estimated that the entire
weight of all the fragments was at least 300 pounds.
The specimens from all these localities were quite similar, and
their specific gravity varied from 3.3 to 3.6. Their composition
was nearly one half silex, about one third oxyd of iron, and one
sixth magnesia, with a little nickel and sulphur.
The same meteor was extensively seen as far north as Ver-
mont, and as far south as New Jersey. The length of its visible
path exceeded 100 miles, and it moved from northwest to south-
east, its path being inclined downward about 30 to the horizon,
and when it exploded its elevation was only about eight miles.
The time of flight was probably between five and ten seconds.
Hence the velocity relative to the earth was about fifteen miles
per second.
476. The Guernsey, Ohio, Aerolite. On the first of May, 1860,
about half an hour after noon, an aerolite exploded over Guern-
sey County, Ohio. A great number of distinct detonations were
heard, like the firing of a cannon, after which the sounds became
blended together, and were compared to the roar of a railway
train. The elevation of this meteor above the earth's surface was
computed at forty-one miles, and its path was nearly horizontal.
The entire weight of all the fragments which descended from this
meteor was estimated at 700 pounds. Their specific gravity was
3.54, and their composition very similar to that of the Weston
meteor.
477. The Braunau, Bohemia, Aerolite. On the 14th of July,
1847, about four o'clock in the morning, at Braunau, in Bohemia,
there were heard two heavy explosions, which followed each oth-
er in quick succession. Two streams of fire were seen to descend
to the earth, and, upon examination, a fresh hole three feet deep
was found in the earth, and at the bottom of the hole a mass of
iron, which for six hours after the fall continued so hot that it
could not be held in the hand. This mass weighed forty -two
pounds, and is preserved in the cabinet at Vienna. Another
mass, weighing thirty pounds, fell upon a roof, and broke through
large pieces of timber.
SHOOTING-STARS, METEORS, AND AEROLITES. 243
^he specific gravity of this meteor was 7.71. Its composition
was ninety -two per cent, of iron, and five per cent, of nickel, with
a small quantity of cobalt, arsenic, etc.
478. The Orgueil, France, Aerolite. On the evening of May
14th, 1864, a very bright fire-ball was seen in France, throughout
the whole region from Paris to the Pyrenees. Loud detonations
were heard in the neighborhood of Montauban, and a large num-
ber of stones fell near the village of Orgueil. The passage of the
meteor was witnessed by a large number of intelligent observers.
It was first seen at an altitude greater than fifty-five miles ; it ex-
ploded at an altitude of about twenty miles ; and it was descend-
ing in a line inclined 20 or 25 to the horizon. The length of
its visible path was 112 miles; and the time of flight was esti-
mated at five or six seconds, indicating a velocity of not less than
fifteen or twenty miles per second. The stones were hot when
they were first picked up. Their specific gravity was 2.567.
479. Number of Aerolites. There are eighteen well-authenti-
cated cases in which aerolites have fallen in the United States
during the last sixty years, and their aggregate weight is 1250
pounds. The entire number of known aerolites, the date of whose
fall is well determined, is 261. There are also on record seventy-
four cases of aerolites in which the day and month are not given,
and sometimes even the year is uncertain. Besides these there
have been found eighty -six masses, which, from their peculiar
composition, are believed to be aerolites, although the date of
their fall is unknown. The weight of these masses varies from
a few pounds to several tons. The entke number of aerolites of
which we have any knowledge is therefore about 420.
The actual number of aerolites which have reached the earth
must have been far greater than this. Many must have fallen
upon the ocean, or upon uninhabited lands where they were un-
observed. During the past fifty years the fall of 115 aerolites
has been recorded. If we suppose aerolites to have fallen over
the entire globe at the same rate as has been observed over the
more populous portions of Europe and America, we should have
an average of over 300 annually. Now we can not suppose that
even in Europe more than half the whole number are actually
seen to fall; hence we conclude that more than 600 aerolites fall
244
METEOROLOGY.
annually on various parts of the earth's surface. If we suppose
their average weight to equal that of those which have fallen in
the United States, we should have for the entire globe eighteen
tons of aerolites annually. See Tables XXXY. and XXXYI.
480. Chemical Composition of Aerolites. Aerolites are composed
of the same elementary substances as occur in terrestrial minerals,
not a single new element having been found in their, analysis.
Of the sixty -three elements now admitted by chemists, the follow-
ing twenty or twenty-two have been found in aerolites.'
Me
1. Aluminium.
2. Calcium.
3. Chromium.
4. Cobalt.
tals.
9. Manganese.
10. Nickel.
11. Potassium.
12. Sodium.
Metalloids.
1. Carbon.
2. Oxygen.
3. Phosphorus.
4. Silicium.
5. Copper.
6. Iron.
13. Strontium.
14 Tin.
5. Sulphur.
6. Arsenic?
7. Lithium.
15. Titanium.
7. Chlorine?
8. Magnesium.
Aerolites differ greatly in the proportions of these ingredients.
Some of them contain ninety -six per cent, of iron, while others
contain less than one per cent. Some contain eighteen per cent,
of nickel, and others less than one per cent. On the contrary,
others consist mostly of silica, magnesia, lime, etc. It is common,
therefore, to divide aerolites into two groups, viz., meteoric iron
and meteoric stones.
The specific gravity of aerolites varies from 1.70 (that of March
15th, 1806, at Alais, France) to 7.8 (that of May 26th, 1751, at
Agram, in Austria).
481. Peculiarities of Aerolites. While aerolites contain no ele-
ments but such as are found in terrestrial minerals, their appear-
ance is quite peculiar, and the grouping of the elements, that is,
the compounds formed by them, are so peculiar as to enable us
by chemical analysis to distinguish an aerolite from any terres-
trial substance.
Iron ores are very abundant in nature, but iron in the metallic
state is exceedingly rare in nature. Now aerolites invariably
contain metallic iron, sometimes ninety to ninety -six per cent.
SHOOTING-STARS, METEORS, AND AEROLITES. 245
This iron is perfectly malleable, and may be readily worked into
cutting instruments. This meteoric* iron always contains a cer-
tain amount of nickel, generally eight or ten per cent, with small
quantities of cobalt, copper, tin, and chrome. This composition
has never been found in any terrestrial mineral. Moreover, when
the fragments of meteoric iron which are dispersed through those
aerolites which are mostly earthy are extracted and submitted to
analysis, they show the same composition, viz., about ninety of
iron, with eight or ten of nickel, etc.
Many of the other constituents of aerolites are similar to those
which are found in volcanic rocks, such as olivine (a silicate of
magnesia), magnetic pyrites, chrome-iron, etc.
All aerolites, without exception, contain a substance called
schreibersite, though often in very small quantities. This sub-
stance is a compound of iron, nickel, and phosphorus, and has
never been found except in aerolites. Fig. 97 represents an iron
meteor found near Lockport, New York, in 1818.
Fig. 97.
482. Widmannstaten Figures. Meteoric iron possesses a highly
crystalline structure. If the surface be carefully polished, and
the mass be heated to a straw-yellow, after cooling, the surface
will be covered with groups of regular triangles formed by lines
nearly parallel to each other, intersected by others at angles of
sixty degrees. These figures were first discovered by an Aus-
trian iron -master, Widmannstaten, in the year 1808, and they
have received the name of their discoverer.
It was afterward discovered that the same figures could be de-
veloped by the use of acids. For this purpose, nitric acid is di-
luted with an equal volume of water, and the iron, having been
previously cut and polished, is placed in the solution, the parts
not required to be acted upon being coated with asphaltum.
246
METEOROLOGY.
After five or six minutes the iron is taken out of the acid, care-
fully washed and dried. Figure 98 shows the crystalline struc-
ture of the meteoric iron of Elbogen, preserved in the cabinet of
Vienna.
Fig. 98.
Ordinary iron will not exhibit these Widmannstaten figures,
but iron melted directly out of some volcanic rocks does exhibit
them.
483. Periodicity of Aerolites. The falls of aerolites exhibit some
indications of periodicity, and these periods correspond with those
of ordinary shooting-stars. There are on record eleven cases in
which aerolites have been seen to fall near the time of the annual
display of the August meteors, Art. 463 ; that is, four per cent, of
all the recorded aerolite falls have occurred within three days of
the maximum display of August meteors. This number is more
than double that which we should expect if aerolites and shoot-
ing-stars had no connection with each other.
There are on record seven cases in which aerolites have fallen
between December 7th and 13th, which is also a period of unus-
ual display of shooting-stars, Art. 467 ; and there are also three
cases in which aerolites have fallen from November llth to 13th.
These numbers are greater than should be expected if shooting-
stars and aerolites were entirely independent of each other.
It is not probable that such a coincidence of dates is accidental,
SHOOTING-STARS, METEOES, AND AEROLITES. 247
and hence we are led to conclude that aerolites form portions of
the nebulous rings or groups from which shooting-stars are de-
rived.
483. Are Aerolites formed in our Atmosphere f -^Various hypoth-
eses have been proposed to account for the origin of aerolites. It
has been conjectured that they are formed in our atmosphere like
rain or hail. This supposition is inadmissible, because, allowing
the aerolite to be once formed, there is no known force which
could impel it in a direction nearly horizontal with a velocity of
several miles per second.
484. Have Aerolites been ejected from Terrestrial Volcanoes ? It
has been conjectured that aerolites are masses ejected from terres-
trial volcanoes. This supposition is inadmissible, because the
greatest velocity with -which stones have ever been ejected from
volcanoes is less than two miles per second, and the direction of
this motion must be nearly vertical, while aerolites frequently
move in a direction nearly horizontal, and with a velocity of sev-
eral miles per second. This argument is unanswerable, and there-
fore it is superfluous to add that the composition of aerolites is
different from that of any known terrestrial mineral.
485. Have Aerolites been ejected from Lunar Volcanoes? It has
been conjectured that aerolites have been ejected from volcanoes
in the moon with a velocity sufficient to carry them out of the
sphere of the moon's attraction into that of the earth's attraction.
It has been computed that a velocity of projection of 8000 feet
per second would be sufficient to produce such an effect.
The following are some of the objections to this hypothesis :
1. In order that a body projected from the moon may reach the
earth's surface, it must describe about the earth a conic section
whose distance at perigee is less than the earth's radius. Hence
there are limits to the direction in which the aerolite must have
left the moon, and also to the force with which it must have been
projected. If a body was projected from near the moon's centre,
or from its eastern hemisphere, since it would retain the moon's
orbital velocity, its resulting velocity would be such that its peri-
gee distance would exceed 4000 miles. If the body was project-
ed with a small force, it would not get beyond the sphere of the
248 METEOROLOGY.
moon's attraction ; and if the velocity was too great, the perigee
distance would exceed 4000 miles. It has been computed that a
change of y^ part in the force of projection would cause a change
of more than 4000 miles in the perigee distance, and for a given
force of projection a change of -^ part in the mass of the body
would produce a like effect.
Hence it has been estimated that if an indefinite number of
bodies, having different masses, were expelled from the moon in
all directipns and with different velocities, not one in a million
could reach the earth. But it is computed that 600 aerolites fall
to the earth annually, Art. 479. Hence the lunar hypothesis re-
quires us to conclude that more than 600 millions of aerolites are
annually expelled from the moon. But the lunar volcanoes are
to all appearance nearly, if not entirely, extinct ; and although the
moon has long been carefully watched with the most powerful
telescopes, in only one or two instances have astronomers sus-
pected that they had discovered any indications of change. We
can not, therefore, admit that lunar volcanoes have ejected rocks
in such quantities as to account for the known aerolites.
2. The observed velocities of some aerolites are incompatible
with the theory that they are satellites of the earth. In order
that a body may revolve around the earth, its velocity must not
be less than 5 miles, nor greater than 7 miles per second. If
the velocity were less than 5, the body would fall to the earth ;
and if the velocity was greater than 7, the body would recede
from the earth, never to return. Now the velocity of the Orgueil
meteor, Art. 478, certainly exceeded 7 miles per second, and there-
fore it was not a satellite to the earth. There are but few cases
in which the velocity of aerolites has been even rudely deter-
mined ; but detonating meteors seem to have the same origin as
aerolites,,, and the average velocity of detonating meteors is cer-
tainly greater than 7 miles per second.
3. Aerolites appear to be subject to a periodicity depending
upon the season of the year, which shows that they are satellites
of the sun and not of the earth.
Although, then, we can not pronounce it impossible that a
small body projected from a lunar volcano may occasionally have
fallen to the earth, it is certain that aerolites generally can not
have had this origin, and there is no reason to suppose that any
aerolite has ever been derived from this source.
SHOOTING-STAES, METEOES, AND AEEOLITES. 249
486. Conclusions. A comparison of all the facts which are
known respecting shooting-stars, detonating meteors, and aerolites
leads to the conclusion that they are all minute bodies revolving
like the comets in orbits about the sun, and are encountered by
the earth in its orbital motion. The visible path of aerolites is
somewhat nearer to the earth's surface than that of ordinary
shooting-stars, a result which may be ascribed to their greater
density. It is probable, also, that their velocity is somewhat
smaller, a result which may be due to their descending into an
atmosphere of greater density, which causes, therefore, greater re-
sistance.
These three classes of bodies exhibit alternate periods of maxi-
mum and minimum abundance, and the times of maximum for
the several classes correspond somewhat with each other, indicat-
ing that these bodies are collected in groups, and the three classes
of bodies are grouped in a somewhat similar manner. The Au-
gust meteors move in orbits which require more than a century
to complete, and comprehend bodies differing greatly in size and
probably also in density. Their magnitudes range from comets
whose diameter is perhaps 100,000 miles to minute atoms which,
in a single second, are dissipated by the heat resulting from their
collision with our atmosphere. Their density ranges from that of
metallic iron to earthy bodies having but feeble cohesion, which
are dissipated into fine dust by the heat of collision with our at-
mosphere ; and it is possible that the rarest of them may consist
of solid or liquid matter in a state of minute subdivision, like a
cloud of dust or smoke.
The periodic meteors of November probably comprehend bod-
ies having an equal range of magnitude, and perhaps also of den-
sity.
TABLE I. MILLIMETRES CONVERTED INTO INCHES. 251
TABLE I.
TO CONVERT MILLIMETRES INTO ENGLISH INCHES.
Mil-
lime-
tres.
[nclies.
Mil-
ime-
tres.
Inches.
Mil-
ime-
tres.
Inches.
Mil-
ime-
tres.
Inches.
Mil-
ime-
tres.
Inches.
Mil-
ime-
tres.
Inches.
!
0.039
5o
1.969
5oo
19.685
68 9
27. 126
7 34
28.898
779
30.670
2
.079
60
2.362
5io
20.079
6 9 o
.166
7 35
. 9 38
7 8o
.709
3
.118
70
2.756
520
20.473
6 9 i
.2o5
7 36
977
781
749
4
.i5 7
80
3.i5o
53o
20.867
6 9 2
.245
73 7
2 9 .0l6
782
.788
5
.197
9
3.543
54o
21 .260
6 9 3
.284
7 38
.o56
7 83
.827
6
.236
100
3. 9 3 7
55o
21.654
6 9 4
.323
7 3 9
,o 9 5
784
.867
7
.276
I IO
4.33i
56o
22.048
6 9 5
.363
74o
.i34
785
.906
8
.3i5
I2O
4.724
570
22.441
6 9 6
.402
7 4 1
.174
786
. 9 45
9
.354
i3o
5.n8
58o
22.835
6 9 7
44i
742
.213
787
. 9 85
10
.3 9 4
i4o
5.5i2
5 9 o
23.229
6 9 8
.481
743
.252
,788
31.024
ii
.433
i5o
5.906
600
23.622
699
.520
744
,2 9 2
789
.o64
12
.472
160
6.299
610
24.016
700
.56o
7 45
.331
79
.io3
i3
.512
170
6.6 9 3
620
24.410
701
5 99
746
.3 7 I
791
.142
i4
.55i
1 80
7.087
63o
24.804
702
.638
747
.410
792
.182
i5
.5 9 i
190
7 .48o
64o
25.197
7 o3
.678
748
.449
79 3
.221
16
.63o
200
7-874
65o
25.591
704
.717
74 9
48 9
794
.260
*7
.669
210
8.268
660
25. 9 85
7 o5
. 7 56
7 5o
.528
79 5
.3oo
18
.709
22O
8.662
661
26.024
706
.796
7 5i
.56 7
79 6
.339
J 9
748
230
9 .o55
662
.o63
707
.835
7 52
.609
797
.379
20
.787
24o
9.449
663
.io3
708
.8 7 5
7 53
.646
79 8
.418
21
.827
2 5o
9. 843
664
.142
709
. 9 i4
754
.686
7 99
.457
22
.866
260
10.236
665
.182
710
. 9 53
7 55
.725
800
497
23
.goS
2 7
io.63o
666
.221
711
. 99 3
7 56
7 64
810
.890
24
.945
280
ii .024
667
.260
712
28.032
7 5 7
.8o4
820
32.284
25
.984
290
n.4i8
668
.300
7 i3
.071
7 58
.843
83o
.678
26
1.024
3oo
11.811
669
.33 9
7 i4
.III
7 5 9
.882
84o
33.072
27
.o63
3io
12.205
670
.3 7 8
7 i5
.i5o
7 6o
. 9 22
85o
.465
28
. 1 02
320
12.599
671
.4i8
716
.189
7 6i
. 9 6l
860
.85 9
29
.i4a
33o
12.992
672
.45 7
717
,22 9
762
So.ooi
870
34.253
3o
.181
34o
i3.386
6 7 3
497
718
.268
7 63
.o4o
880
.646
3i
.220
35o
13.780
6 7 4
.536
719
.3o8
764
079
8 9 o
35.o4o
32
.260
36o
i4.i73
6 7 5
.5 7 5
720
.34 7
7 65
.119
9 oo
.434
33
.299
3 7 o
i4.56 7
676
.6i5
721
.386
7 66
.i58
Proportional
34
.33 9
38o
14.961
677
.654
722
.426
767
.19-7
Parts.
35
.3 7 8
390
i5.355
678
.6 9 3
72 3
.465
768
. 2 3 7
Mill.
Inches.
36
.4i 7
4oo
i5. 7 48
679
.733
724
.5o4
769
.276
O. I
0.004
37
.45 7
4io
i 6. i 42
680
.772
72 5
.544
770
.3i6
0.2
0.008
38
.496
420
i6.536
681
.812
726
.583
771
.355
0.3
O.OI2
3 9
.535
43o
16.929
682
.85i
727
.623
772
3 9 4
0.4
0.016
4o
.5 7 5
44o
i 7 . 323
683
.890
728
.662
77 3
.434
o.5
O.O2O
4i
.614
45o
17.717
684
. 9 3o
729
.701
77 4
4 7 3
0.6
0.024
42
..654
46o
i8.ni
685
. 9 6 9
7 3o
. 7 4i
77 5
.512
0.7
0.028
43
.6 9 3
4 7 o
i8.5o4
686
27.008
7 3i
.780
77 6
.552
0.8
o.o3i
44
. 7 3 2
48o
18.898
687
.o48
7 32
.8i 9
77 7
.5 9 i
o. 9
o.o35
45
.772
4go
19.292
688
.087
7 33
.85 9
778
.63o
I.O
o.o3g
One millimetre equals 0.03937079 English inch.
252
TABLE II. METRES CONVERTED INTO FEET.'
TABLE II.
TO CONVERT METRES INTO ENGLISH FEET.
Me-
tres.
Feet.
Me-
tres.
.Feet.
Me-
tres.
Feet.
Me-
tres.
Feet.
Me-
tres.
Feet.
1
Me-
tres.
Feet.
I
3.28
46
150.92
9 1
298.56
i36
446.20
181
5 9 3.84
226
74i.48
2
6.56
47
154.20
9 2
3oi.84
i3 7
449.48
182
5 97 .i2
22 7
744.76
3
9'- '84
48
i5 7 .48
93
3o5. 12
i38
452.76
i83
600. 4o
228
748.o5
4
I3.I2
49
160.76
94
3o8.4o
i3 9
456.o4
1 84
603.69
229
7 5i.33
5
i6.4o
5o
i 64 . o4
9 5
3n .69
i4o
459'. 33
i85
606.97
230
7 54.6i
6
19.69
5i
167.33
96
3i4-97
i4i
462.61
186
610.25
23l
757.89
7
22.97
52
170.61
97
3i8. 2 5
142
465.89
187
6i3.53
232
761.17
8
26.25
53
i 7 3.8 9
98
321.53
i43
469.17
188
616.81
233
7 64.45
9
29.53
54
177.17
99
3 2 4. 81
i44
472.45
189
620.09
234
767.73
10
32. 81
55
180.45
IOO
328.09
i45
475.73
190
623.37
235
771.01
ii
36. 09
56
i83. 7 3
101
33i.3 7
i46
479.01
191
626.65
236
774.29
12
3 9 .3 7
5 7
187.01
IO2
334.65
i4 7
482.29
192
629.93
23 7
7 77 .5 7
i3
42.65
58
190.29
io3
337.93
i48
485. 5 7
i 9 3
633.21
238
78o.85
M
45. 9 3
5 9
i 9 3.5 7
io4
34 I .21
149
488.85
194
636.49
2 3 9
784.13
i5
49.21
60
196. 85
io5
344.49
i5o
492.13
196
63 9 . 7 8
24o
787.42
16
52.49
61
200. i 3
106
347.78
i5i
495.42
196
643.o6
24 1
790.70
i?
55.78
62
203.42
107
35i.o6
i5 2
498.70
197
646.34
242
79 3. 9 8
18
59.06
63
206.70
1 08
354.34
i53
501.98
198
649.62
243
7 9 7 .26
*9
62.34
64
209.98
109
357.62
i54
5o5. 2 6
199
652.90
244
8oo.54
20
65.62
'65
213.26
no
360.90
i55
5o8.54
200
656.i8
245
8o3.82
21
68.90
66
216.54
III
364.i8
i56
611.82
2OI
659.46
246
807. 10
22
72.18
67
219.82
112
36 7 .46
i5 7
616.10
2O2
662.74
24 7
8io.38
23
7 5.46
68
223.10
U3
370.74
i58
5i8.38
203
666.02
248
8i3.66
24
78.74
69
226.38
n4
374.02
i5 9
521.66
2O4
669.30
249
816.94
25
82.02
7
229.66
ii5
3 77 .3o
160
524-94
205
672.58
25o
820.22
26
85. 3o
7 1
232.94
116
B8o.58
161
528.22
206
6 7 5.8 7
25l
823. 5i
27
88.58
72
236.22
117
383. 87
162
53i.5i
207
679. i5
252
826.79
28
91.87
73
2 3 9 .5i
118
38 7 .i5
i63
534.79
208
682.43
253
83o.o7
29
9 5.i5
74
242.79
119
390.43
1 64
538.07
209
685. 7 i
254
833.35
3o
9 8.43
75
246.07
120
3 9 3. 7 i
i65
54i.35
210
688.99
255
836.63
3i
101 .71
76
249.35
121
3 9 6. 99
166
544.63
211
692.27
256
839.91
32
104.99
77
252.63
122
400.27
167
547.91
212
695.55
25 7
843. I9
33
108.27
78
255.91
123
4o3.55
168
55i .19
2l3
698.83
Proportional
34
in. 55
79
25 9 .i 9
124
4o6.83
169
554.47
214
7 O2. II
Parts.
35
n4.83
80
262.47
125
4io.n
170
55 7 . 7 5
2l5
7 o5.3 9
Met.
Feet.
36
ii8.ii
81
265. 7 5
126
4i3.39
171
56i.o3
216
7 o8.6 7
0. 1
0.33
37
121.39
82
269.03
I2 7
4i6.6 7
172
564.3i
217
711.96
O.2
0.66
38
124.67
83
272.31
128
419.96
173
567.60
218
7 i5.24
0.3
0.98
3 9
127.96
84
275.60
I2 9
423.24
1 74
570.88
219
7 i8.5 2
0.4
i.3i
4o
i3i.24
85
278.88
i3o
426.52
i 7 5
574.16
22O
721.80
o.5
i.64
4i
134.52
86
282.16
i3i
429.80
176
577.44
221
7 25.o8
0.6
1.97
42
137.80
87
285.44
132
433.o8
177
580.72
222
7 28.36
0.7
2.30
43
i4i.o8
88
288.72
i33
436.36
178
584. oo
223
7 3i.64
0.8
2.62
44
i44.36
89
292.00
1 34
43 9 . 64
179
587.28
224
7 34. 9 2
0.9
2.95
45
147.64
9
295.28
i35
442.92
180
590.56
225
7 38.2o
i .0
3.28
One metre equals 3.2808992 English feet.
TABLE III. KILOMETRES CONVERTED INTO MILES. 253
TABLE III.
TO CONVERT KILOMETRES INTO ENGLISH MILES.
Kil-
ome-
tres.
Miles.
Kil-
ome-
tres.
Miles.
Kil-
ome-
tres.
Miles.
Kil-
ome-
tres.
Miles.
Kil-
ome-
tres.
Maes.
Kil-
ome-
tres.
Miles.
I
0.621
46
28.584
9 1
56.546
i36
84.5o8
181
112.470
226
i4o.432
2
1.243
47
29.205
92
57.167
i3 7
85.129
182
113.092
227
i4i .064
3
i.864
48
29.826
9 3
57.789
1 38
85. 7 5i
i83
n3.7i3
228
141.676
4
2.486
49
3o.448
94
8.410
i3 9
86.3 7 2
1 84
ii4.334
229
142.297
5
3.107
5o
31.069
9 5
59.o3i
i4o
86.994
i85
n4.956
230
142.918
6
3.728
5i
31.691
96
5 9 .653
i4i
87.615
186
116.577
23l
i43.53 9
7
4.35o
52
32.312
97
60.274
142
88.236
187
116.198
232
144.161
8
4.971
53
32. 9 33
98
60.895
i43
88.858
188
116.820
233
144.782
9
5.592
54
33.555
99
61 .517
i44
89.479
189
117.441
234
i45.4o3
10
6.214
55
34.176
IOO
62.i38
i45
90. 100
190
ii8.o63
235
146.026
ii
6.835
56
34.797
101
62.760
i46
90.722
191
118.684
236
i46.646
12
7 .457
57
35.4i9
1 02
63.38i
147
91.343
192
119. 3o5
2 3 7
147.268
i3
8.078
58
36.o4o
io3
64.002
i48
91.965
193
119.927
238
147.889
i4
8.699
5 9
36.662
io4
64.624
149
92.586
194
120.548
239
i48.5io
i5
9.321
60
37.283
io5
65.245
i5o
93.207
196
121.170
240
149. i32
16
9.942
61
37.904
1 06
65. 867
i5i
93.829
196
121.791
241
149.763
l l
io.563
62
38.526
107
66.488
i5 2
94.45o
197
122. 4l2
242
i5o.3 7 5
1 8
n.i85
63
39.147
1 08
67. 109
i53
95.071
198
I23.o34
243
160.996
19
ii. 806
64
3 9 . 7 68
109
6 7 . 7 3i
1 54
9 5.6 9 3
199
123. 65
244
161 .617
20
12.428
65
40.390
IIO
68.352
i55
9 6.3i4
200
124.276
245
162.239
21
i 3. 049
66
4i.on
Hi
68.973
i56
96.936
201
124.898
246
162.860
22
13.670
67
4i.633
112
6 9 .5 9 5
i5 7
97.557
2O2
126.519
247
i53.48i
23
14.292
68
42.254
n3
70.216
i58
98.178
203
126. i4i
248
i54.io3
24
i4^9 l3
69
42.8 7 5
n4
70.838
169
98.800
2O4
126.762
249
164.724
25
i5.535
7
43.497
u5
71.459
160
99.421
205
127.383
260
155.346
26
i6.i56
?i
44.n8
116
72.080
161
ioo.o43
206
128.006
261
166.967
27
16.777
72
44.740
117
72.702
162
i 00.664
2O7
128.626
262
i56.588
28
17.399
73
45.36i
118
73.323
i63
101 .285
208
129.248
2 53
167.210
29
18.020
74
45.982
119
73.944
1 64
101 .907
209
129.869
264
167. 83i
3o
i8.64i
75
46.6o4
I2O
7 4.566
i65
102.528
210
i3o.49o
2 55
168.462
3i
19.263
76
47.225
121
75.187
166
io3.i49
211
l3l . 112
266
169.074
32
19.884
77
47.846
122
75.809
167
103.771
212
i3.i. 7 33
267
169.696
33
20.5o6
78
48.468
123
76.430
1 68
104.392
2l3
i3 2 .354
Proportional
34
21.127
79
49.089
124
77.o5i
169
io5.oi4
2l4
132.976
Parts.
35
21. 7 48
80
49.711
125
77.673
170
io5.635
215
133.697
Kil.
Miles.
36
22.370
81
5o.332
126
78.294
171
io6.256
216
134.219
0. I
0.062
3 7
22.991
82
5o. 9 53
127
78.916
172
106.878
217
i34.84o
0.2
0. 124
38
2 3'.6i3
83
5i.5 7 5
128
79.537
i 7 3
107.499
218
i35.46i
o.3
0.186
3 9
24.234
84
52.196
129
8o.i58
1 74
I 08. 121
219
i36.o83
0.4
0.249
4o
24.855
85
52.8i8
i3o
80.780
i 7 5
108.742
220
i 36. 704
o.5
o.3n
4i
25.477
86
53.439
i3i
8i.4oi
! 7 6
109. 363
221
i3 7 .326
0.6
0.373
42
26.098
87
54.o6o
132
82.022
177
109.985
222
137.947
0.7
0.435
43
26.719
88
54.682
i33
82.644
178
110.606
223
138.568
0.8
0.497
44
27.341
89
55.3o3
1 34
83.265
179
in .227
224
139.190
0.9
0.55 9
45
27.962
9
55.924
i35
83.887
1 80
in .849
225
139.811
I.O
0.621
One kilometre equals 0.6213824 English mile.
254 TABLE IV. FRENCH FEET CONVERTED INTO ENGLISH FEET.
TABLE IV.
TO CONVERT FRENCH FEET INTO ENGLISH FEET.
Fr.
English.
Fr.
English.
Fr.
English.
Fr.
English.
Fr.
English.
Fr.
English.
!
1. 066
46
49.O25
9 1
9 6. 9 85
i36
i44. 9 44
181
192.903
226
24o.863
2
2 . l32
47
So.ogi
9 2
9 8.o5o
i3 7
146.010
182
193.969
227
241 .929
3
3.197
48
5i.i5 7
9 3
99. 116
i38
147.076
i83
195.035
228
242.996!
4
4.263
4 9
52.222
94
100. 182
i3g
i48.i4i
1 84
196. 101
229
244.060;
5
5.329
5o
53.288
9 5
101 .248
i4o
149.^07
i85
197.167
230
245. 126
6
6.395
5i
54.354
9 6
io2.3i4
i4i
160.273
186
198.232
23l
246. 192
7
7.460
52
55.420
97
103.379
142
i5i.33 9
187
199.298
232
247.258
8
8.526
53
56.486
98
io4.445
i43
i52.4o4
188
200.364
233
248.323
9
9/592
54
5 7 .55i
99
io5.5n
1 44
153.470
189
201. 43o
2 34
249.389
10
10.658
55
58. 617
IOO
106.576
i45
i54.536
190
2O2 .495
235
2 5o.455
ii
11.723
56
5 9 .683
101
107.642
i46
i55.6o2
191
2o3.56i
236
25l .521
12
12.789
5 7
60.749
102
108.708
147
i56.66 7
192
204.627
2 3 7
252.586
i3
i3.855
58
6i.8i4
io3
109.774
i48
i5 7 . 7 33
i 9 3
205.693
2 38
253.652
i4
14.921
5 9
62.880
io4
no.84o
i4 9
i58. 799
194
206.758
239
254.718
i5
15.986
60
63.946
io5
in .905
!50
i5 9 .865
196
207.824
24o
255.784
16
17.052
61
65.oi2
1 06
112.971
i5i
i6o. 9 3i
196
208.890
24 1
256.849
17
18.118
62
66.077
107
n4.o37
l52
161 . 99 6
i 97
209.956
242
267.915
18
19.184
63
67.143
1 08
n5.io3
i53
163.062
198
211 .O2I
243
268.981
J 9
2O.25o
64
68.209
109
116.168
1 54
164.128
i 99
212.087
244
260.047
20
2i.3i5
65
69.275
no
117.234
i55
i65. i 9 4
200
2i3.i53
245
261 . n3
21
22.38l
66
70.340
III
n8.3oo
!56
166.269
201
214.219
246
262. 178
22
23.447
67
71.407
112
119.366
i5 7
i6 7 .325
2O2
2i5.285
247
263.244
23
2 4.5i3
68
72.472
ii3
120.431
i58
168.391
203
216. 35o
248
264.3io
24
2 5.5 7 8
69
73.538
n4
121.497
i5 9
169. 45 7
204
217.416
249
265.3 7 6
25
26.644
7o
74.6o4
ii5
122.563
160
I 7 O.522
205
218.482
25o
266.44i
26
27.710
7i
75.669
116
123.629
161
I 7 1.588
206
219.548
25l
26 7 .5o 7
27
28.776
72
76.735
IJ 7
124.695
162
172.654
207
220. 6l3
252
268. 5 7 3
28
29.841
73
77.801
118
125.760
i63
173.720
208
221 .679
253
269.639
2 9
30.907
74
78.867
119
126.826
1 64
174.785
209
222.745
254
270.704
3o
81.973
7 5
79.932
120
127.892
i65
i 7 5.85i
210
223.8II
255
271. 77 o
3i
33.o3 9
76
80.998
121
128.958
166
I 7 6.9i 7
211
224.877
256
2 7 2.836
32
34.io4
77
82.064
122
i3o.o23
167
i 77 . 9 83
212
225.942
25 7
2 7 3. 9 02
33
35.170
78
sa. i3o
123
i3i .089
168
i 79 .o4 9
2l3
227.008
Proportional
34
36.236
79
84.i 9 5
124
!3 2 .i55
i6 9
i8o.n4
214
228.O74
Parts.
35
37.302
80
85.26i
125
133.221
170
181.180
2l5
229.140
Fr.
English.
36
38.368
81
86.327
126
134.286
171
182.246
216
23O.2O5
O. I
O. I0 7
37
39.433
82
8 7 .3 9 3
127
135.352
172
i83.3i2
2I 7
23i .271
0.2
0.213
38
4o.499
83
88.458
128
i36.4i8
173
i84.3 77
218
232. 33 7
0.3
0.320
3 9
4i.565
84
89.524
I2 9
i3 7 .484
1 7 4
i85.443
219
233. 4o3
0.4
0.426
4o
42.63i
85
90.590
i3o
i38.54 9
i 7 5
i86.5o 9
220
234-468
o.5
0.533
4i
43.696
86
91.656
i3i
139. 6i5
176
i8 7 .5 7 5
221
235.534
0.6
o.63 9
42
44.762
87
92.722
132
i4o.68i
177
i88.64o
222
236. 600
0.7
o. 7 46
43
45.828
88
93.787
i33
141.747
178
i8 9 . 7 o6
223
23 7 .666
0.8
0.853
44
46.8 9 4
89
9 4.853
1 34
i42.8i3
i7 9
190.772
224
238.731
0.9
0.969
45
47.959
90
9 5. 9 i 9
i35
143.878
180
191.838
225
239.797
i .0
i .066
One French foot equals 1.066766 English foot.
TABLE V. CENTESIMAL AND FAHRENHEIT THERMOMETERS. 255
i
TABLE V.
TO COMPARE THE CENTESIMAL THERMOMETER WITH FAHRENHEIT'S.
Centes.
Fahren.
Centes.
Fahren.
Centes.
Fahren.
Centes.
Fahren.
Proportional Parts.
o
O
o
Centes. Fahren.
IOO
2I2-.0
5o
122.0
25
7 7 .0
O
+ 32.0
99
210.2
49-5
121 . I
24.5
76.1
I
30.2
o.i 0.18
98
208.4
49
I2O.2
24
7 5.2
2
28.4
0.2 o.36
97
2O6.6
48.5
II9.3
23.5
74.3
3
26.6
o.3 o.54
96
204.8
48
Il8.4
23
7 3.4
4
24.8
0.4 0.72
9 5
203.0
47-5
II7.5
22.5
72.5
5
23.0
o.5 0.90
94
201.2
47
II6.6
22
71.6
6
21.2
o . 6 i .'08
93
199-4
46.5
n5.7
21.5
70.7
7
19.4
0.7 1.26
92
197.6
46
u4.8
21
6 9 .8
.8
I 7 .6
0.8 i.44
9 1
i 9 5.8
45.5
n3. 9
20. 5
68. 9
9
i5.8
0.9 1.62
9
194.0
45
/ /
n3.o
20
68.0
(!
10
14.0
i.o i. 80
89
192 . 2
44.5
1 12 . I
I O . D
67.1
12 .2
88
190.4
44
III. 2
i 9
66.2
12
IO.4
87
188.6
43.5
II0.3
i8.5
65.3
13
8.6
86
186.8
43
io 9 .4
18
64.4
14
6.8
85
i85.o
42.5
108.5
i 7 .5
63.5
15
5.o
84
i83.2
42
107.6
62.6
16
3.2
83
181.4
4i.5
106.7
i6.5
61.7
17
+ 1.4
82
179.6
4i
io5.8
16
60.8
18
- 0.4
81
177.8
4o.5
io4. 9
i5.5
59.9
1 9
2.2
80
176.0
4o
io4.o
i5
r ' *
5g.o
20
- 4.0
79
174.2
39.5
io3.i
i4.5
58.1
21
5.8
78
172.4
3 9
IO2.2
14
5 7 . 2
22
- 7.6
*o
77
170.6
38.5
IOI.3
i3.5
56.3
23
- 9-4
76
168.8
38
100.4
i3
55.4
-24
II. 2
o
P '
75
167.0
3 7 .5
99.5
12,5
54.5
25
i3.o
"
74
i65.2
98.6
12
53.6
26
14.8
i'
73
i63.4
36.5
97-7
ii. 5
5 2 . 7
27
16.6
E,
72
161.6
36
96.8
II
5i.8
28
18.4
A'
i5 9 .8
35.5
95.9
io.5
5o9
29
2O. 2
70
i58.o
35
95.0
IO
5o.o
-3o
22. O
to
69
i56.2
34.5
94.1
9 .5
49.1
3i
23.8
+
68
i54.4
34
93.2
9
48.2
32
25.6
CnKO
67
162.6
33.5
92.3
8.5
4 7 .3
33
27.4
-3
66
i5o.8
33
91.4
8
46.4
-34
29.2
*=d
65
149.0
32.5
9 o.5
7 .5
45.5
35
-3i.o
&
64
147.2
32
8 9 .6
7
44.6
36
32.8
g
63
i45.4
3i.5
88.7
6.5
43.7
-3 7
34.6
I
62
i43.6
3i
87.8
6
42.8
38
36.4
ft
61
i4i.8
3o.5
86. 9
5.5
41.9
-3 9
38.2
60
i4o.o
3o
86.0
5
4i .0
4o
4o.o
5 9
i38.2
2 9 .5
85.i
4.5
4o.i
4i.8
58
i36.4
29
84.2
4
3 9 . 2
-42
43.6
57
i34.6
28.5
83.3
3.5
38.3
43
-45.4
56
i3 2 .8
28
82.4
3
3 7 .4
-44
47.2
55
i3i.o
27.5
8i.5
2.5
36.5
-45
49.0
54
129.2
27
80.6
2
35.6
-46
5o.8
53
127.4
26.5
79-7
i.5
34-7
-4 7
52.6
52
125.6
26
78.8
i
33.8
-48
-54.4
5i
123.8
2 5.5
77-9
o.5
32. 9
-4 9
56.2
256 TABLE VI. REAUMUR AND FAHRENHEIT THERMOMETEKS.
TABLE VI.
TO COMPAKE REAUMUR'S THERMOMETER WITH FAHRENHEIT'S.
Reaum.
Fahrenheit
Reaum.
Fahrenheit
Reaum.
Fahrenheit.
Reaum.
Fahrenheit.
O
O
O
80
212.0
4o
122. OO
20
77.00
O
4-32.0
79
209.75
3 9 .5
120.87
I 9 .5
75.8 7
I
+ 29.75
78
207.5
3 9
119.75
'9
74. 7 5
2
+ 27.5
77
2O5.25
38.5
II8.62
i8.5
73.62
3
+ 25.25
76
2O3.O
38
II7.5O
18
72. 5o
4
4-23.0
75
200.75
3 7 .5
n6.3 7
i 7 .5
7 i.3 7
5
4-20.75
74
198.5
37
ii5. 2 5
17
70.25
6
4-18.5
73
196.25
36.5
114.12
i6.5
69. 12
7
4-16.25
72
194.0
36
i i 3. oo
16
68.00
8
4-14-0
7 1
191.75
35.5
111.87
i5.5
66.87
9
+ II-75
70
189.5
35
110.75
i5
65. 7 5
10
+ 9-5
69
187.25
34.5
109.62
14.5
64.62
ii
+ 7-25
68
i85.o
34
io8.5o
i4
63. 5o
12
+ 5.o
67
182.75
33.5
107.37
i3.5
62.37
13
+ 2. 7 5
66
i8o.5
33
io6.25
i3
6i.25
-i4
+ o.5
65
178.25
32.5
io5. 12
12.5
60. 12
-i5
- i. 7 5
64
176.0
32
104.00
12
Sg.OO
16
- 4.o
63
i 7 3. 7 5
3i.5
102.87
ii. 5
57.87
17
6.25
62
171.5
3i
101.75
II M,
56. 7 5
18
8.5
61
169.25
3o.5
100.62
io.5
55.62
19
10.75
60
167.0
3o
99. 5o
10
54. 5o
20
i3.o
69
i64.75
29.5
98.37
9 .5
53. 3 7
21
i5. 2 5
58
162.5
29
97.25
9
52.25
22
-i 7 .5
5 7
160.25
28.5
96.12
8.5
5l.I2
23
19. 7 5
56
i58.o
28
95.00
8
5o.oo
-24
22. O
55
i55. 7 5
2 7 .5
93.87
7 .5
48.87
25
24.25
54
i53.5
27
92.75
7 '
47.75
26
26.5
53
i5i.25
26.5
91 .62
6.5
46.62
2 7
-28. 7 5
52
149.0
26
90. 5o
6
45. 5o
28
3i.o
5i
146.75
25.5
8 9 .3 7
5.5
44. 3 7
2 9
33.25
5o
144.5
2 5
88.25
5
43.25
3o
35.5
4 9
142.25
24.5
87.12
4.5
42. 12
3i
-37.75
48
i4o.o
24
86.00
4
4 1 .00
3 2
4o.o
47
i3 7 . 7 5
23.5
84.87
3.5
39.87
33
42.25
46
135.5
23
83. 7 5
3
38. 7 5
-34
-44.5
45
i33.25
22.5
82.62
2.5
37.62
35
-46. 7 5
44
i3i.o
22
8i.5o
2
36. 5o
36
49.0
43
128.75
21.5
80.37
i.5
35. 3 7
-3 7
5i.25
42
126.5
21
79.25
I
34.25
38
53. 5o
4i
124.25
20. 5
78.12
o.5
33.12
-3 9
55. 7 5
4o
122. O
26
77.00
o
32. OO
4o
58.0
Proportional Parts.
O O O O O
O
Reaumur c
.1 0.2 o.3 0.4 o.5
0.6 0.7 0.8
0.9 i .0
Fahrenheit . . c
.22 O.45 0.67 0.96 I.I
2 i.35 1.57 i. 80
2 . O2 2.25
x Reaumur=:(32 -|-?a; ) Fahrenheit.
4
TABLE VII. HEIGHT OF A COLUMN OF AIK, ETC. 257
TABLE VII.
HEIGHT OF A COLUMN OF AIK CORRESPONDING TO A TENTH OF AN INCH
IN THE BAROMETER.
Barom.
40
45
50
55
60
65
70
75
80
85
90
Inches.
Feet.
Feet.
Feet.
Feet.
Feet.
Feet.
Feet.
Feet.
Feet.
Feet.
Feet.
22. O
121 .5
122.8
124.2
125.5
126.8
128.2
129.5
i3o.8
I32.I
133.5
134.8
.2
120.4
121.7
123. 1
124.4
125-7
127.0
128.3
129.6
i3o. 9
132.2
i33.6
.4
119.3
I2O.6
121 .9
123.2
124.6
125.9
127 .2
128.5
I2 9 .8
i3i.i
i3 2 .4
.6
118.2
Iig.S
120.8
122. I
123.4
124.7
I26.O
127.3
128.6
I2 9 . 9
l3l.2
.8
117.2
II8.5
119.8
121 .1
122.3
123.6
124.9
126.2
127.5
128.8
iSo.o
23.0
116.2
II7.5
118.7
120.0
121 .3
132.6
123.8
125. I
126.4
127.6
129.9
.2
Il5.2
II6.5
117.7
Iig.O
I2O.2
121. 5
122.7
124.0
125.3
126.5
127.8
.4
114.2
ii5.5
116.7
IlS.O
119.2
120.5
121 .7
123.0
124.2
125.4
126.7
.6
II3.2
n4.4
n5.7
116.9
IlS.I
119.4
1 20. 6
121. 8
123.
124.3
125.5
.8
112. 3
ii3.5
n4.8
116.0
II7.2
118.4
119.7
120.9
122.
123.3
124.6
24.0
III .4
112. 6
n3.8
n5.o
116.2
117.4
118.7
119.9
121.
122.3
123.5
.2
iio.5
111.7
112.9
114. i
n5.3
u6.5
117.7
n8. 9
120.
121. 3
122.5
.4
109.5
110.7
111.9
ii3.i
ii4.3
n5.5
116.7
117.9
IIO.
120.3
121. 5
.6
108.6
109.8
III.O
112. 2
ii3.4
ii4-6
ii5.8
116.9
118,
ii 9 .3
120.5
.8
107.8
108.9
IIO. I.
in. 3
112 .5
n3. 7
ii4.8
116.0
II7.2
118.4
ii 9 .5
25.0
106.9
108.1
109.2
110.4
in .6
112.7
ii3. 9
u5.i
116.2
117.4
118.6
.2
106.0
107.2
108.4
ip 9 .5
1 10.7
in. 8
u3.o
n4. i
ii5.3
116.5
117.6
.4
IO5.2
106.4
107.5
108.7
109.8
III.O
112. I
ii3.3
ii4.4
ii5.6
116.7
.6
io4-4
io5.5
106.7
107.8
108.9
I IO. I
III .2
112. 4
ii3.5
ii4.6
ii5.8
.8
io3.6
104.7
io5.8
107.0
108.1
109.2
110. 4
in. 5
1 12. 6
ii3.8
"4. 9
26.0
102.8
103.9
io5.o
106.1
107.3
108.4
109.5
no. 6
in. 8
112. 9
ii4.o
.2
IO3.O
io3.i
104.2
io5.3
io6.5
107 .6
108.7
109.8
no. 9
112. O
iiS.i
.4
IOI .2
102.3
io3.4
io46
105.7
106.8
107.9
109.0
no. i
III .2
112. 3
.6
100.5
101 .6
102.7
io3.8
104.9
1 06 .0
107. 1
108.2
io 9 .3
no. 4
in .4
.8
99-7
100.8
101 .9
io3.o
io4.i
io5 .2
106.3
107.4
io8.5
io 9 .5
no. 6
27.0
99.0
IOO. I
IOI .2
I O2. 2
io3.3
io4.4
io5.5
106.6
107.6
108.7
io 9 .8
.2
9 8.3
99.3
IOO.4
101 .5
102.6
io3.6
104.7
io5.8
106.8
107-9
io 9 .o
-.4
97.5
98.6
99-7
100.7
ioi .8
1 02 .9
103.9
io5.o
106.1
107.1
108.2
.6
96.8
97-9
98.9
IOO.O
IOI . I
1 02 . i
103.2
104.2
io5.3
io6.3
107.4
.8
96. i
97.2
98.2
99.3
loo.S
IOI .4
102.4
io3.5
io4.5
io5.6
106.6
28.0
95.4
96.5
97.5
98.6
99.6
100. 6
ioi .7
102.7
io3.8
io4.8
io5. 9
.2
94.8
95.8
96.8
97-9
98.9
99.9
IOI .O
102.0
io3.o
104.1
io5. i
.4
94.1
9 5.i
96.1
97.2
98.2
99.2
IOO. 2
ioi. 3
102.3
io3.3
io4.3
.6
93.4
94.4
9 5.5
96.5
97.5
9 8.5
99.5
100.6
ioi .6
IO2.6
io3.6
.8
92.8
9 3.8
94.8
9 5.8
96.8
97.8
98.8
99.8
100. 8
ioi. 8
IO2.8
29.0
92. i
9 3.i
94.1
96.1
96.2
97.2
98.2
99- 2
IOO. 2
IOI .2
IO2.2
.2
9 i.5
9 2.5
93.5
94.5
9 5.5
96.5
97.5
9 8.5
99 .5
ioo. 5
IOI .5
.4
90.9
91.9
92.9
9 3. 9
94.8
95.8
96.8
97 .8
9 8.8
99 .8
ioo. 8
.6
90.3
9 i.3
92.2
93.2
94.2
95.2
96.2
97- 2
9 8.2
99 .i
IOO. I
.8
89.7
90.6
91 .6
92.6
93.6
94-5
9 5.5
9 6.5
97 .5
9 8.5
99.4
3o.o
89.1
90.0
91 .0
92.0
92.9
93.9
94-9
9 5. 9
9 6.8
97.8
98.8
.2
88.5
89.4
90.4
91.4
92.3
9 3.3
94.3
9 5.2
9 6.2
97.2
98:1
.4
87.9
88.8
89.8
90.8
91.7
92.7
93.6
9 4.6
9 5.6
96.5
97 .5
.6
8 7 .3
88.2
89.2
90.2
91. i
92. i
93.0
9 4-o
9 5.o
9 5. 9
9 6.8
.8
86.7
87.6
88.6
89.6
90.5
9 i.5
92.4
9 3.4
94.3
95.2
9 6.2
E
258 TABLE VIII. FOE SEDUCING BAROMETRIC OBSERVATIONS
TABLE VIII.
FOB REDUCING BAROMETRIC OBSERVATIONS TO THE FREEZING POINT.
Temp.
27
27.5
28
28.5
29
29.5
30
30.5
31 | Tem P-
+ .069
.071
.072
.073
.074
.076
.077
.078
.080
o
I
.067
.068
.069
.071
.072
.o 7 3
.074
.076
.077
I
2
.064
.066
.067
.068
.069
.070
.072
.073
.074
2
3
.062
.o63
.064
.o65
.067
.068
.069
.070
.071
3
4
.oSg
.061
.062
.o63
.064
.o65
.066
.067
.068
4
5
.057
.o58
.059
.060
.061
.062
.o63
.o65
.066
5
6
+ .o55
.o56
.057
.o58
.059
.060
.061
.062
.o63
6
7
.052
.o53
.o54
.o55
.o56
.067
.o58
.oSg
.060
7
8
.o5o
.o5i
.052
.o53
.o54
.o54
.o55
.o56
.57
8
9
.047
.o48
.049
.o5o
.o5i
.o52
.o53
.o54
.o54
9
10
.o45
.o46
.047
.047
.o48
.049
.o5o
.o5i
.052
10
ii
+ .042
.o43
.o44
.o45
.o46
.046
.047
.048
.049
ii
12
,o4o
.o4i
.o4s
.042
.o43
.o44
.o45
.o45
.o46
12
i3
.o38
.o38
.089
.o4o
.o4o
.041
.042
.o43
.o43
i3
i4
.o35
.o36
.o3 7
.037
.o38
.o38
.039
.o4o
.o4o
i4
i5
.o33
.033
.o34
.o35
.o35
.o36
.o36
.o3 7
.o38
!5
16
-}-.o3o
.o3i
.032
.032
.o33
.o33
.o34
.o34
.o35
16
l l
.028
.028
.029
.o3o
,o3o
.o3i
.o3i
.032
.032
J 7
18
.025
.026
.026
.027
.027
.028
.028
.029
.029
18
'9
.023
.024
.024
.024
.025
.025
.026
.026
.027
'9
20
.021
.021
.021
.022
.022
.023
.023
.023
.024
20
21
+ .618
.019
.019
.019
.020
.020
.020
.021
.021
21
22
.016
.016
.016
.017
..017
.017
.018
.018
.018
22
23
.oi3
.014
.014
.014
.014
.oi5
.oi5
.oi5
.oi5
23
24
.on
.on
.01 1
.012
.012
.012
.012
.012
.oi3
24
25
.009
.009
.009
.009
.009
.009
.009
.010
.010
25
26
+ .006
.006
.006
.006
.007
.007
.007
.007
.007
26
27
4-.oo4
.oo4
.oo4
.004
.oo4
.oo4
.oo4
.oo4
.oo4
27
28
-j-.ooi
.001
.001
.001
.001
.001
.OOI
.OOI
.001
28
2 9
.001
.001
.001
.001
.001
.001
.OOI
.OOI
.OOI
29
3o
.oo4
.oo4
.oo4
.oo4
.oo4
.004
.oo4
.oo4
.oo4
3o
3i
.006
.006
.006
.006
.007
.007
.007
.007
.007
3i
32
.008
.009
.009
.009
.009
.009
.009
.010
.010
32
33
.on
.Oil
.on
.012
.012
.012
.012
.012
.012
33
34
.oi3
.oi4
.oi4
.oi4
.014
.oi5
.oi5
.oi5
.oi5
34
35
.016
.016
.016
.017
.017
.017
.018
.018
.018
35
36
.018
.019
.019
.019
.020
.020
.020
.021
.021
36
37
.021
.021
.021
.022
.022
.022
.023
.023
.024
37
38
.023
.023
.024
.024
.025
,O25
.026
.026
.026
38
3 9
.025
.026
.026
.027
.027
.028
.028
.029
.029
3 9
4o
.028
.028
.029
.029
.o3o
.o3o
.o3i
.o3i
.032
4o
4i
.o3o
.o3i
.o3i
.032
.o33
.o33
.o34
.o34
.o35
4i
42
.o33
.o33
.o34
.o34
.o35
.o36
.o36
.o3 7
.037
42
43
.o35
.o36
.o36
.o3 7
.o38
.o38
.oSg
.o4o
.o4o
43
' 44
.o3 7
.o38
.089
.o4o
.o4o
.o4i
.042
.042
.043
44
45
.o4o
.o4i
.o4i
.o4s
.043
o44
o44
.045
.o46
45
TO THE FREEZING POINT.
259
TABLE VIII.
FOB REDUCING BAROMETRIC OBSERVATIONS TO THE FREEZING POINT.
Temp.
27
27.5
28
28.5
29
29.5
30
30.5
31
Temp.
45
.o4o
.o4i
.o4i
.042
.o43
.o44
.o44
.o45
.o46
45
46
.042
.o43
.o44
.o45
.o45
.046
.047
.o48
.049
46
4?
.o45
.046
.046
.047
.o48
.049
.o5o
.o5i
.o5i
47
48
.047
.048
.049
.o5o
.o5i
.052
.052
.o53
.o54
48
49
.o5o
.o5o
.o5 i
.o52
.o53
,o54
.o55
.o56
.o5 7
4 9
5o
.062
.o53
.o54
.0.55
.o56
.o5 7
.o58
.059
.060
5o
5i
.o54
.o55
.o56
.057
.o58
.059
.060
.061
.062
5i
52
.057
.o58
.069
.060
.061
.062
.o63
.064
.o65
52
53
.oSg
.060
.061
.o63
.064
.o65
.066
.067
.068
53
54
.062
.o63
.064
.o65
.066
.067
.068
.070
.071
54
55
.064
.o65
.066
.068
.069
.070
.071
.072
.073
55
56
.066
.068
.069
.070
.071
.073
.074
.075
.076
56
5 7
.069
.070
.071
.073
.074
.075
.076
.078
.079
57
58
.071
.o 7 3
.074
.075
.077
.078
.079
.081
.082
58
5 9
.074
.076
.076
.078
.079
.080
.082
.o83
.o85
5 9
60
.076
.077
.079
.080
.082
.o83
.o85
.086
.087
60
61
.078
.080
.081
.o83
.084
.086
.087
.089
.090
61
62
.081
.082
.084
.o85
.087
.088
.090
.091
.093
62
63
.o83
.o85
.086
.088
.089
.091
.093
.094
.096
63
64
.086
.087
.089
.090
.092
.094
.095
.097
.098
64
65
.088
.090
.091
.093
.095
.096
.098
. IOO
.101
65
66
.090
.092
.094
.096
.097
.099
. IOI
.102
.104
66
67
.093
.095
.096
.098
.100
. IO2
.io3
.io5
.107
67
68
.095
.097
.099
.101
.102
. io4
.106
.108
. 109
68
69
.098
. IOO
. IOI
.io3
.io5
.107
.109
.no
.112
69
70
. IOO
.102
.104
.106
.108
.109
.in
.ii3
.u5
70
?i
.102
.104
.106
.108
.110
. 112
.n4
.116
.118
7J
72
.io5
. 107
. 109
.in
.n3
.n5
.117
.119
. I2O
72
73
.107
.I0 9
.in
.ii3
.n5
.117
.119
. 121
.123
73
74
. no
.112
.114
.116
.118
. I2O
.122
.124
.126
74
75
. 112
.114
.116
.118
. 1 20
. 122
.125
.127
.129
75
76
-.114
.117
.119
.121
.123
.125
.127
.129
.i3i
76
77
.117
.119
.121
.123
. 126
.128
.i3o
.132
.i34
77
78
.119
. 122
. 124
.126
.128
.i3o
.x33
.135
.i3 7
78
79
.122
.124
. 126
.128
.i3i
.i33
.i35
.i3 7
. i4o
79
80
.124
.126
. 129
.i3i
.i33
.i36
.i38
. i4o
.i43
80
81
. 126
. 129
.i3i
.i34
.i36
.i38
.i4i
.i43
.i45
8l
82
. 129
.i3i
.i34
.i36
.i38
.i4i
.i43
.i46
.i48
82
83
.i3i
.i34
.i36
.iSg
.i4i
.i43
.i46
.i48
.i5i
83
84
.134
.i36
.iSg
.141
i44
.i46
.i4 9
.i5i
.i54
84
85
.136
.i3 9
.141
.i44
.146
.149
.i5i
.i54
.i56
85
86
.i38
,x4i
.i44
.i46
.149
.i5i
.i54
.i56
.i5 9
86
87
.141
.i43
.i46
149
.i5i
.i54
.167
.i5 9
.162
87
88
.i43
.i46
.149
.i5i
.i54
.i5 7
.159
.162
.i65
88
89
.i46
.i48
.161
.i54
.i56
.169
. 162
.i65
.167
89
90
.i48
.161
.i53
.i56
.i5 9
.162
.164
. 167
. 170
90
260
TABLE IX. ALTITUDES WITH THE BAROMETER.
TABLE IX.
ALTITUDES WITH THE BAROMETER. Part I.
Inches.
Feet
Inches.
Feet.
Inches.
Feet.
Inches.
Feet.
II.
1396.9
16.0
III86.3
21 .O
18291 .0
26.0
23871.0
.1
1633.3
. I
II349.I
.1
i84i5.i
. I
28971.3
.2
1867.6
.2
iiSio.g
.2
18538. 7
.2
24O7I .2
.3
2099.9
.3
11671.7
.3
18661.6
.3
24170.7
.4
233o.i
.4
n83i.5
.4
i8 7 84.o
.4
24269.8
.5
2558.3
.5
11990.3
.5
18905.8
.5
24368.6
.6
2784.5
.6
12148.2
.6
1902-7.0
.6
24467.0
-7
3008.7
-7
I23o5. i
-7
19147-7
7
24565.1
.8
323i.i
.8
12461 .0
.8
i926 7 .8
.8
24662.7
9
345i.6
9
12616. I
9
i 9 38 7 .4
9
24760.0
12. O
3670.2
17.0
12770.2
22.0
19506.4
27.0
24857.0
. I
3887.0
.1
12923.5
.1
19624.9
.1
24953.6
.2
4lO2.O
.2
i3o 7 5.8
.2
19742.9
?
25049. 8
.3
43i5.3
.3
13227.3
.3
19860.3
.3
25145.7
4
4526.9
.4
i33 77 . 9
4
I 9977- 2
.4
2524l .2
.5
4736.7
.5
13527.6
.5
20093.6
.5
25336.4
.6
4944.9
.6
13676.5
.6
20209.4
.6
2543i.2
7
5i5i.4
-7
i3824.5
7
20324.8
7
25525.7
.8
5356.4
.8
13971.7
.8
20439.6
.8
25619.9
9
5559. 7
9
i4n8.o
9
2o554-o
9
25713.7
i3.o
5 7 6i.4
18.0
14263.6
23.0
2066-7.8
28.0
25807.1
.1
5961.6
. i
i44o8.3
. I
20781.1
. i
25900.3
.2
6i6o.3
.2
i4552.3
.2
20894.0
.2
25993.1
.3
6357.5
.3
14695.4
.3
21006.4
.3
26o85. 6
.4
6553.2
.4
i483 7 .8-
4
2in8.3
.4
26177.7
.5
6 7 47.5
.5
14979.4
.5
21229.7
.5
26269.6
.6
6940.3
.6
i5i2o.3
.6
2i34o.6
.6
2636i. i
7
7 i3i. 7
7
i526o.3
7
2i45i . i
7
26452.3
.8
7 321. 7
.8
i53 99 . 7
.8
2i56i.i
.8
26543.2
9
75io.3
9
15538.3
9
21670.6
9
26633.7
14.0
7697.6
19.0
15676.2
24.0
21779.7
29.0
26724.0
. i
7883.6
. i
i58i3.3
. i
21888.4
. i
26813.9
.2
8068.2
.2
15949.8
.2
21996.6
.2
26903.5
.3
825i.5
.3
i6o85.5
.3
22104.3
.3
26992.8
4
8433.6
.4
16220.5
.4
222II .6
.4
27081 .9
.5
86i4-4
.5
16354.8
.5
223l8.4
.5
27170.6
'.6
8794.0
.6
i6488.5
.6
22424.8
.6
27259.0
7
8972.3
7
16621 .4
7
2253o.8
7
27347.1
.8
9149.5
.8
16753.7
.8
22636.4
.8
27434.9
9
9325.5
9
16885.3
9
22741 .5
9
27522.5
i5.o
95oo.3
20. o
17016.3
25.0
22846.3
3o.o
27609.7
. i
9673.8
.1
17146.6
.1
22950.6
.1
27696.6
.2
9846.2
.2
17276.3
.2
2 3o54.4
.2
2 77 83.3
.3
10017.5
.3
I74o5.3
.3
23i5 7> 9
.3
2*7869. 7
.4
10187.7
.4
I7 533. 7
.4
23261 .0
.4
27955. 7
,5
io356.8
.5
17661.4
.5
23363.6
.5
28041.6
.6
io524.8
.6
17788.6
.6
23465.9
.6
28127 - 1
7
10691 .8
7
17915. i
7
23567.7
7
28212.3
.8
10857.7
.8
i8o4i .0
.8
23669.2
.8
28297.3
9
IIO22.5
9
18166. 3
-9
23770.3
9
28382.0
TABLE IX. ALTITUDES WITH THE BAROMETER.
261
TABLE IX.
ALTITUDES WITH THE BAROMETER Part II.
T T'.
Feet
T i".
Feet.
T T'.
Feet.
T T'.
Feet
T T'.
Feet
1
2.3
17
39.8
33
77 .3
49
114.7
65
152.2
2
4.7
18
42.1
34
79.6
5o
117.0
66
i54.5
3
7.0
J 9
44.5
35
81.9
5i
119.4
67
1 56. 8
4
9.4
20
46.8
36
84.3
52
121 .7
68
159.2
5
11.7
21
49.2
37
86.6
53
124. i
69
i6i.5
6
i4o
22
5i.5
38
8 9 .o
54
126.4
7
i63. 9
7
16.4
23
53.8
3 9
9 i.3
55
128.7
7 1
166.2
8
18.7
24
56.2
4o
9 3.6
56
iSi.i
72 .
168.6
9
21 .1
25
58.5
4i
9 6.o
5 7
i33.4
73
170.9
IO
23.4
26
60.9
42
98.3
58
i35.8
74
i 7 3.3
ii
25.8
27
63.2
43
100.7
5 9
i38.i
75
i 7 5.6
12
28.1
28
65.5
44
io3.o
60
i4o.4
76
177.9
i3
3o.4
29
67.9
45
io5.3
61
142.8
77
i8o.3
i4
32.8
3o
70.2
46
107.7
62
i45.i
78
182.6
i5
35.i
3i
72.6
4 7
IIO.O
63
i47.5
79
i85.o
16
3 7 .5
32
74-9
48
112. 4
64
149-8
80
187.3
Parts III., IV, and V.
Approximate
Altitude.
Part III.
Positive from Lat. o to 45.
Negative from Lat. 45 to 90.
Part IV.
Always positive.
Part V.
Always positive.
Latitude.
Height of Barometer at Lower Station.
10
20
3o
4o
45
d
00
.s
ji
<N
.s
xt
cq
^ri
to
<N
A
oo
9
80
7
60
5o
Feet.
I,OOO
2,OOO
3,OOO
4,000
5,OOO
Feet
2.6
5.3
7-9
10.6
13.2
Feet.
2.5
5.0
7 .5
IO.O
12.4
Feet.
2.0
4.1
6.1
8.1
IO. I
Feet.
1.3
2.6
4.o
5.3
6.6
Feet.
0.5
0.9
1-4
1.8
2.3
Feet,
o
o
o
O
o
Feet.
2.5
5.2
7-9
10.8
i3. 7
Feet.
1.3
2.5
3.8
5.i
6.4
Feet.
I.O
2.0
3.0
4.0
5.o
Feet.
0.8
1.5
2.3
3.i
3.8
Feet.
0.6
I.I
i-7
2.2
2.8
Feet
0.4
0.7
I.I
i.4
1.8
Feet.
O.2
0.3
0.5
0.7
0.8
6,OOO
7,OOO
8,OOO
9,OOO
IO,OOO
i5. 9
i8.5
21 .2
23.8
26.5
14.9
17.4
19.9
22.4
24.9
12.2
14.2
16.2
i8.3
20.3
7-9
9 .3
10.6
11.9
13.2
2.8
3.2
3.7
4.i
4.6
o
o
o
o
16.7
19.9
23.1
26.4
29.8
7.6
8.9
IO.2
n.4
12.7
6.0
7-
8.
9
IO.
4.6
5.4
6.2
6.9
7-7
3.3
3. 9
4.4
5.o
5.5
2.1
2.5
2.8
3.2
3.5
i .0
1.2
i.3
i.5
!-7
II,OOO
I2,OOO
1 3,000
i4,ooo
i5,ooo
29.1
31.8
34.4
3 7 .i
39.7
27.4
2 9 . 9
32.4
34.9
37.3
22.3
24-4
26.4
28.4
3o.4
i4.6
:5. 9
17.2
i8.5
19.9
5.i
5.5
6.0
6.4
6.9
o
o
33.3
36. 9
4o.6
44.4
48.3
14.0
i5.3
i6.5
17.8
19.1
ii.
12.
i3.
i4.
i5.i
8.5
9.2
IO.O
10.8
ii. 5
6.1
6.6
7.2
7-7
8.3
3. 9
4.2
4.6
i.l
1.8
2.0
2.2
2.3
2.5
16,000
17,000
18,000
19,000
20,000
42.4
45.o
47-7
5o.3
53.o
39.8
42.3
44.8
4 7 .3
49-8
32.5
34.5
36.5
38.6
4o.6
21 .2
22.5
23.8
25.2
26.5
7-4
7.8
8.3
8.7
9.2
o
o
o
52.3
56.4
6o.5
64.8
69.2
20.3
21.6
22.9
24.1
2 5.4
16.1
17.1
18.1
19.2
2O. 2
12.3
i3.i
i3.8
i4.6
i5.4
8.8
9 .4
9.9
io.5
ii .0
5.6
6.0
6.3
6.7
7.0
2. 7
2.8
3.o
3.2
3.3
21,000
22,000
23,OOO
24,000
25,000
55.6
58.3
60.9
63.6
66.2
52.3
54.8
5 7 .3
59.8
62.2
42.6
44-7
46.7
48. 7
50.7
2 7 .8
29 . I
3o.5
3i.8
33.i
9-7
IO. I
10.6
II.
ii. 5
o
o
o
o
7 3.6
7 8.2
82.9
87.6
9 2.5
26.7
28.0
29.2
3o.5
3i.8
21.2
22.2
23.2
24.2
25.2
16.1
16.9
17.7
i8.5
19.2
ii. 6
12. I
12.7
13.2
i3.8
7-4
7-7
8.1
8.4
8.8
3.5
3. 7
3.8
4.o
4.i
262 TABLE X. MEAN HEIGHT OF THE BAROMETER, ETC.
TABLE X.
MEAN HEIGHT OF THE BABOMETEB IN THE DIFFERENT MONTHS.
George-
town.
Havana.
Natchez.
St.Loui3.
Philadel-
phia.
Boston.
Toronto.
Port
Bowen.
Renn.
Harbor.
Jan. . .
29.942
30.129
29.779
29.602
29.961
29.976
29.618
29.716
29.773
Feb...
.966
29.928
. 7 33
.586
.908
. 9 5 7
.614
.886
.843
March
.967
.960
. 7 23
.55 9
.942
.886
.622
30.107
745
April.
.945
.906
.664
.490
.924
.897
.65 7
.068
.898
May..
. 9 33
.85 1
.623
.465
.886
.907
.565
.o5i
. 9 3 7
June .
.962
.948
.608
.478
.891
.878
.5 77
29.888
. 7 i4
July..
.971
.948
.646
.523
. 9 i5
.936
.58 9
.817
736
Aug. .
.954
.816
.624
.542
.943
.965
.638
.683
.689
Sept. .
.943
.821
?49
.56i
.971
'.994
.647
.689
.653
Oct. . .
9l4
.85i
.722
.588
.949
9 64
.663
.784
. 7 5o
Nov...
.885
.971
.765
.588
.941
. 9 55
.626
.899
. 7 53
Dec. . .
.910
3o.o65
.816
.601
9 5 9
. 9 53
.643
.869
-748
Year
2 9 . 9 3 9
29.933
29 .696
29.548
2 9 . 9 32
2 9 . 9 3 9
29.621129.886
29.766
Chris-
tianborg.
Aden.
Cairo.
Constan-
tinople.
Paris.
Green-
wich.
St. Pe-
tersburg.
Arch-
angel.
Ham-
merfest.
Jan. . .
29.829
29.823
3o.Ol6
30.009
29.877
29.760
So.022
29.728
29.606
Feb...
.84o
.844
.016
29.982
.886
.8o4
.o44
743
.456
March
.8 7 4
.776
29.899
3o.oi4
-111
7 6 9
29.961
.698
.704
April .
9 3 9
.701
.926
29. 833
. 7 3 2
.760
.969
.767
.816
May..
.971
.610
.85 2
.899
749
.771
. 9 58
774
.888
June .
.968
.528
.702
.852
.8*4
.787
.015
.690
.818
July..
.920
.482
.682
.845
. 7 85
.801
.853
.677
.816
Aug. .
.882
.512
.688
.860
794
789
.919
.672
.771
Sept. .
.862.
.632
.792
. 9 5i
.788
.819
.969
. 7 63
. 7 4o
Oct. . .
.849
.778
.910
3o.o33
.702
.696
. 9 54
. 7 i3
.658
Nov...
.862
.876
. 9 5i
.io4
. 7 54
.754
.845
.680
.684
Dec. . .
.838
.890
994
.oi4
.729
.821
. 9 3i
.676
.568
Year
29.886
29.704129.856
29.943
29. 782
29.777
29.944
29.711
29.710
Singa-
pore.
Madras.
Bombay.
Canton.
Benares.
Pekin.
Tiflis.
Nerts-
chinsk.
Jakutsk.
Jan. . .
29.917
29.999
29.941
3o. 176
29.741
3o.4 9 8
28.64o
27.989
29.896
Feb...
. 9 i5
.970
.925
.099
.642
.429
.627
.960
.868
March
.884
.890
.872
.018
.5 7 5
.254
.534
.8 7 4
74 9
April.
.886
.828
799
29.849
.423
.091
477
744
.620
May..
.872
.709
.752
.761
,33i
29.940
.468
.5 97
4 7 2
June .
.858
.688
.649
. 7 3o
.179
.788
.421
.690
.366
July..
.868
.711
.656
.656
.161
746
.392
.56i
.383
Aug. .
.880
747
.722
.65 9
.265
.860
446
.65o
.435
Sept. .
.886
. 7 83
.791
.5 9 6
.370
30.079
.555
. 7 58
.711
Oct. . .
.898
.863
.833
.911
.543
.268
.662
.858
.670
Nov...
.866
943
.896
30.071
.649
.410
.689
.898
.828
Dec. . .
.-684
.964
944
.123
744
.482
.628
.901
3o.o6o
Year 129.884129.842
29.816
2 9 .8 9 5
29.468
3o.i54
28.538
27 . 782^9 . 679
TABLES XI, XII. MEAN HEIGHT OF THE BAROMETER, ETC. 263
TABLE XL
MEAN HEIGHT OF THE BAROMETER FOK ALL HOUBS OF THE DAY.
Equator.
Cumana.
Calcutta.
Philadel-
phia.
Toronto.
Padua.
Green-
wich.
St. Pe-
tersburg.
Van
Renns'r.
Midnight
1A.M.
2
3
4
5
29.626
.616
.5 9 8
.692
.58o
.693
29.798
. 7 85
.772
.760
. 7 5i
.766
29.876
.868
.866
.863
.862
.861
29.941
.940
938
936
938
94l
29.616
.6l5
.6i5
.6i5
.616
.621
29.804
.800
.798
79 5
794
794
29.785
.778
773
77
.768
.768
29.957
. 9 55
.954
.962
.952
. 9 5i
2976b
.768
. 7 6f
.766
. 7 66
. 7 66
6
7
8
9
10
11
.6o5
.626
.644
.662
.652
.638
.771
.787
.8o3
.8x5
.816
.804
.870
.888.
.$17
.926
.gSo
.926
. 9 5l
. 9 6l
.967
.970
.968
.960
.63i
.63 9
.646
.648
.648
.64i
.796
.800
.8o4
.807
.810
.807
- -771
777
.783
.789
.792
.791
.953
.954
9 5 7
.960
.961
.961
. 7 66
. 7 66
. 7 62
. 7 62
7 64
7 64
Noon
1P.M.
2
3
4
5
.621
.602
.689
.5 7 3
.568
.579
.787
.764
744
. 7 3i
. 7 23
. 7 3i
.906
.891
.85 9
.848
.83 9
.84o
.943
.927
. 9 l5
.908
.908
.910
.629
,618
.608
.6o5
.6o3
.6o4
.806
.798
.791
.786
. 7 83
.782
.786
.781
.776
774
.772
.771
. 9 5 9
.966
. 9 55
.954
. 9 53
. 9 53
. 7 63
7 5 9
7 5 9
.761
.763
.765
6
7
8
9
10
11
.5 9 5
.6o4
.617
.636
.64o
.64o
.741
.756
.772
.788
799
.810
.843
.844
.865
.892
.8 9 5
.886
.916
.925
.934
. 9 4i
.945
.950
.608
.611
.616
.620
.620
.620
.784
.789
.796
.801
.8o5
.8o5
774
.780
. 7 85
.788
.790
.789
.964
.955
9 5 7
9 5 7
958
.958
.767
.768
.769
.770
.771
.769
Mean
29.616
29.777
29.877
29.939
29.621
29.797
29.780
29.966
29.766
TABLE XII.
DEPRESSION OF MERCURY IN GLASS TUBES.
Diameter
of Tube.
Ivoiy.
Young.
Laplace.
Poisson.
Caven-
dish.
Pouillet.
Daniell,
Boiled
Tubes.
Diameter
of Tube.
Inch.
Inch.
Inch.
Inch.
Inch.
Inch.
Inch.
Inch.
Inch.
o.o5
0.2949
0,2964
O.
0.2796
O.
O.
O.
o.o5
.10
.i4o4
.i4a4
.i3 9 4
.i36 7
. i4o
. 1390
.070
. 10
.i5
.o865
.0880
.o854
.o83o
.092
.0868
.o44
.15
.20
.o583
.0589
.o58o
.o559
.067
.o58o
.029
.20
.25
.0409
.0404
.O4l2
.0394
.o5o
.0407
.020
.25
.3o
,0293
.0280
.0296
.0281
.o36
.0296
.014
.3o
.35
.0212
.0196
.O2l6
.0204
.025
.0216
.010
.35
.4o
.01 54
.0139
,0l59
.0149
.oi5
.0169
.007
.4o
.45
.0112
.0100
.0117
.OIO9
.010
.01 17
.oo5
.45
.5o
.0082
.0074
.0087
.0080
.007
.0086
.oo3
.5o
.60
.oo43
.0045
.oo46
.oo4i
.006
.0047
.002
.60
.70
.OO23
.0024
.0020
.0026
.70
.80
.0012
.ooi3
.OOIO
.ooi3
.86
264 TABLE XIII. DRY AND SATURATED AIR COMPARED.
TABLE XIII.
TO COMPABE THE WEIGHT OF A CUBIC FOOT OF DRY AIR AND OF SAT-
URATED AIR.
Temp.
Dry Air.
Saturated.
Excess.
Temp.
Dry Air.
Saturated.
Excess.
Degrees.
Grains.
Grains.
Grains.
Degrees.
Grains.
Grains.
Grains,
O
6o3^2I
602.77
0.44
45
548.16
546.06
2. IO
I
601.87
601 .4o
0.47
46
547. o5
544-88
2.17
2
6OO.52
600. o3
0.49
47
545.97
543.75
2.22
3
599.20
598.69
o.5i
48
544-85
542.55
2.30
4
5 97 .8 7
597.34
o.53
49
543. 7 5
54i.36
2.39 .
5
5 9 6.55
596.01
o.54
5o
542.65
54o.2i
2.44
6
5 9 5.24
594.69
o.55
5i
54i.55
539.04
2.5l
7
593.94
593.36
o.58
52
540.48
537.87
2.61
8
592.63
592.04
0.59
53
539.41
536. 71
2.70
9
591.33
590.72
0.61
54
538.33
535.55
2.78
10
590.04
589.40
o.64
55
53 7 .2 7
534.39
2.88
ii
588. 7 5
588.07
0.68
56
536.i 9
533.22
2.97
12
58 7 .48
586. 7 8
0.70
57
535.12
532.o6
3.o6
i3
586.21
585.49
0.72
58
534.o 7
530.92
3.i5
i4
584-93
584.i8
0.75
5 9
533.o3
529.77
3.26
i5
583. 67
582.89
0.78
60
53i. 97
528.62
3.35
16
582.4i
58i.6i
0.80
61
53o.93
527.48
3.45
r 7
58i.i5
58o.33
0.82
62
529.88
526.32
3.56
18
579.91
579.06
o.85
63
528.84
525. 17
3.67
'9
578.67
577.79
0.88
64
527.81
524-o3
3. 7 8
20
577.44
5 7 6.54
0.90
65
526.78
522.90
3.88
21
5 7 6.2i
575.27
0.94
66
5 2 5. 76
52i. 7 5
4.01
22
574.98
574.01
0.97
67
524.75
520. 61
4.i4
23
5 7 3. 7 6
572.76
.00
68
523.72
519.46
4.26
24
5 7 2.55
571 .5o
.o5
69
522.70
518.29
4-4i
25
5 7 i.33
570.26
.07
70
521.70
517.17
4.53
26
570.13
569.01
. 12
7 1
520.70
5i6.o2
4.68
27
568.92
567.77
.i5
72
519.69
514.87
4.82
28
56 7 . 7 3
566.53
.20
73
518.70
5i3. 7 5
4-95
29
566.54
565.3i
.23
74
517.70
5i2.6i
5.09
3o
565.35
564.o8
.2 7
75
516.71
5i.i.46
5. 2 5
3i
564.17
562.86
.3i
76
5i5.73
5io.3s
5.4i
32
563.oo
56i.64
.36
77
5i4.74
Sog.iS
5.56
33
56i.84
56o.42
.42
78
5i3. 77
5o8.o4
5. 7 3
34
560.67
559.20
4 7
79
5i2.8o
506.91
5.89
35
55 9 .5i
558.01
.5o
80
5 1 1. JB2
5o5.74
6.08
36
558.35
556.79
.56
81
510.87
5o4.6i
6.26
3?
557,21
555.6i
.60
82
509.89
5o3.45
6.44
38
556.o5
554.4o
.65
83
508.93
502.32
6.61
3 9
554.91
553.20
7 1
84
507.97
5oi . 16
6.81
4o
553.77
552. oo
-77
85
507. o3
5oo.o5
6.98
4i
552.65
55o.8o
.84
86
506.07
498.87
7.20
42
55i.52
549-63
.89
87
SdS.ii
497.71
7-4o
43
55o.39
548.44
. 9 5
88
5o4.i9
4 9 6.58
7.61
44
549.27
547.26
2.OI
89
5o3.25
495.44
7.8!
45
548'. 1 6
546.06
2. IO
9
502.32
494.28
8.o4
TABLE XIV. HEIGHT OF BAKOMETEE, ETC.
265
TABLE XIV.
HEIGHT OF BAROMETER CORRESPONDING TO TEMPERATURE OF BOILING WATER.
Temp.
Barom.
Temp. Barom.
Temp.
Barom.
Temp.
Barom.
Temp.
Barom.
Inches.
O
Inches.
Inches.
Inches.
O
Inches.
188.0
18.197
igS.O
20.253
198.0
22.5oi
2o3.O
24.952
208.0
27.622
. I
.236
. I
.296
. I
.548
. I
25.oo3
. I
.678
.2
.276
.2
.33 9
.2
.595
.2
.o55
.2
. 7 33
.3
.3i5
.3
.382
.3
.642
.3
. 106
.3
.789
.4
.355
.4
.426
.4
.689
.4
.i58
.4
.845
.5
.3 9 5
.5
.46 9
.5
. 7 36
.5
.210
.5
.901
.6
.434
.6
.512
.6
.784
.6
.26l
.6
9 5 7
7
.474
-7
.556
7
.83i
7
.3i3
.7
28.oi3
.8
.5i4
.8
.5 99
.8
.879
.8
.365
.8
.069
9
.554
9
.643
9
.926
9
.4i 7
9
.126
189.0
5 9 4
194.0
.687
199.0
974
2O4.O
.469
209.0
.182
. i
.634
.1
. 7 3i
. i
23. O22
. I
.521
. i
.239
.2
.674
.2
. 77 5
.2
.070
.2
.5 7 3
.2
.295
.3
. 7 i4
.3
.819
.3
.118
.3
.626
.3
.352
.4
. 7 55
.4
.863
.4
.166
.4
.678
.4
.409
.5
79 5
.5
.907
.5
.214
.5
73o
.5
.466
.6
.835
.6
.961
.6
262
.6
. 7 83
.6
.5 2 3
.7
.876
7
.996
7
.3n
7
.836
7
.58o
.8
.917
.8
21 .o4o
.8
.35 9
.8
.888
.8
.63 7
9
9 5 7
9
.084
9
.407
9
.941
9
.6 9 5
190.0
.998
igS.o
. 129
200. o
.456
2O5.O
994
2IO.O
.752
. i
19.039
.1
.i 7 4
.1
.5o5
. I
26.047
.1
.810
.2
.080
.2
.218
.2
.553
.2
. IOO
.2
.867
.3
.121
.3
.263
.3
.602
.3
.i53
.3
.925
4
. l62
.4
.3o8
4
.65i
.4
.206
.4
. 9 83
.5
.203
.5
.353
.5
.700
.5
.259
.5
29.041
.6
.244
.6
.3 9 8
.6
.749
.6
.3i3
.6
.099
7
.285
7
.443
7
.798
7
.366
7
.i5 7
.8
.326
.8
.488
.8
.847
.8
.420
.8
.215
9-
.368
9
.533
9
.897
9
.473
9
.274
191.0
.409
196.0
.5 7 8
2OI .O
946
206.0
.527
211 .O
.332
.1
.45o
. i
.623
. I
.996
. i
.58i
. I
.391
.2
.492
.2
.669
.2
24.o45
.2
.635
.2
449
.3
.534
.3
. 7 i4
.3
.095
.3
.689
.3
.5o8
.4
.5 7 5
.4
.760
.4
.145
4
743
.4
.56 7
.5
.617
.5
.806
.5
.i 9 5
.5
797
.5
.626
.6
.65 9
.6
.85i
.6
.24?
.6
.852
.6
.685
7
.701
7
.897
7
.295
7
.906
.7
744
.8
743
.8
.943
.8
.345
.8
.961
.8
.8o3
9
. 7 85
9
.989
9
.3 9 5
9
27.015
9
.863
192.0
.827
197.0
22.035
202. o
.445
207.0
.070
212.0
.922
. i
.869
. i
.081
.1
.495
. i
.125
.1
.982
.2
.912
.2
.128
.2
.546
.2
.180
.2
So.otfi
.3
.954
.3
.174
.3
.5 9 6
.3
.235
.3
. IOI
.4
.996
.4
.221
.4
.647
.4
.290
.4
.161
.5
20.039
.5
.267
.5
.697
.5
.345
.5
.221
.6
.082
.6
.3i4
.6
748
.6
.4oo
.6
.281
7
. 124
7
.36i
7
799
7
.456
7
.341
.8
.167
.8
.407
.8
.85o
.8
.5n
.8
4oi
9
.210
9
.454
9
.901
9
.566
9
.462
266 TABLE XV. DIURNAL VARIATION OF TEMPERATURE.
TABLE XV.
DIURNAL VABIATION OF TEMPERATURE AT NEW HAVEN, CONNECTICUT.
Hours.
Jan.
Feb.
Mar.
Apr.
May.
June.
July.
Aug.
Sept.
Oct.
Nov.
Dec.
Year.!
O
Q
o
o
o
o
Midn'ht
2.3
2.9
3. 7
4.6
5.4
5.8
5.2
4-7
4.8
4.i
2.6
2.2
4.0
i
2.6
3.3
4.3
5.4
6.3
6.9
6.2
5.6
5.6
4.8
3.2
2.5
4.7
2
3.o
3.8
4.8
6.1
7.2
8.0
7.0
6.3
6.3
5.5
3.7
2.8
5.4
3
3.3
4.3
5.4
6.7
8.0
8.7
7 .5
6.8
6.8
6.0
4.i
3.2
5. 9
4
3. 7
4.8
5.8
7.3
8.5
8.9
7-7
7.2
7.2
6.5
4.5
3.5
6.3
5
4.i
5.2
6,2
7 .5
8.4
8.2
7-4
7-i
7 .3
6.8
4.8
3.8
6.4
6
4.3
5.3
6.1
7- 1
6.6
6.1
6.1
6.4
6.8
6.6
4.8
4.o
5. 9
7
4.4
5.i
4. 9
5.3
3.6
3.2
3. 7
4.i
4-7
5.3
4.5
4.o
4.4
8
3.8
3. 7
2.3
2.O
o.5
O.O
0.9
i.3
i7
2.S
3.o
3.2
2.1
9
i.3
o.5
-o.5
-I.I
-2. I
-2.7
-I .2
-I .2
-0.6
o.5
i .0
-o.-6
10
-1.6
-2.5
-3.2
-3. 9
-4.2
-4.7
-3.8
-3.4
-3.8
-3.5
-2.2
-1.6
-3.2
ii
-3.6
-4.2
-4. 9
-5.5
-5.8
-6.1
-5.6
-5.3
-5.6
-5.6
-4.2
-3.5
-5.o
Noon
-5.2
-5.6
-6.1
-6.8
-7.0
-7.1
-6.7
-6.5
-6.9
-7.0
-5.6
-5.o
-6.3
i
-6.1
-6.6
-7.0
-7-7
-7.9
-7.9
- 7 .5
-7.3
-7.7
-7.7
-6.4
-5.8
-7.1
2
-6.3
-6.9
- 7 .5
-8.3
-8.5
-8.3
-7.8
-7.7
-8.0
-8.1
-6.6
-6.1
- 7 .5
3
-5. 9
-6.8
- 7 .3
-8.3
-8.5
-8.2
-7.7
-7.6
-7.9
-7.7
-6.2
-5.5
4
-4. 7
-5.8
-6.6
-7.8
-8.0
-7.6
-7.2
-7.1
-7.1
-6.6
-4.6
-4.o
-6u
5
-2.8
-3.8
-4. 7
-6.6
-6.8
-6.5
-6.1
-5. 9
-5.8
-4.5
-2.9
-2.1
-4-9
6
-i.4
-2.O
-2.5
-4.o
-4. 7
-4.3
-4.2
-3. 9
-4.0
-2.8
-1.6
-I .O
-3.o
7
-o.3
-0.6
-0.9
-i.5
-1.6
-2.2
-2.O
-1.9
-1.9
-I .2
-o.5
-O.2
-I .2
8
0.5
0.4
0.6
0.6
0.6
O.I
-O. I
0.0
O.I
0.2
0.4
0.5
O.C
9
i.i
I .2
'7
2.O
2.2
1.8
1.6
1.6
'7
1.5
i . i
I.O
i.5
10
i.5
1.8
2.4
3.o
3.5
3.3
2.9
2.8
2.8
2.4
1.6
i.5
2.5
ii
1.9
2.4
3.o
3.8
4.4
4.6
4.i
3. 9
3. 9
3.3
2. I
1.8
3.3
Da. ext.
-I.O
-0.8
-0.6
-o.4
O.O
o.3
-O. I
-o.3
-o.3
-0.6
-0. 9
-i.i
-0.6
7-i
-0.8
-0.7
-i.i
-1.2
-2. I
-2.4
-1.9
-1.6
-i.5
-I .2
-0. 9
-0.9
-i.4
7.2
-i .0
-0.9
-i.3
-1.5
-2.4
-2.6
-2.1
-1.8
-1.6
-1.4
-I .O
-i . i
-1.6
8.1
-i .1
-i.4
-2.4
-2.8
-3.7
-4.0
-3.3
-3.o
-3.0
-2.7
-1.7
-i .3
-2.5
8.2
-1.3
-1.6
-2.6
-3.i
-4-o
-4.2
-3.5
-3.2
-3.2
-2.9
-1.8
-i.4
-2.7
9.8
0.9
o.5
O.O
-O.2
-^0.8
-i.3
-0.9
-0.6
-0.6
-O.2
0.4
0.7
-0.2
6.6
i.5
1.6
1.8
i.5
0.9
0.9
i .0
I .2
i.4
I. 9
1.6
i 5
i.4
7-7
2.0
2.2
2.0
1.9
i .0
o.5
0.8
I . I
i.4
2.0
2.O
1.9
1.6
8.8
2 . I
2. I
i.4
i .3
0.6
O.O
o.4
0.7
0.9
I .2
i-7
1.8
I .2
9.9
I .2
0. 9
0.6
0.4
0.0
-o.4
0.0
0.2
O.2
0.4
0.8
i .0
0.4
10.10
O.O
-0.3
-0.4
-0.4
-0.4
-0.7
-0.4
-0.3
-0.5
-0.5
-o.3
-O.I
-0.4
6.2.6
-I.I
-I .2
-i.3
-1.7
-2.2
-2.2
-1.9
-1.8
-1.7
-i.4
-i .1
-i .0
-1.6
6.2.8
-0.5
-0.4
-o.3
-O.2
-O.4
-0.7
-0.6
-o.4
-0.4
-0.4
-o.5
-o.5
-o.4
6.2.9
-o.3
-O.I
O.I
o.3
O. I
-O. I
O.O
O. I
O.2
O.O
-O.2
-o.4
O.O
6.2. IO
-O. I
O. I
o.3
0.6
o.5
0.4
0.4
o.5
o.5
o.3
-O. I
-O.2
o.3
7.2.9
-0.3
-O.2
-o.3
-o.4
-0.9
-i .1
-0.8
-0.6
-o.5
-0.4
-0.3
-0.4
-o.5
7.2.9.9
0. I
O.I
0.2
0.2
-0. I
-o.4
-O.2
-O. I
O.O
0.0
O.O
0.0
O.O
TABLE XVI. DIURNAL VARIATION OF TEMPERATURE. 267
TABLE XVI.
DIURNAL VARIATION OF TEMPERATURE AT GREENWICH, ENGLAND.
Hours.
Jan.
Feb.
Mar.
Apr. May.
June.
July.
Aug.
Sept.
Oct.
Nov.
Dec.
Year.
o
O
o
o
o
p
o
Midn'ht
1 .0
1.6
2. 9
4.8
5.4
6.2
5.o
5.i
4.0
2.9
i-7
0.9
3.5
i
0.9
1.8
3.o
5.2
6.0
7- 1
5.5
5.5
4.5
3.o
1.8
.0
3.8
2
.2
2.0
3.3
5. 7
6.4
8.0
6.0
6.0
5.5
3.4
2.0
.2
4.2
3
.3
2.1
3.6
6.2
6.7
8.7
6.4
6.3
6.4
3.6
2 .O
.3
4.5
4
.6
2.3
3. 9
6.6
6.7
9 .3
6.6
6.5
6.6
3.8
2. I
.4
4.8
5
.8
2.2
4.o
6.7
6.3
8.8
6.2
6.5
6.2
3.8
2.0
4
4-7
6
9
2.3
3. 9
6.0
4.8
6.4
4.5
5.5
5.3
3.5
I. 9
.4
3.9
7
2. I
3.6
4.3
2.6
3.o
2.5
3.3
4.o
2.8
i-7
.5
2.8
8
.5
1.6
2.5
2.0
o.5
o.o
o.o
0.9
2. I
1.6
i .0
.3
1.2
9
.0
0.7
0.2
-0.9
-2.0
-2.5
-2.0
-1.6
-0.4
o.o
0.4
0.9
-0.5
10
O.2
-0.5
-1.9
-3.2
-4.0
-4.5
-4.o
-3.5
-3.0
-2.O
-0.6
o.o
-2.2
ii
-1.3
-2. I
-3.5
-5.3
-5.5
-5-. 8
-5.4
-5.4
-5.o
-3.8
-2.O
-i.3
-3. 9
Noon
-2.3
-3.2
-5.o
-6.8
-6.7
- 7 .3
-6.4
-6.5
-6.4
-5.i
-3.1
-2.1
-5.i
i
-2.9
-3. 9
-5.8
-7-9
- 7 .5
-8.1
-6.7
- 7 .5
-7.1
-5.5
-3.5
-s.4
-5. 7
2
-3.o
-3. 9
-5.8
-8.2
-7.7
-8.6
-6.7
-7-7
-7.1
-4. 9
-3.6
-2.3
-5.8
3
-2.5
-3.6
-5.5
-7-7
- 7 .3
-8.4
-6.5
-7.0
-6.6
-3. 7
-3.o
-1.9
-5.3
4
-1.9
-2.8
-4.5
-6.7
-6.1
-7-4
-5.8
-5.5
-5.5
-2.8
-2. I
-i.3
-4.4
5
-i.i
-1.6
-3.3
-5.4
-4-8
-6.1
-4. 9
-3.6
-4.2
-1.7
-I .2
-0.8
-3.2
6
-0.6
-0.6
-1.8
-3.5
-3.o
-4.5
-3.5
-2.O
-2.5
-0.8
-0.4
-0.4
-2.O
7
-0.3
o.3
-0.4
-i.i
-I .0
-2.4
-i.5
-0.5
-0.6
0.0
O. I
-O.I
-0.6
8
O. I
0.6
0.9
0.7
0.9
o.o
o.3
I.O
I.O
0.7
0.6
0.2
0.6
9
0.4
i .0
!-7
2.O
2.3
1.8
1.9
2.4
1.8
i.3
I.O
0.4
i.5
10
0.6
i.3
2.3
3.2
3.5
3.6
3.3
3.3
2.7
1.9
i.3
o.5
2.3
ii
0.7
1.5
2.6
4-1
4.5
5.o
4.2
4.3
3.4
2.4
i.5
0.8
2.9
Da. ext.
-0.5
-0.8
-0.9
-0.7
-o.5
o.3
-O.I
-0.6
-o.3
-0.8
-0.7
-o.4
-o.5
7- 1
-o.5
-0.9
-1 .1
-1.8
-2.4
-2.6
-2. I
-2. I
-i.5
-1.4
-0.9
-o.4
-i.5
7.2.
-o.5
-0.9
-I . I
-1.9
-2.5
-2.8
-2.1
-2.2
-i.5
-i .0
-0.9
-o.4
-i.5
8.1
-0.7
-1.6
-2.9
-3.5
-4.o
-3.4
-3.3
-2.5
-1.9
-i.3
-o.5
-2.2
8.2
-0:7
-i . i
-1.6
-3.i
-3.6
-4.3
-3.3
-3.4
-2.5
-1.7
-1.3
-o.5
-2.3
9.8
o.5
0.6
o.5
-O.I
-o.5
-I .2
-o.-8
-o.3
o.3
o.3
o.5
o.5
0.0
6.6
0.6
0.9
I.O
I .2
0.9
0.9
o.5
i-7
i.4
1.3
0.8
o.5
0.9
7-7
0.8
I .2
1.6
1.6
0.8
o.3
o.5
i.4
i-7
i.4
0.9
0.7
I . I
8.8
0.8
I.I
?
1.3
0.7
o.o
O. I
0.9
i.5
i . i
0.8
0.8
0.9
9.9
0.7
0.8
0.9
o.5
O.I
-d.3
0.0
0.4
0.7
0.6
0.7
0.6
o.5
10. 10
o.4
0.4
O.2
o.o
-O.2
-o.4
-0.4
-O.I
-O.I
o.o
o.4
O.2
o.o
6.2.6
-0.6
-0.7
-1.2
-1.9
-I. 9
-2.2
-1.9
-i.4
-i.4
-0.7
-0.7
-0.4
-i.3
6.2.8
-0.3
-o.3
-o.3
-o.5
-0.7
-0.7
-0.6
-0.4
-0.3
-O.2
-0.4
-0.2
-o.4
6.2.9
-0.2
-O.I
-O. I
-O.I
-O.2
O.I
-O. I
o.o
o.o
O.O
-O.2
-0.2
O. I
6.2.10
-O.2
-O. I
O. I
o.3
0.2
o.5
o.4
o.3
o.3
O.2
-O. I
-O.I
O. I
7.2.9
-O.2
-O.2
-0.2
-0.6
-0.9
-I .2
-0.8
-0.7
-o.4
-0.2
-0.3
-O. I
-o.5
7.2.9.9
-O. I
O. I
o.3
o.o
-0. I
-0.5
-O. I
-O. I
O. I
O.I
o.o
O.O
0.0
268
TABLE XVII. MEAN TEMPERATURES
TABLE XVII.
MEAN TEMPERATURES FOR EACH MONTH, SEASON, AND THE YEAR.
Place.
Lat.
Long.
Alt.
Jan.
Feb.
March.
April.
Paramaribo, Dutch Guiana . .
St Vincent West Indies
/
5 44
i3 10
17 58
19 12
19 25
o /
55 i3
60 3i
7 6 5o
96 9
99 6
Feet.
5o
5o
6990
o
7 8.2
80.0
7 5. 7
70.0
52.5
o
78.0
79.3
76.0
71.6
56.4
o
78.9
79-7
75.9
7 3.4
61.1
o
79.2
8i.3
78.1
77.2
63.o
Mexico City
23 9
24 32
29 18
29 48
29 5 7
82 23
81 4 7
94 4 7
81 35
90 o
60
IO
20
IO
7 i .4
68.1
54-2
5 7 .o
54.8
7 4.o
69.4
60.2
5 9 . 9
56.4
74.i
72.7
69.2
63.3
62 .9
76.6
7 5.3
71.6
68.8
69.0
Key West, Florida
Galveston Texas ... ...
St Augustine Florida
New Orleans, Louisiana
Bermuda, Atlantic Ocean. . . .
San Diego California
32 2O
32 42
32 46
35 4i
3 7 32
64 5o
n 7 i3
79 56
106 i
77 27
i5o
20
6846
120
56.8
5l.Q
5o.3
3i.4
33. 7
58.8
53.3
52.4
33.2
39.8
5 9 .4
56.o
58. 7
40.7
47.1
62.8
61.2
65.4
5i.3
54.7
Charleston, South Carolina. . .
Santa Fe New Mexico
Richmond, Virginia
San Francisco, California ....
3 7 48
38 3 7
38 53
3 9 6
3 9 18
122 26
90 1 5
77 o
84 3o
76 3 7
100
45o
80
.543
36
4 9 .8
32. 9
34.1
33.i
32.8
52.3
35.o
36. 7
34.i
34.2
53. 7
44.4
45.3
43.5
42.3
57.0
58.3
55. 7
54.i
52.7
Washington DC
Cincinnati Ohio .
Baltimore, Maryland
Philadelphia, Pennsylvania . .
New York City, New York . .
Salt Lake City, Utah
3 9 58
4o 43
4o 46
4i 18
4i 3i
7 5 10
7 4 o
112 6
72 55
81 5i
4o
23
435i
5o
660
3i.8
30.2
2 7 . I
26.5
26.2
3a.3
3o.4
34.o
28.1
29.0
4i .0
38.3
3 9 . 7
36.i
35.6
5i.8
48.6
5o.2
46.8
47-7
New Haven, Connecticut ....
Cleveland Ohio
4i 54
42 12
42 2O
42 21
42 53
8 7 38
io4 48
83 2
71 3
78 55
691
45i 9
58o
5o
600
23.6
3i.o
2 7 .O
2 7 .8
23.4
24.7
32.6
26.6
27.9
21 .1
32.3
36.8
35.4
36.2
35.5
46.i
47.6
46.3
46.4
40.7
Fort Laramie Dacotah
Detroit Michigan
Buffalo New York
Toronto, Canada . . . .'
43 4o
44 3 9
44 53
46 ii
46 3 9
79 23
63 3 7
93 8
123 48
84 43
34i
20
820
5o
600
24.3
22.6
i3. 7
43.o
I 7 .2
23.1
23. 7
17.6
43.6
16.2
3o.4
30.9
3i.4
45.7
25.1
4i.3
38. 9
46.3
52.8
38.3
Halifax, Nova Scotia
Fort Snelling, Wisconsin ....
Astoria, Oregon
Fort Brady, Michigan
Quebec, Canada
46 49
4 7 33
53 5 7
5 7 3
5 7 10
7 I 12
52 28
IO2 I 7
i35 18
61 5o
3oo
i4o
9 oo
5o
5o
10.4
23.3
- 7.0
3o.o
- 2.9
i3.8
20.9
- 4.6
3o. 7
- 0.7
26.4
24.2
15.2
34.i
7.6
3 9 .o
33.4
3i.o
3 9 . 9
22.7
St. Johns, Newfoundland ....
Cumberland House
Sitka, Aliashka
Nain, Labrador
Godhaab, Greenland
64 i o
65 12
69 5 9
7 4 4 7
78 3 7
52 24
123 i3
92 i
no 48
7 o 53
5oo
5
12.4
-22.3
-28.7
-3i.3
-28.2
12.6
-16.7
-32.0
-3 2 .4
-26.4
i5.6
- 5.4
-28.7
-18.2
-34-9
22.0
12.4
- 2.6
- 8.2
-10.3
Fort Franklin, Brit. America .
Boothia Felix, Arctic Regions
Melville Island, Arc. Regions .
Van Rensselaer Harbor, A. R.
FOB EACH MONTH, SEASON, AND THE YEAR.
269
TABLE XVII.
MEAN TEMPERATURES FOR EACH MONTH, SEASON, AND THE YEAR.
May.
June.
July.
Aug.
Sept.
Oct.
Nov.
Dec.
Spring.
Summer.
Autumn.
Winter.
Year.
79-9
82.2
80.3
80.4
66.1
o
79 .5
82.2
80.6
81.9
65.4
80.0
82.2
81.7
8i.5
65.4
O
82.0
82.8
81.0
82.4
64-9
o
83.4
83.3
80.7
81.0
64.3
o
83.3
82.7
79 .8
7 8.4
60.2
o
81.5
82.1
78.7
75.4
55.8
O
79-7
8o.4
76.7
71.1
52. O
o
79.3
81.1
76,2
77.0
63.4
o
80.5
82.4
7 8.1
81.9
65.2
O
82.7
82. 7
8l.I
7 8.3
60. 1
78.6
79-9
79-7
70.9
53.6
8o.3
8l.5
78.8
77.0
60.6
78.0
79.0
81.1
7 3.5
74.8
81 .0
8i.4
83.5
79 .3
79-9
8i.5
82.7
85.6
80.9
81.6
81.6
82.8
85. 9
8o.5
81.2
8o.4
81.6
82.7
78.6
7 8.5
7 8.8
77-7
68.6
71.9
6 9 .8
7 5.i
74-7
60. i
64.i
60.2
7 3.5
70. 7
5 7 .5
5 7 .2
56.o
76.2
7 5. 7
74.0
68.5
68.9
8i.3
82.3
85.o
8o.3
81.0
7 8.1
7 8.o
7 o.5
7 i.5
6 9 .3
73.0
69.7
5 7 .3
58.i
55. 7
77-2
76.4
71.7
69.6
68.7
69.1
62.7
?3.4
5 7 .i
65.4
7 3.2
67.4
79.0
68.8
7 3.8
7 5. 7
72. 7
81.7
72.6
77.6
76.6
73- 7
80.9
70.0
7 4.8
76.8
70.9
76.9
61.9
67.1
7 3.o
65.5
67.9
5i.3
5 7 .5
65.8
56. 9
3s!e
44.2
60.6
5i. 7
52.5
30.2
38.i
63.7
60.0
65.8
49.7
55. 7
7 5.2
71. a
80.6
70.4
7 5.4
71.9
64.4
68.1
5o.6
56.3
58.8
52.3
5i. 7
3i.6
3 7 . 2
67.4
62.0
66.6
5o.6
56.. 2
56.5
66.4
66.3
63.6
63.i
5 7 . 7
74.0
7 4.4
7i.4
7 i.6
58.8
7 8.5
7 8.3
7 6.5
76.7
5g.o
7 6.5
7 6.3
7 4.2
74- 7
5 9 . 9
68.7
67.7
66.0
67.8
59.8
55.4
56.7
53.2
55. 7
55.6
4o. 9
44.8
42.5
45.i
5i.3
33.6
3 7 .3
33.8
35.6
55. 7 .
56.4
55.8
53. 7
52. 7
58. 7
7 6.3
7 6.3
7 4-o
74.3
58.4
55.o
56.4
53. 9
56.2
5l.2
33.8
36.i
33. 7
34.2
56.o
55.4
56.i
53.8
54.3
62.5
5 9 .3
63.o
5 7 .3
56.6
7 i.5
68.3
7 i.3
67.0
66.3
76.0
7 4.8
76.9
71.7
71.9
7 3.2
7 3.2
7 5.o
7 o.3
68.8
63.8
65.8
67.1
62.5
62 .4
54.5
54.5
55.6
5i.i
49-3
44.o
43.3
4i. 7
4o.3
3 7 . 9
34.5
33.5
3i.3
3o.4
2 9 .6
5i.8
48. 7
5i.o
46. 7
46.6
73.6
72.1
74.4
70.0
69.0
54.i
54.5
54.8
5i.3
49.9
32. 9
3i.4
3o.8
28.4
28.3
53.i
5i.-7
52. 7
49.0
48.5
56.3
56.i
56.o
56.5
55.3
62.7
6 7 .3
65.6
66.2
67.4
70.8
74-7
69.7
71.6
7 i.5
68.5
73.8
6 7 .5
69.4
70.0
60. i
64.2
60.0
62.2
5 9 . 9
48.5
50.9
47-7
5i.5
48. 7
3 7 . 9
35.8
38.2
4i .0
3 7 .2
2 9 .3
28.0
26. 9
3i.i
22.8
44-9
46.8
45.9
46.3
43.8
6 7 .3
71.9
6 7 .6
69. i
69.6
48.8
5o.3
48. 7
5i.6
48.6
25.9
3i.i
26.8
28.9
22.4
46. 7
5o.i
47.2
48. 9
46.i
bi.5
48.o
5 9 .o
55.o
49.3
6i.4
56.o
68.4
5 9 .5
58.4
66.8
62.0
7 3.4
61.6
64-7
66.3
64.4
70.1
63.o
62.9
58.i
58.4
58. 9
58. 7
54.6
45.2
48.o
4 7 .i
55.4
43.5
36.6
38.5
3i. 7
46.4
32.5
26.2
2 7'7
i6. 9
40.7
21.5
4i.i
3 9 .3
45.6
5i.i
3 7 .6
64.8
60.8
70.6
61.6
62.0
46.6
48 .3
45. 9
53. 7
43.5
24.5
24. 7
16.1
42.4
i8.3
44.3
43.2
44.6
52.2
4o.4
53.2
39.3
5i.3
46.o
32.8
64.5
48.o
58.8
5 2 .5
4i.8
69.0
56.2
61.8
55.i
48.2
68.1
5 7 . 9
5 9 .5
55.i
5i.i
56.8
53.o
45.8
5o.o
42.2
43-9
44.5
35.o
44.i
32.2
32. 9
34.0
17.2
37.7
22.3
i5.o
2 5.3
5.6
35. 9
3.4
39.5
32.3
32.5
4o.o
21.7
67.2
54.o
60.0
54.2
47.0
44.6
43.8
32. 7
43-9
32.2
i3.o
23.2
- 2.0
32.2
- 0.4
4o.6
38.3
3o.8
42.6
25.1
32.2
35.2
i5.6
:6.8
i3.4
39. i
48.o
34.2
36.2
3o.i
41.9
52. I
4i.3
42.4
38.2
4o.8
5o.6
38. 7
32.6
3i.8
35.6
4r.o
25.4
22.5
i3.4
29.8
22.5
9 .I
-2.8
-3.6
21 . 9
-O.I
- 5.4
-21.1
-21 . 9
i 7 .5
-10.9
-22.4
-21.6
-3i.i
23.3
i4-o
- 5.2
- 3.2
-10.6
4o.6
5o.2
38.o
3 7 .i
33.4
29. I
21 . I
9 *z
-o.5
-4.o
14.1
-16.7
-27.7
-28.4
-28.6
26.8
17.2
3. 7
1.2
-2.5
270
MEAN TEMPEKATUKE OF CERTAIN LOCALITIES.
TABLE XVm. PLACES WHOSE MEAN TEMPERATURE is ABOVE 80 FAH.
Place.
Latitude.
Longitude.
Tempera-
ture.
No. of
Years.
Niger Africa
/
5 9
O /
6
85.27
j
Maracaibo, South America ....
Kouka, Central Africa
10 43
i3 10
71 52
i4 3o
84. 7 5
83.63
I
2
Coburg Peninsula, Australia. . .
Pondichery, India
ii 5
ii 56
i32 i5
82.79
82 58
I
Calcutta, India
22 35
/y ^^
88 20
82 4i
/
Madras India
i3 4
80 10
81.94
2O
Samarang, Java
6 5o
i i o 3o
81 .87
I
Nagpoor India
21 8
81 5g
3
Rio Berbice, British Guiana . . .
St Vincent, West Indies
6 29
i3 10
/y i i
57 24
60 3i
8i.56
81 52
i
6
Porto Rico, West Indies
18 20
66 1 3
81 38
5
Guinea Africa
5 3o
81 38
St Domingo West Indies
18 29
70 o
81 29
St. Christopher, West Indies . . .
Bombay, India
17 44
18 56
64 49
72 54
81.27
8l .27
St Thomas West Indies
18 21
64 56
81 23
Anjarakandy India
1 1 4o
75 Ao
8 1 07
Cobbe Africa
i4 1 1
28 8
80 96
2
Colombo, Ceylon
6 57
80 o
80.75
I
Trincomalee Ceylon
8 34
8l 22
80 75
Demerara British Guiana
6 45
58 2
80 71
Para Brazil
i 28
AS 20
80 70
/
Singapore, Malacca
I 17
io3 5o
80 68
6
Upper Park Camp, Jamaica . . .
17 58
76 5o
80. 63
i
Fort Dundas Australia
II 25
l32 25
80. 63
i
Christiansborg, Africa
5 24
o 16
80.42
4
Paramaribo, Dutch Guiana. . . .
Benares India
5 44
25 18
55 i3
82 56
80. 3o
80.26
2
3
Kingstown West Indies
i3 8
60 37
8o.25
I
Cawnpore India
26 2O
80 22
80.21
I
Upper Esrvpt . .
y
26 o
33 4o
80. 10
I
TABLE XIX. PLACES WHOSE MEAN TEMPERATURE is BELOW 18 FAH.
Place.
Latitude.
Longitude.
Tempera-
ture.
No. of
Years.
Van Rensselaer Harbor
o /
78 37
o /
70 53
2.46
2
Melville Island
74 Al
1 10 48
4-1.24
I
Ustjansk, Siberia
70 55
i38 24
2.75
2
Port Bowen, Arctic Regions . . .
Boothia Felix, Arctic Regions. .
73 i4
69 5 9
88 56
92 i
3.53
3. 7 o
I
2
Igloolik, North America
69 21
81 53
5.55
I
Fort Hope, North America. . . .
Winter Island
62 32
66 ii
86 56
83 ii
6. 10
8.82
Nishne Kolymsk Siberia .
68 32
160 56
o.5o
2
Jakutsk, Siberia
62 2
129 44
ii.53
J 7
Fort Enterprise, North America
Karische Pforte, Nova Zembla .
Yucon, Russian America
64 20
70 36
66 o
n3 6
- 5 7 4 7
147 o
13.90
14.90
16.80
3
i
i
Matoshkin Schar, Nova Zembla
Fort Franklin, Great Bear Lake
Fort Churchill, Hudson Bay . . .
7 3 19
65 12
5g 2
57 20
123 i3
93 10
i6. 9 3
17.18
17.45
i
2
2
TABLES XX., XXI. MONTHLY RANGE OF TEMPERATURE. 271
TABLE XX. PLACES HAVING A SMALL MONTHLY RANGE OF TEMPERATURE.
Place.
Latitude.
Longitude.
Hottest
Month.
Coldest
Month.
Differ-
ence.
No. of
Years.
Coinmewine, South America. .
Buitenzorff Java ....
O '
5 38
6 3?
o /
54 42
i 06 49
O
79.2
77.8
O
77.0
75.2
2.2
2.6
2
3
Souttea Asia
81.2
78,5
2.7
I
Puerto d'Espana, S. America .
10 38
i 17
61 34
io3 5o
79.5
82.2
76.5
78.5
3.o
3.7
I
6
Kingstown St Vincent
i3 8
60 3?
81.8
78.1
* 7
i
Kandy Ceylon
7 17
80 4o
74.6
70.6
4.0
3
St Vincent West Indies
i3 10
60 3i
83.3
79.3
4.0
6
Caraccas, South America ....
Samarang Java
10 3i
6 5o
67 5
T no 3o
7 3.5
84.2
69.4
80. 1
4.i
4.1
i
i
Bogota South America . .
4 36
?4 1 4
61 .0
57.6
4.3
i
Tovar South America
10 3i
67 3o
66.0
61.5
4.5
i
Barbadoes West Indies
i3 4
69 37
80.6
76. 1
4.5
i
St. Bartholomew, West Indies .
La Guayra, South America. . .
Freetown West Africa
17 53
10 37
8 3o
62 54
67 7
i3 10
83.3
81.1
82.0
78.7
76.5
77.0
4.6
4.6
5.o
i
i
i
Batavia Java
60
106 53
80.0
75.0
5.o
i
Trevandrum, Hindostan
83?
7 4 5o
82.7
77-7
5.o
8
Raiatea Society Islands
1 6 4o
i56 16
80.9
75.7
5.2
i
Antigua, West Indies
17 8
61 48
81.9
7 6.5
5.4
i
Paramaribo, Dutch Guiana. . .
Guatemala, Central America. .
St. Thomas, West Indies
5 44
i4 36
18 21
55 i3
90 28
64 56
83.4
71.2
83. 7
78.0
65.8
78.2
5.4
5.4
5.5
2
I
I
Upper Park Camp, Jamaica . .
17 58
76 5o
83.o
77.5
5.5
I
TABLE XXI. PLACES HAVING A GREAT MONTHLY RANGE OF TEMPERATURE.
Place.
Latitude.
Longitude.
Hottest
Month.
Coldest
Month.
Differ-
ence.
No. of
Years.
Jakutsk, Siberia
/
62 2
o /
129 44
O
62.2
43.8
106 .0
17
Ustjansk Siberia
70 55
i38 24
52.7
38. Q
QI 6
4
Fort Churchill, Hudson Bay . .
Nertchinsk Russia
5g 2
5i 18
93 10
I I O 2O
58.o
64 o
28.0
21.3
86.0
85 3
I
i4
Udskoi Ostrog Siberia
54 3o
i 34 58
61 . o
21 . 6
82 6
i
Utkinsk, Russia ...
57 45
5o 22
73 .Q
6.0
80.8
i
Kirgis, Russia
5o o
60 o
67.5
i3.o
8o.5
i
Uralsk, Russia
5i ii
5l 22
78.4
O. 2
78.6
3
Fort Simpson, British America
Cumberland House, Br. Amer.
62 ii
53 5 7
121 32
IO2 17
63.5
61.8
-i3.5
13.2
77.0
75.0
2
I
Melville Island, Br. America. .
Fort Franklin, Great Bear Lake
Boothia Felix, Br. America. . .
Barnaul, Russia
74 4 7
65 12
69 5 9
53 20
no 48
123 i3
92 I
83 27
42.4
52.1
4i.3
67.5
32.5
22.3
32.0
5.6
74-9
74.4
7 3.3
73.i
I
I
3
6
Nischney Tugilsk, Russia ....
5 7 56
60 8
70.3
1.2
69.1
2
Bogoslowsk, Russia . . .
5o 45 '
5o 5o
66.0
3.i
6O . I
6
Tomsk, Russia
56 3o
y "V
85 10
65.3
3.5
68.8
5
Irkutsk Russia
52 17
io4 17
64.8
3.3
68.1
10
Igloolik, British America ....
Orenburg, Russia
69 21
5o 46
81 53
55 6
3 9 .i
67.2
28.2
0.8
6 7 .3
66.4
i
8
272
TABLES XXII., XXIII. TEMPERATURE, ETC.
TABLE XXII. PLACES HAVING SMALL ABSOLUTE RANGE OF TEMPERATURE.
Place.
Latitude.
Longitude.
Highest.
Lowest
Range.
Barbadoes, West Indies
Pulo Penang, Malacca Strait .
Curasao, South America ....
San Luis de Maranha, Brazil. .
Surinam, Dutch Guiana
/
13 5
5 25
12 6
2 3l
5 38
/
5 9 3 7
100 19
69 20
44 18
55 20
86
90
9i
92
90
7 2
7 <
7 5
76
70
i4
i4
16
16
20
La Guayra Venezuela
10 36
67 7
Q I
70
21
Cayenne Guiana
4 56
r' '
02 J7
8 7
65
22
Amboyno, E. Archipelago . . .
Tahiti South Pacific
3 4i
17 2Q
v * /
I2O iy
149 3o
9 1
QO
68
65
23
25
Maracaibo, Venezuela
10 43
71 52
99
70
2 9
Singapore Malacca ...
I 17
io3 5o
o5
66
20
Quito, Ecuador
o i4
78 45
72
43
2Q
Lima, Peru
12 3
77 8
86
5 7
2O
St. Helena, South Atlantic . . .
Port Louis, Isle of France . . .
Martinique, West Indies
15 55
20 10
i4 4o
5 43
5 7 3o
61 3
82
9 l
9 5
52
60
63
3o
3i
32
Trinidad, Caribbean Sea ....
St. Bartholomew, West Indies.
Paramaribo, Guiana
10 3 9
17 54
5 45
61 23
62 54
55 i3
9 3
97
o4
61
64
61
32
33
33
Funchal Madeira
32 38
16 56
85
5i
34
Vera Cruz Mexico . .
IO 12
06 o
06
61
35
Fort Dundas, Australia .....
* -
II 25
132 25
IOO'
63
37
TABLE XXIII. PLACES HAVING LARGE ABSOLUTE RANGE OF TEMPERATURE.
Place.
Latitude.
Longitude.
Highest.
Lowest.
Range.
Barnaul Asia
/
53 20
/
83 2 7
06
6 7
i63
Jakutzk Siberia
62 2
129 44
86
76
162
Nijnei Taguilsk, Ural Mts. . . .
Boo"oslowsk Ural Mts
5 7 56
5o 45
60 8
5o 5o
9 5
QI
60
63
i55
1 54
Fort Reliance, Brit. America. .
62 46
V "V
109 o
8j
70
i5i
Zlatouste Ural Mts
55 ii
59 45
88
57
i45
Nertchinsk Siberia . .
5i 18
119 20
94
5o
1 44
Catherinenburg, Ural Mts. . . .
Moscow, Russia
56 5o
55 45
60 34
3 7 34
9
o4
-48
47
142
i4i
Montreal Canada
45 3i
7 3 32
1 02
38
i4o
Lowville, New York
43 4 7
7 5 33
IOO
4o
i4o
Quebec Canada
46 4o
7 I 12
QO
4o
i3g
Fort Howard Wisconsin
44 3o
88 5
IOO
38
i38
Nijnei Kolymsk, Siberia
Enontakis, Lapland
68 3 2
68 3o
160 56
20 4 7
72
79
65
58
i3 7
i3 7
Kazan, Russia
55 48
4o 7
Q7
4o
i3 7
Fort Snelling, Wisconsin ....
Montgomery, New York ....
Tomea, Lapland
44 53
4i 32
66 27
93 10
74 o
23 55
IOO
io4
77
-3 7
33
58
i3 7
i3 7
i35
Lougan, Russia
48 35
39 21
101
33
1 34
Granville, New York
44 20
7 3 1 7
IO2
3i
i33
St. Louis, Missouri
38 3 7
QO l5
108
25
i33
Kinderhook, New York
42 22
73 43
IO2
3o
132
Chicago, Illinois
4i 53
87 3 7
I O2
3o
132
Albany, New York
42 3 9
7 3 44
99
32
i3i
TABLES XXIV., XXV. HEIGHT OF THE SNOW LINE, ETC. 273
TABLE XXIV.
HEIGHT OF THE SNOW LINE ABOVE THE SEA.
Mountains.
Latitude.
Height.
Mountains.
Latitude. Height.
/
78 N.
71 10
70 3o
67
65
62
60 55
5g 3o
59 3o
56 3o
5o
4545 '
43 20
43
4245
89 42
38 33
3? 3oN.
Feet.
o
2 400
3,I. 7 8
3,835
3,o8o
5,i55
4,470
5,249
5,423
3,5io
7,246
8,890
10,818
12,467
8,676
14,170
10,705
Q.485
Bolor Mountains . . .
Hindu Kho
'
37 3oN.
34 3o
3o
28
!9
i3 10
8 5
446
2 i5N.
o4i S.
16
18
33
42 3o
53 3oS.
Feet.
17,010
i5, 7 35
17,392
14,280
i4,868
i4,o65
14,920
i5,3 2 5
i5,38i
15,960
15,924
i7,25o
i8,524
14,708
6,000
3.707
North. Cape
Mountains of Norway
SuHtelma, Lapland. .
Iceland.
Himalaya, nort'
Himalaya, sout'
Cordilleras of
Mexico
Mountains of
Abyssinia
Sierra Nevada
of Merida
Volcano of Tol
Puraci, S. Amei
Nevados of Qiu
Cotopaxi
i side
i side
ima . .
ica. .
to. . .
Mountains of Norway
Aldan Mount- )
ains, Siberia j * * * *
Kamtschatka
Mountains of Norway
Unalaschka, W. )
America J * ' '
Altai Mountains. . . .
Alps .
Caucasus
Rocky Mountains. . .
Pyrenees
Arequipa, Bolivia . . .
Paachata, Bolivia. . .
Portillo, Chili
Ararat
Mount Argaeus
Cordilleras, Chjli . . .
Magellan Strait .
Etna .
TABLE XXV.
FACTORS FOR MULTIPLYING THE EXCESS OF THE DRY-BULB OVER THE WET-
BULB THERMOMETER, TO FIND THE EXCESS OF THE TEMPERATURE OF THE
AIR ABOVE THAT OF THE DEW-POINT.
t
,e
,0 .
(
5
^
33
-? 1
|
3 a
? S
|
if
|
a S
1
Is
|
"3 p
'o^
Q^
1
Q^
1
I-N
SP
1
Q
1
o
IO
8.78
25
6.53
4o
2.29
55
.96
70
77
85
.65
II
8.78
26
6.08
4i '
2.26
56
94
7i
.76
86
.65
12
8.78
27
5.6i
42
2.23
57
.92
72
. 7 5
87
.64
i3
8. 77
28
5.12
43
2.20
58
.90
73
74
88
.64
i4
8.76
29
4.63
44
2.18
5 9
.89
74
. 7 3
89
.63
i5
8. 7 5
3o
4.i5
45
2.l6
60
.88
75
.72
9
.63
16
8.70
3i
3.70
46
9.14
61
.87
76
7 1
9 1
.62
17
8.62
32
3.3 2
47
2.12
62
.86
77
.70
9 2
.62
18
8.5o
33
3.oi
48
2.10
63
.85
78
.69
9 3
.61
I 9
8.34
34
2.77
49
2.08
64
.83
79
.69
94
.60
20
8.i4
35
2.60
5o
2.06
65
.82
80
.68
95
.60
21
7.88
36
2.5o
5i
2.O4
66
.81
81
.68
96
.5 9
22
7.60
37
2.42
52
2. O2
67
.80
82
.67
97
.59
23
7.28
38
2.36
53
2.OO
68
79
83
.67
98
.58
24
6.92
3 9
2.32
54
I. -98
69
.78
84
.66
99
.58
274
TABLE XXVI. RELATIVE HUMIDITY OF THE AIR.
TABLE XXVI.
RELATIVE HUMIDITY OP THE AIR.
Temp.
of Air.
"~6~
8
9
IO
Difference of Temperature of the Air and of the Dew Point.
100
IOO
IOO
IOO
IOO
1
2
3|4
5
~8o"
80
80
80
80
6
7 6
7 6
7 6
7 6
7 6
^1
72
73
73
73
73
8
6 9
6 9
69
69
7 o
9
66
66
66
66
66
10il2
14 16 18
20|22i24
9
96
96
96
96
9 1
9 1
9 1
9 1
9 1
87
8 7
8 7
87
8 7
83
83
83
83
83
63
63
63
63
63
I 1
5 7
5 7
58
58
52
52
52
52
52
47
47
47
48
48
43
43
43
43
43
39
3 9
39
3 9
3 9
35
35
35
35
36
32
32
32
32
32
II
12
i3
i4
i5
IOO
IOO
IOO
IOO
IOO
96
9 5
95.
9 <
9 5
91
9 1
9 1
9 1
91
8 7
87
8 7
87
87
83
83
83
83
83
80
80
80
80
80
7 6
7 6
7 6
76
7 6
76
76
76
76
7 6
73
73
73
73
73
7 o
70
7
7
7
70
70
69
9
69
66
66
66
66
67
63
63
63
64
64
58
58
58
58
58
53
53
53
53
53
48
48
48
48
48
43
44
44
44
44
3 9
4o
4o
4o
4o
36
36
36
36
36
33
33
33
33
33
16
I?
18
J 9
20
IOO
IOO
IOO
IOO
IOO
9 <
95
9*
9
96
9 1
9 1
9 1
9 1
9i
87
87
87
87
8 7
83
83
83
83
83
80
80
80
80
80
73
73
73
73
73
66
66
66
66
66
64
64
63
63
63
58
58
58
58
58
53
53
53
53
53
48
48
48
48
48
44
44
44
44
44
4o
4o
4o
4o
4o
36
36
36
37
3 7
33
33
33
33
33
21
22
23
24
25
IOO
IOO
IOO
IOO
IOO
96
96
96
9
96
9'
9 1
9 1
9 1
9 1
8 7
8 7
8 7
87
88
83
83
83
84
84
80
80
80
80
80
~8o~
80
80
81
81
"87
81
81
82
82
76
76
7 6
76
j76_
77
77
77
77
77
77
78
78
78
78
73
73
73
73
73
73
73
74
74
74
74
74
75
75
75
69
70
70
70
7
70
70
70
70
7 1
7i
7i
7i
72
72
66
66
66
67
67
67
67
67
67
68
"68~
68
68
69
69
63
63
64
64
64
64
64
64
64
65
"65"
65
65
66
66
58
58
58
58
58
53
53
53
53
53
48
48
48
48
48
44
44
44
44
44
4o
4o
4o
4o
4o
3 7
37
37
37
37
33
33
34
34
34
26
27
r>
2o
29
3o
IOO
IOO
IOO
IOO
IOO
96
96
96
9
96
92
92
92
92
9 2
92
92
92
92
92
88
88
88
88
88
84
84
84
84
84
58
58
< 9
< 9
5 9
53
53
53
54
54
49
49
49
4 9
49
44
44
45
45
45
4o
4i
4i
4i
4i
37
37
3 7
3 7
3 7
34
34
34
34
34
3i
32
33
34
35
IOO
IOO
IOO
IOO
IOO
9
96
96
96
96
88
88
89
89
89
84
85
85
85
85
5 9
60
60
60
60
54
54
55
55
55
49
5o
5o
5o
5i
45
45
46
46
46
4i
4i
42
42
42
37
38
38
38
38
34
34
35
35
35
36
37
38
3 9
4o
IOO
IOO
IOO
IOO
IOO-
9
96
96
96
96
96
96
96
96
96
92
92
92
92
92
89
89
89
89
89
89
89
8 9
89
89
89
89
89
89
89
85
85
85
85
86
"86"
86
86
86
86
~86~
86
86
86
86
82
82
82
82
82
"87
82
82
83
83
~83~
83
83
83
83
79
79
79
79
79
79
79
79
79
80
IkT
80
80
80
80
75
76
76
76
76
76
76
76
76
j76_
77
77
77
77
77
72
72
73
73
73
69
69
70
70
70
66
67
67
67
67
61
61
61
62
62
~6T
62
62
63
63
56
56
56
56
57
5 7
57
58
58
58
5i
5i
5i
52
52
46
47
47
47
48
42
43
43
43
43
39
39
39
3 9
4o
4o
4o
4i
4i
4i
35
36
36
36
36
~36~
37
3 7
37
38
4i
42
43
44
45
IOO
IOO
IOO
IOO
IOO
9 3
9 3
9 3
9^
9 3
73
73
73
73
74
74
74
74
74
74
70
70
7i
7i
7 1
7i
7i
7i
7 1
7i.
67
68
68
68
68
52
53
53
53
53
"54"
54
54
54
54
48
48
48
49
49
49
49
5o
5o
5o
44
44
44
45
45
45
45
46
46
46
46
47
48
i 9
5o
IOO
IOO
IOO
IOO
IOO
96
9
96
96
96
9 3
9 3
9^
9 3
9 3
68
68
68
68
69
63
63
63
63
63
58
58
58
< 9
5 9
4i
42
42
42
42
38
38
38
39
3 9
TABLE XXVI. EELATIVE HUMIDITY OF THE AIR.
275
TABLE XXVI.
RELATIVE HUMIDITY OF THE AIR.
Temp,
of Air.
Difference of Temperature of the Air and of the Dew Point.
1
96
96
9 6
2
93
93
93
3
4
il
83
83
83
83
83
6
80
80
80
80
80
77
77
77
77
77
8
74
74
74
74
74
7 1
7 1
72
72
72
10
6 9
69
69
69
69
12|14
54
55
55
55
55
18i20
22
24
52
53
54
55
IOO
IOO
IOO
IOO
IOO
89
9
9
86
'86
86
86
86
64
64
64
64
64
5 9
5 9
5 9
5 9
5o
5o
5i
5i
5i
46
47
47
47
47
43
43
43
43
43
3 9
39
4o
4o
56
58
5 9
60
IOO
IOO
IOO
IOO
IOO
9 6
96
96
93
93
93
9 3
93
9
9
9
9
86
86
87
87
87
83
83
83
84
84
80
80
80
81
81
77
77-
78
78
78
75
75
75
75
72
72
72
72
72
69
69
69
70
70
64
64
64
65
65
60
60
60
55
55
55
55
56
5i
5i
5i
5i
52
52
52
52
52
47
47
47
48
48
44
44
44
44
44
44
44
45"
45
45
4o
4o
4i
4i
4i
61
62
63
64
65
IOO
IOO
IOO
IOO
IOO
96
97
97
97
97
97
97
97
97
97
93
93
93
93
9 3
9
9
9
9
9
87
87
87
87
87
84
84
84
84
84
84
84
84
84
84
84
84
84
84
84
81
81
81
81
81
~sT
81
81
81
81
78
78
78
78
78
75
75
75
75
72
72
72
73
73
70
70
70
70
70
65
65
65
65
65
60
60
60
60
61
56
56
56
56
56
48
48
48
48
49
4i
4i
4i
4i
42
66
67
68
6 9
70
IOO
IOO
IOO
IOO
IOO
93
93
9
9
9
9
9
87
87
87
87
87
87
87
87
87
87
78
78
78
78
78
75
76
76
76
76
73
73
73
73
73
70
70
70
65
65
66
66
66
61
61
61
61
61
56
57
57
57
57
52
53
53
53
53
49
49
49
49
49
45
45
45
46
46
42
42
42
42
42
7 1
72
73
74
75
IOO
IOO
IOO
IOO
IOO
97
97
97
97
97
93
93
93
9 3
93
9
9
9
9
9
81
81
81
82
82
79
79
79
79
79
76
76
76
76
76
73
73
73
74
74
74
74
74
74
74
7 1
7 1
7 1
7 1
66
66
66
66
66
61
61
62
62
62
57
57
58
53
53
53
53
54
49
i 9
5o
5o
5o
46
46
46
46
46
43
43
43
43
43
76
77
78
79
80
IOO
IOO
IOO
IOO
IOO
97
97
97
97
97
94
94
9 4
94
94
9
9
9
9 1
9 1
87
87
88
88
88
85
85
85
85
85
"85"
85
85
85
85
"85"
85
85
85
85
82
82
82
82
82
79
79
79
79
79
76
76
76
77
77
7 1
7 1
7 1
72
66
67
67
67
67
62
62
62
62
62
58
58
58
58
58
54
54
54
54
54
5o
5o
5o
5o
5i
47
47
47
47
47
43
43
44
44
44
81
82
83
84
85
IOO
IOO
IOO
IOO
IOO
97
97
97
97
97
97
97
97
97
97
94
94
94
94
94
94
9 4
9 4
9 4
94
9 1
9 1
9 1
9 1
9 1
9 1
9 1
88
88
88
88
88
"88"
88
88
88
88
82
82
82
82
82
"82"
82
82
83
83
79
79
79
79
80
~8o~
80
80
80
80
77
77
77
77
77
77
77
77
77
77
74
74
74
74
75
75
75
75
75
75
72
72
72
72
7 2
72
72
72
72
73
67
67
67
67
67
67
68
68
68
68
63
63
63
63
63
~63~
63
63
63
64
58
58
5 9
5 9
5 9
P
5 9
54
55
55
55
55
~55~
55
55
55
56
5i
5i
5i
5i
sr
TT
52
52
52
52
47
47
48
48
48
48
48
48
48
49
44
44
44
44
45
45
45
45
45
45
86
87
88
89
9
IOO
IOO
IOO
IOO
IOO
9 1
92
9 3
9 i
9 5
IOO
IOO
IOO
IOO
IOO
97
97
97
97
97
94
9 4
9 4
9 4
94
9 1
9 1
9 1
9 1
9 1
88
88
88
88
88
85
85
85
86
86
83
83
83
83
83
80
80
80
80
80
77
78
78
78
78
75
75
75
75
75
73
73
73
73
73
68
68
68
68
68
64
64
64
64
64
60
60
60
60
60
56
56
56
56
56
52
52
52
52
53
4 9
49
49
4 9
49
45
46
46
46
46
276 TABLE XXVII. ELASTIC FORCE OF AQUEOUS VAPOR,
TABLE XXVII.
ELASTIC FORCE OF AQUEOUS VAPOR.
Tempera-
ture.
Force of
Vapor.
Tempera-
ture.
Force of
Vapor.
Tempera-
ture.
Force of
Vapor.
Tempera-
ture.
Force of
Vapor.
Tempera-
ture.
Force of
Vapor.
O
Inch.
Inch.
Inch.
O
Inch.
Inch.
-3o
.009
47
.323
69
.708
8l.4
I.07I
91.4
.473 <
25
.OI2
4 7 .5
.329
6 9 .3
.716
8l.6
.078
91.6
.482
20
.Ol6
48
.335
69.7
. 7 25
8l.8
.o85
91.8
.4 9 I
15
.021
48.5
.S4i
7
.733
82.0
.'092
92.0
.5oi
10
.027
49
.348
70.3
740
82.2
.099
92.2
.5io
5
.o34
49-5
.354
70.7
. 7 5i
82.4
. 106
92.4
.52o
.043
5o
.36i
7 1
. 7 58
82.6
.n4
92.6
.520
+ 2
.o48
5o.5
.36 7
7 i.3
. 7 66
82.8
. 121
92.8
.539
4
.o52
5i
3 7 4
71.7
. 77 6
83.o
.128
93.0
.548
6
.057
5i.5
.38i
72
. 7 84
83.2
.135
93.2
.558
8
.062
52
.388
72.3
79 2
83.4
.i43
93.4
.568
10
.068
52.5
.3 9 5
72.7
.8o3
83.6
.i5o
93.6
.5 77
12
.075
53
.4o3
73
.811
83.8
.i58
93.8
.58 7
i4
.082
53.5
4io
7 3.3
.820
84.o
.i65
9 4.o
5 97
16
.090
54
.4i8
7 3. 7
.83i
84.2
.i 7 3
94-2
.6o 7
18
.098
54.5
.425
74
.83 9
84.4
.180
94.4
.617
20
.108
55
.433
74.3
.848
84.6
.188
94.6
.627
21
.n3
55.5
44i
74.7
.859
84.8
.i 9 5
94.8
.63 7
22
.118
56
.449
75.0
.868
85.o
.203
95.0
64 7
23
.123
56.5
45 7
75.2
.8 7 3
85.2
.211
95 .2
.65 7
24
.129
5?
.465
75.4
879
85.4
.219
9^.4
.66 7
25
.i35
5 7 .5
.474
7 5.6
.885
85.6
.226
95.6
.6 77
26
.i4i
58
.482
7 5.8
.891
85.8
.234
9 5.8
.688
27
.i4 7
58.5
.491
7 6.o
.897
86.0
.242
96.0
.698
28
.i53
5 9
.5oo
76.2
.903
86.2
.250
96.2
. 7 o8
29
. 160
5 9 .5
.509
7 6.4
.909
86.4
.258
96.4
.719
3o
.167
60
.5i8
7 6.6
. 9 i5
86.6
.266
96.6
7 2 9
3i
.174
6o.5
.527
7 6.8
.921
86.8
2 7 4
96.8
74o
32
.181
61
.536
77-
9 2 7
8 7 .o
.282
97-
.761
33
.188
6i.5
.546
77-2
.933
87.2
.290
97 .2
.761
34
. 196
62
.556
77-4
. 9 3 9
87.4
.298
97-4
772
35
.204
62.5
.566
77 .6
946
87.6
.3o 7
9 7 .6
. 7 83
36
.212
63
.5 7 6
77-8
.952
87.8
.3i5
97-8
794
3?
.220
63.3
.582
78.0
. 9 58
88.0
.323
98.0
.8o5
38
.229
63.7
.Sgo
78.2
9 64
88.2
.332
98.2
.816
3 9
.238
64
.5 9 6
7 8.4
97i
88.4
.34o
98.4
.82 7
4o
.248
64.3
.602
78.6
977
88.6
.349
98.6
.838
4o.5
.252
64-7
.611
78.8
.984
88.8
.35 7
9 8 - 8 ,
.849
4i
. 2 5 7
65
.617
79.0
99
8 9 .o
.366
99.0'
.861
4i.5
.262
65.3
.624
79.2
997
8 9 .2
.3 7 5
99.2
.872
42
.267
65.7
.632
79-4
.oo3
8 9 .4
.383
99.4
.883
42.5
.272
66
.63 9
79.6
.010
8 9 .6
.3 9 2
99.6
.895
43
.277
66.3
.646
79.8
.016
8 9 .8
.4oi
99.8
.906
43.5
.283
66.7
.655
80.0
.023
90.0
.4io
100.
.918
44
.288
67
,662
80.2
.o3o
90.2
.419
IOO.2
.929
44-5
.294
6 7 .3
.668
80.4
.o3 7
90.4
42 7
100.4
94i
45
.299
67.7
.678
80.6
.o43
90.6
.436
100.6
. 9 53
45.5
.3o5
68
.685
80.8
.o5o
90.8
.446
100.8
.965
46
.3n
68.3
.692
81.0
.o5 7
91 .0
1.455
IOI .O
977
46.5
.3i 7
68.7
.701
81.2
i .064
91.2
1.464
101 .2
.988
TABLE XXVIII. PRESSURE AND VELOCITY OF THE WIND. 277
TABLE XXVIII.
FOB COMPARING THE PRESSURE AND VELOCITY OF THE WIND.
Pressure,
oz. per
sq. foot.
Velocity,
miles
per hour.
Pressure,
Ibs. per
sq. foot.
Velocity,
miles
per hour.
Pressure,
Ibs. per
sq. foot.
Velocity,
miles
per hour.
Pressure,
Ibs. per
eq. foot.
Velocity,
miles
per hour.
Pressure,
Ibs. per
sq. foot.
Velocity,
miles
per hour.
0.08
I .OOO
6.75
36. 742
I 7 .00
69.160
28.25
76.166
3 9 . 2 5
88.600
0.25
1.767
7.00
3 7 .4i6
17.76
5 9 .58i
28.50
7 5.498
3 9 .5o
88.881
o.5o
2.5oo
7 . 2 5
38.o 7 8
18.00
60.000
28.76
7 5.828
3 9 . 7 5
89.162
0.76
3.061
7 .5o
38. -729
18.25
6o.4i5
29.00
76.167
4o.oo
89.442
i .00
3.535
7 . 7 5
3 9 .3 7 o
18.60
60.827
29.26
76. 485
40.26
89.721
2
5. ooo
8.00
4o.ooo
18.76
6i.23 7
29.60
76.811
4o.5o
9O.OOO
3
6.123
8.25
40.620
19. oo
6i.644
2 9 . 7 5
77 .i36
40.76
90.277
4
7.071
8.5o
4l.23i
19.26
62.048
3o.oo
77-459
4i .00
90.553
5
7.905
8. 7 5
4i.833
19.60
62.449
3o.25
77.781
41.26
90.829
6
8.660
9.00
42.426
19.76
62.849
3o.5o
78.102
4i.5o
91 .io4
7
9.354
9.25
43.0H
20.00
63.245
30.76
78.421
4i.75
91.378
8
to. ooo
9.60
43.588
20.25
63.63 9
Si.oo
78.740
42.00
91 .65i
9
10.606
9 . 7 5
44.i58
2O.5o
64.o3i
3i. 2 5
79.066
42.26
91.923
10
ii . 180
IO.OO
44- 7 2i
20.76
64.420
3i.5o
79.372
42.60
92.196
ii
11.726
10.25
45.2 7 6
21 .OO
64.807
3i. 7 5
79.686
42.76
92.466
12
12.247
10. 5o
45.825
21.25
65. 192
32. OO
80.000
43.00
92.736
i3
12.747
10.75
46.368
21. 5o
65.5 7 4
32.26
8o.3n
43.25
93.006
i4
13.228
11.00
46.904
21.76
65. 9 54
32. 5o
80.622
43.5o
9 3.2 7 3
i5
i3.6 9 3
11.25
4 7 .434
22.00
66.332
32. 7 5
80.932
43. 7 5
9 3.54i
Pounds.
n.5o
4-7.968
22..2S
66.708
33.oo
81.240
44.oo
93.808
I
14.142
11.75
48.4 7 6
22. 50
67.082
33.25
8i.54 7
44.25
94.074
1.25
i5.8n
I2.OO
48.989
22.76
67.453
33.5o
8i.853
44.5o
94.339
i.5o
17.320
12.25
49.497
23.00
67.823
33.76
82.168
44.76
94.604
i. 7 5
18.708
12. 5o
5o.ooo
23.25
68.190
34.oo
82.462
45.00
94.868
2.OO
20.000
12.75
60.497
23. 5o
68.556
34.25
82.764
46.26
95.i3i
2.25
2I.2I3
1 3. oo
60.990
23. 7 5
68.920
34.5o
83.o66
45.5o
9 5.3 9 3
2.60
22.360
i3.25
61.478
24.00
69.282
34.76
83.366
45. 7 5
96. 655
2.75
23.452
i3.5o
61.961
24.26
69.641
35.00
83.666
46. oo
96.916
3.oo
24.494
i3.75
52.44o
24.60
70.000
35.25
83.964
46.25
96.176
3.25
25.495
14.00
62.916
24. 7 5
70.356
35.5o
84.261
46. 5o
96.436
3.5o
26.457
i4.25
53.385
26.00
70.710
35.76
84.667
46. 7 5
96.696
3. 7 5
27. 386
i4.5o
53.85i
26.26
7i.o63
36.oo
84.862
47.00
96.963
4.oo
28.284
i4.75
54.3i3
26.60
7i.4i4
36.25
85.146
47.26
97.211
4.25
29.154
i5.oo
54.77 2
26.76
7 i. 7 63
36. 5o
85.44o
47.60
97.467
4.5o
3o.ooo
i5. 2 5
55.226
26.00
72.111
36.76
86.732
47.75
97.724
4. 7 5
3o.822
i5.5o
66.677
26.26
72.466
37.00
86.023
48.oo
97.979
5.oo
3l.622
16.76
66.124
26.60
72.801
37.26
86.3i3
48.26
98.234
5.25
32.4o3
16.00
56.568
26.76
7 3.i43
3 7 .5o
86.602
48. 5o
98.488
5.5o
33.i66
16.26
67.008
27.00
7 3.484
37.75
86.890
48. 76
98.742
5. 7 5
33. 9 n
16.60
57.445
27.26
73.824
38.oo
87.177
49.00
98.994
6.00
34.64i
16.76
67.879
27.60
74.161
38.25
87. 464
49.26
99.247
6.25
35.355
17.00
68.309
27.76
74.498
38. 5o
87.749
49.60
99.498
6.5o
36.o55
17.26
68.736
28.00
7 4.833
38. 7 5
88.o34
4 9 . 7 5
99.749
6. 7 5
36. 7 42
17.60
69. 160
28.26
76.166
Sg.oo
88.3 I7
60.00
100.000
278
TABLE XXIX. AVERAGE AMOUNT OF RAIN
TABLE XXIX.
AVERAGE AMOUNT OF RAIN FOR EACH MONTH, SEASON, AND THE YEAR.
Station.
Lat.
Long.
Alt.
Jan.
Feb.
March.
April.
May.
Paramaribo, Dutch Guiana . .
Caraccas Venezuela . . .
O f
5 44
10 22
16 10
19 12
23 9
o /
55 13
67 12
61 5o
96 9
82 23
Feet.
5o
5o
Inches.
.l8. 7 4
I .OO
21. 30
5.io
4-97
Inches.
16.54
O.25
17.76
0.00
3.08
Inches.
20.75
1. 10
22.64
o.oo
4.08
Inches.
21.10
I .20
21.38
o.5o
2.28
Inches.
23.23
17.00
18.11
3i.4o
IO. II
Matouba Guadeloupe
Vera Cruz Mexico
Havana, Cuba
Key West Florida
24 32
27 47
28 o
29 48
29 5 7
81 48
97 27
82 28
81 35
90 o
IO
20
20
25
10
2.20
3.96
2. 2O
2.09.
5.6i
I .22
s.3 7
3.01
i.63
2.90
.2.83
1.25
3.3 7
2.34
3.90
1.34
4.oi
i. 9 5
i.56
3.29
3.92
4.68
3.24
2.OO
4. 10
Corpus Christi Texas
Fort Brooke Texas ....
St Augustine Florida
New Orleans Louisiana
Mobile Alabama . .
3o 42
32 6
32 42
32 46
35 4i
88 i
81 5
117 i3
79 56
106 i
3o
3o
160
3o
6846
8.89
2.76
o.83
2.33
o.3i
5.o 7
2.53
2.OI
3.3 9
o.5 7
5.86
3.6 9
i .4o
3.02
1.29
4.95
2. II
0.77
1.72
0.80
3.43
5. 20
0.57
3.66
0.74
Savannah Georgia
San Dieo'O California
Charleston, South Carolina. . .
Santa Fe New Mexico . .
Nashville Tennessee
36 9
36 5o
3 7 -32
3 7 48
38 35
86 49
76 19
io5 23
122 27
121 28
533
8
8365
i5o
5o
5.oi
3.26
O.23
3.23
2.98
3.98
2. 7 4
0.72
3.3i
2.36
4.91
3.33
0.94
4.6j
3-97
5.20
2.80
0.42
3.72
i.44
4.94
3.64
2.14
o.48
0.87
Norfolk Virginia
Fort Massachusetts, New Mex.
San Francisco, California ....
Sacramento California. .....
St Louis Missouri
38 3 7
38 53
3 9 6
3 9 5 7
4o 3 2
90 1 5
77 o
84 3o
75 10
80 2
48 1
78
55o
3o
74
2 .o3
4.45
3.35
3.09
2.18
2.28
2. 7 5
3.5i
2.94
2 . I 7
3.4o
2.5 7
3.93
3.43
2. 7
3.93
4.o3
3.66
3.64
3.io
4-97
3.85
4.55
3.90
3.58
Washington, D. C
Cincinnati Ohio
Philadelphia, Pennsylvania . .
Pittsburg Pennsylvania
New York City, New York . .
Salt Lake City, Utah
4o 43
4o 46
4i 18
42 12
42 2O
74 o
112 6
72 55
io4 48
83 2
23
435i
60
45i 9
58o
2. 7 8
1.23
3.53
O.2 7
2.18
2.92
1.99
3. 97
O. 7 I
1.38
3.44
2.34
3.49
i.3 7
2.86
3.33
1.66
3.3i
1.93
2.92
4.78
1.34
4.23
5.3 9
2.73
New Haven, Connecticut ....
Fort Laramie Dacotah ....
Detroit Michigan ......
Boston Massachusetts ....
42 21
42 4o
42 44
43 4
43 8
71 3
73 45
124 29
87 54
77 5i
7 1
i3o
5o
5 9 3
5o6
2.39
2. 77
8.81
i.3c<
1.88
3.i 9
2.62
6.35
0.80
i .4o
3.4 7
2.82
8.24
i .60
1.81
3.64
3.12
5.64
2.4o
1.97
3. 7 4
3.85
5.24
2.5o
3.o4
Albany New York
Fort Orford, Oregon
Milwaukee Wisconsin
Rochester New York
Toronto, Canada
43 3 9
44 53
45 6
45 3i
46 ii
79 2 3
93 10
64 25
73 33
123 48
34i
820
5o
i . 7 o
o. 7 3
3.2 7
3.21
2 7 .OO
i .09
O.52
2.54
4.19
10.95
1.6!
i.3o
5. 9 4
3.32
6.10
2.5 7
2.14
4.20
2.48
4.38
2.98
3.i 7
3.3o
2.82
5. 9 5
Fort Snelling, Wisconsin ....
Wolfville, Nova Scotia. . .
Montreal, Canada
Astoria, Oregon
Fort Brady, Michigan
46 3 9
47 10
4? i5
4 7 33
5 7 3
84 43
122 25
68 35
52 28
i35 18
600
3oo
5 7 5
i4o
20
1.84
9.54
3. 7 3
4. 7 4
7.80
i.i3
5.i6
2.60
2. 7 5
7 .3 2
i.3 7
4.56
1.77
4.8o
6.20|
i.83
4-77
i .06
3.76
6.83
2.24
1.86
2.63
4.i3
5.2 9
Steilacoom, Washington Ter. .
Fort Kent, Maine
St. Johns, Newfoundland ....
Sitka, Aliashka
279
TABLE XXIX.
AVERAGE AMOUNT OF RAIN FOR EACH MONTH, SEASON, AND THE YEAR.
June.
July.
Aug.
Sept.
Oct.
Nov.
Dec.
Spring.
Summer.
Autumn.
Winter.
Year.
Inches.
16.34
1 6. oo
3 9 .53
21 .20
25.28
Inches.
5.8 9
i4.o4
27. 9 5
39.70
5. 9 3
Inches.
1.77
21 . l4
IO.2O
35.90
6.90
Inches.
o.63
39. 37
i3.i5
38.90
ii . i4
Inches.
1.46
i3.4o
33.ii
8.00
I I .01
Inches.
2.99
26.80
24.13
4.5o
4. 7 4
Inches.
13.03
4.0 7
43. o 7
o.4o
i.83
Inches.
65.08
19.30
62.13
31.90
16.47
Inches.
24.OO
5l.l8
77 .68
116.80
38.ii
Inches.
5.o8
79 .5 7
70.39
5i.4o
26.89
Inches.
48. 3i
5.32
82.13
5.5o
9.88
Inches.
l42.47
i55.3 7
292.33
i83.2o
91.35
5.48
5.63
7.04
4.27
4-97
2.97
4.8 9
II . IO
3.24
6.66
4.33
2.91
IO. IO
3.o3
5.65
6. 12
6. 7 3
6.23
5.85
2. 2O
4.94
2.3 7
2.40
2.42
2.74
1.77
i.o5
2.OO
I .29
4.68
2.09
1.26
2.83
2.08
4.20
8.09
9 . 9 4
8.56
5.90
ii .29
12. 7 8
i3.43
28.24
10.54
17.28
12.83
10. i5
10.63
9.56
9.62
5.5i
7 .5 9
8.o4
5.8o
12. 7 I
39.21
4i . 1 1
55. 4 7
3i.8o
50.90
5.o5
4-84
o.i5
5.oo
1.32
4.36
7 .5 7
O.OI
6.i5
4.i8
8.59
8.3 2
0.39
7 .53
3.4o
4.68
4.26
o.o3
6.34
2.55
2.65
2.55
o.o5
3.o4
i. 60
6.58
i.65
1.16
2.23
1.8 7
4.3i
3.20
3.06
3.68
i .20
14.24
11.00
2.74
8.60
2.83
18.00
20.72
o.55
18.68
8.90
13.91
8.46
1.24
ii. 61
6.62
18.27
8.48
5.90
9.40
2.08
64.42
48.66
io.43
48.29
19.83
4.4i
3. 7 8
0.74
O.O2
O.Og
3.84
5.56
2 .5 9
o.oo
O.II
4-4o
5.70
2.05
O.OI
0.00
4.94
3.93
1.39
0.07
O.OI
3.68
2.82
I . IO
o.63
0.42
3.92
3.4i
6.34
2.o5
3.18
2.96
4-i 7
1.88
4.71
4.42
i4. 10
9-77
3.5o
8.81
6.28
1 4.oo
i5.o8
5.38
o.o3
O.20
I2.3O
10. 16
8.83
2. 7 5
3.6i
12.40
IO. I 7
2.83
II .25
9.76
52. 80
45.18
20.54
22.84
19. 85
6.06
2.93
5.01
3.5 7
3.56
3.86
3.92
4.3 7
4.22
2.97
4.22
3.67
4.32
4.67
3.34
2.67
3.52
3.io
3.53
2.68
3.29
3.55
3.32
3.i8
2.87
3.08
3.09
3.48
3.36
2.68
2.68
'2.87
4.29
4.o3
S.i'3
12. 3o
10.45
12. l4
10.97
9.38
i4.i4
IO.52
13.70
12.46
9.87
8.94
10. 16
9.90
io.o 7
8.23
6.94
10.07
ii. i5
1 0.06
7-48
42.32
4i .20
46.89
43.56
34.9.6
3.46
2.15
3.3o
2.95
3.91
3.1 7
3.09
i'.83
3.20
4.70
o.64
4.i4
0.92
2.18
3.3i
o.85
3.88
i.33
3.3i
3.4o
i.5 7
3.6o
1.26
2.O4
3.5 9
3.85
3. 7 2
i.3 7
2.06
3.93
3.68
3.43
o.65
i.3o
ii.55
5.34
n.o3
8.69
8.5i
H.33
6. 7 8
n.63
5. 7 o
9.29
10. 3o
6.2 7
11.20
3. 9 6
7-4i
9.63
6.90
10.93
1.63
4.86
42.81
25.29
44-79
19.98
30.07
3.i3
4.48
i. 06
4.00
3.25
2.57
4.3 9
o. 16
3.oo
3.oi
5.4 7
3.44
1.78
2.80
2.60
4.27
3.34
2.34
3.20
3.o5
3.73
3.69
7 .3i
i.4o
3.3 9
4.5 7
3.24
IO.2 7
2.10
2.94
4.3i
2.91
14.43
2.OO
2 . IO
10.85
9-79
19.12
6.5o
6.82
ii. 17
12. 3l
3.oo
9.70
8.86
12. 5 7
IO.2 7
19.92
6.80
9 .38
9.89
8.3o
2 9 .5 9
4.20
5.38
44-48
40.67
7 i.63
27.20
3o.44
3.o4
3.63
4.82
2.65
2.85
3. 7 2
4. ii
3.27
3.27
o.oo
2.81
3.i8
5.o4
3.52
i .15
4.46
3.32
3. 7 o
3.53
1.87
2.96
i.35
3.66
3.8 7
6.70
2.91
i.3i
3.o4
3. 9 5
13.20
i.5o
o.6 7
3.6 7
4-4i
6.20
7 . 16
6.61
i3.44
8.62
i6.43
9 .5 7
10.92
12. l3
9.44
4.06
10.33
5.98
10. 4o
H.35
21. 77
4.29
i .92
9-48
ii. 81
44.i5
3i.35
25.43
45.45
4l .22
86.35
2.83
1.97
r.36
5.6 7
3. 79
3. 7 5
o.34
7.72
3.82
4.i5
3.3 9
i.54
2.5 7
5.09
7.81
4.33
2.67
i.36
5. 79
n. 27
3.35
4.43
4-4i
7 .88
12.32
3.08
8. 7 3
3.86
3.25
8.5i
2.21
7.92
3.36
5.25
8.65
5.44
11.19
5.46
12.69
i8.32
9-97
3.85
ii.65
i4.58
16.75
io. 7 6
i5.83
9-64
16.92
32.10
5.i8
22.62
9.71
12.74
23. 77
3i.35
53.49
36.46
56. 9 3
89.94
280
TABLES XXX., XXXI. ANNUAL FALL OF BAIN.
TABLE XXX. PLACES HAVING A SMALL ANNUAL FALL OF RAIN.
Places.
Latitude.
Longitude.
Height.
Amount.
Authority.
Lima Peru
'
12 O
O /
77 2
Feet.
53o
Inches.
O
Arago's Met. Ess., p. 109
Thebes Eoypt
25 43
32 35
O
Wilk'n's Egypt., v. 4, p. 10.
Near Mourzouk, Fezzan .
Tatta, North Africa ....
Cairo, Egypt
26 54
28 3 9
3o 2
i4 12
6 46
3i i5
O
o
i.3i
Gehler, v. 7, p. i25i.
Gehler, v. 7, p. i25i.
Arago Melanges, p. 463.
Kurrachee, Hindostan . .
Kotree Hindostan ....
24 5o
25 2O
67 o
68 14
i.5o
I . 74
Ph. Trans. i85o, p. 36o.
Ph Trans i85o p 36 1
Biscara Algeria
34 5i
5 4o
35o
i. /^
2 . OO
An. Met. i854 p 297
Fort Yuma, California. .
Astrachan Russia
32 43
46 21
n4 36
48 5
I2O
7O
3.24
4.08
Army Reg., p. 675.
Dove Beitrage p i83
Hyderabad, Hindostan. .
Raimsk Russia ...
25 2O
46 4
68 20
61 47
4.5o
5. 00
Johnston's Ph. Atlas.
Dove Beitrage, p. i83.
Aralich Russia
3o 53
44 33
26OO
6.i5
Dove Beitrage, p. i38.
Mendosa, La Plata
Novo Petrowsk, Russia .
32 52
44 27
69 6
5o 8
26OO
n5
6.5o
6.72
Zeitsch. fur Erd'e 1 858, p. 9.
Dove Beitrage, p. i83.
Fort Conrad, New Mex. .
San Louis Rey Cal
33 34
33 i3
107 9
117 3o
45 7 6
20
6.76
6.o5
Army Reg., p. 673.
Armv Resr.
Barnaoul Siberia
53 20
83 27
4oo
7 .4 7
Kupffer's Annales
Taos, New Mexico. . . .
36 21
io5 4s
8000
7.48
Army Reg.
Cumana, Venezuela ....
10 27
64 i5
7.52
Gehler, v. 7, p. i3u.
Sevastopol Russia . . .
44 36
33 32
7 6?
Heis Wochen't 1866, p. 325.
Socorro, New Mexico. . .
Sympheropol, Russia . . .
Bacou Russia
34 10
44 5 7
4o 22
106 54
34 6
4o 4?
456o
780
53
7.86
8. 7 5
9o5
Army Reg., p. 673.
Heis Wochen't 1 858, p. 1 76.
Kupffer's Annales
Fort Fillmore, New Mex.
Albuquerque, New Mex.
32 i3
35 6
106 42
1 06 38
3 9 3 7
5o32
9.23
9.42
Army Reg., p. 673.
Army Reg., p. 6 7 3.
TABLE XXXI. PLACES HAVING A GREAT ANNUAL FALL OF RAIN.
Places.
Latitude.
Longitude.
Height.
Amount.
Authority.
Dherapoonjee, Hindost'n
Matouba, Guadeloupe . .
Vlaranhao, Brazil
o /
25 14
16 10
2 3l
r
91 4o
61 5o
44 18
Feet.
4125
4ooo?
Inches.
592
292
280
Herschel's Met., p. no.
Com. Rend., v. 7, p. 743.
Gehler, v. 7 p 1 3 1 4
Jttray Mullay, Hindost'n
Mahabalishwar, Hindo'n
8 3 9
17 54
77 o
73 38
45oo
43oo
267
254
Ph. Trans. 1860, p. 358.
Ph. Trans. i85o, p. 354.
Sylket, Hindostan
24 53
QI 47
2OQ
Br. As. 1 862, p. 257.
Stye England
54
3 7
1600
2O6
Buchan's Met., p 1 18.
Aracan, Hindostan
Augusta Peak, Hindost'n
Sierra Leone, W. Africa .
20 47
8
8 20
9 3 25
77
i3 8
6200
2OO
194
189
Buchan's Met., p. 117.
Ph. Trans. i85o, p. 362.
Gehler, v. 7, p. i3i4.
Sindola, Hindostan ....
Vera Cruz Mexico
i?
IQ 12
- 73
06 Q
46oo
i85
i83
Ph. Trans. 1860, p. 354.
Mayer's Mexico.
Sandoway, Hindostan . .
Maulmein, Birmah ....
18 25
16 3
94 3o
07 38
178
175
Br. As. 1 852, p. 257.
Johnston's Ph. Atlas.
Attaghery, Hindostan . .
St.Benoit, Isl. of Bourbon
8
20 5l
77
55 3o
22OO
170
162
Ph. Trans. i85o, p. 362.
Dove Beitrage, p. 102.
Marmato, New Granada .
Demerara, Guiana
Caraccas, Colombia ....
Akyab, Hindostan ....
4 4o
6 45
IO 22
20 8
74 42
58 2
67 5
02 52
46 7 8
2730
162
1 56
i55
i55
Br. As. i84o, p. 1 1 6.
Berghaus's Atlas.
Dove Beitrage, p. 90.
Br. As. 1862, p. 257. [368.
Leogane, St. Domingo . .
Buitenzorg, Java
18 3o
6 37
72 3o
I 06 49
i5o
i47
Malte Bran's Geog., v. i, p.
Dove Beitrage, p. 102.
TABLES XXXII., XXXIII. BADIATING POWER, ETC. 281
TABLE XXXII.
COMPARATIVE RADIATING POWER OF DIFFERENT SUBSTANCES AT NIGHT.
i3i6
Copper . .
83o
Rabbit-skin.
I24o
Charcoal in Powder
V
776
White raw Wool on Grass. .
1222
Wood
7?3
1186
Blackened Tin
770
Raw Silk
IIO7
Lead
757
Unwrought white Cotton )
Wool j
1086
Black-lead in Powder
Zinc
697
681
Yellow Cotton
ioo5
Iron
642
Long Grass
IOOO
Paper
6i4
Black Wadding on Grass . .
oo3
Sawdust
610
Lampblack in Powder
961
Slate
578
Flannel
886
Garden-mould
4 7 2
Light blue Lamb's Wool
876
Tin-foil
4?o
Grass less than an Inch in )
River Sand
454
Height C
870
Stone
3qo
Glass
864
Brick
372
Chalk in Powder. .
84o
Gravel. .
288
TABLE XXXIII.
FALL OF THE BAROMETER IN HURRICANES.
Locality.
Date.
Fall in
Inches.
Hours.
Authority.
Near Calcutta ....
Bay of Bengal ....
South Indian Ocean
St. Thomas, W.I. . .
Near Calcutta ....
i833,May 21.
i84o,Apr. 28.
i84o, May 4.
1 837, Aug. 2.
1 8 32, Oct. 7.
2.59
2.O5
2.OO
.69
.60
3
14
6
12
Reid's Law of Storms, p. 271.
Journal Bengal Soc., v. 9, p. ioi4.
Piddington's Horn-Book, p. 21 5.
Poggendorff's Annal., v. 52, p. 25.
Reid's Law of Storms, p. 269.
Near Hong Kong . .
Bay of Bengal ....
Bay of Bengal ....
China Sea
1 867, Sept. 8.
i852,May i4.
1 8 54, Apr. 22.
i845, Oct 9
.5 7
.55
.5o
.5o
i3
8
12
i3
U. S. Steamer Monocacy.
Jour. Bengal Soc., i855, p. 429.
Jour. Bengal Soc., i858, p. 179.
Jour Bengal Soc v 18 p 16
Mauritius
i8i8,Feb 28
.5o
17
Reid's Law of Storms p 1 4 1
Havana, Cuba ....
Macao, China
1 846, Oct. ii.
1 832, Aug. 3.
4 7
.46
6
Piddington's Horn-Book, p. 193.
American Journal, v. 35, p. 217
Calcutta
1842 June 3
.42
18
Jour Bengal Soc 1842 p 1004
Bay of Bengal ....
Aberdeen, Scotland
i85i,Oct. 22.
1839, Jan. 7.
.4o
.4o
7
12
Jour. Bengal Soc., i854, p. 5i3.
Espy's Phil, of Storms, p. 52 1 .
Cape Hatteras ....
Boston Mass
1 853, Sept. 7.
1866, Dec. 27
.35
.26
7
17
Amer. Journal, v. 18, N. S., p. 9.
R T Paine's Journal
China Sea
1 809, Sept. 28
. 20
12
Jour. Bengal Soc., v 11 p 627
Macao, China
1 835, Aug. 5.
.i5
**
American Journal, v. 35, p 211
Chittagong, India. .
Mauritius. .
1 849, May l3 -
1824, Feb. 23.
.06
.o5
a*
Jour. Bengal Soc., 1 854, p. 22.
Reid's Law of Storms, p. i4?.
282
TABLE XXXIV. AURORAS, SOLAR SPOTS, ETC.
TABLE XXXIV.
AURORAS, SOLAR SPOTS, AND VARIATION OF THE MAGNETIC NEEDLE.
Year.
Auroras.
Year.
Auroras.
Solar Spots.
Year.
Auroras.
Solar Spots.
1
!
Year.
Auroras.
Solar Spots.
1
!
i
i
1
i
America.
|
N
!
i685
I
1740
2
I 7 82
29
24
33
8
1824
O
o
7
8
1686
4
i 7 4i
21
i 7 83
i?
22
22
9
1825
I
2
J 7
10
1692
2
1742
i4
2
i 7 84
7
4
4
7
1826
2
O
29
10
i6 9 3
2
1743
9
2
i 7 85
i4
9
18
8
1827
IO
7
4o
ii
1694
2
1744
8
O
1786
4o
55
61
i4
1828
I I
6
52
12
i6 9 5
4
1745
3
O
1787
IO
47
93
i5
1829
18
2
53
i4
1696
4
1746
i
7
1788
IO
38
9 1
i3
i83o
32
b 1
5 9
12
1697
i
1747
7
10
1789
i5
5i
85
i3
i83i
23
2
39
12
1698
9
1748
3
6
1790
4
i3
75
i5
i832
5
2
22
1699
4o
1749
3
10
64
1791
4
12
46
12
i833
12
3
7
1702
i
1760
12
17
68
1792
i
6
53
9
i834
2
9
II
8
1704
i
i 7 5i
2
5
4i
i 79 3
2
8
21
8
i835
6
6
45
IO
1707
12
1762
2
33
1794
2
2
24
8
i836
8
5
97
12
1708
I
i 7 53
I
23
1795
2
2
16
7
1837
25
4i
in
12
1709
3
1754
O
74
1796
I
9
8
i838
28
3 9
83
i3
1710
i
i 7 55
I
O
6
1797
I
6
8
1839
3o
47
68
II
1711
i
1766
2
9
1798
O
3
7
i84o
4o
44
52
9
1714
i
i 7 5 7
o
6
3o
1799
2
O
6
7
1841
35
42
3o
7
1716
ii
i 7 58
2
4
38
1800
3
O
10
7
1842
4 9
ii
I 9
6
1717
12
i 7 5 9
8
5
48
1801
4
O
3i
8
i843
38
IO
9
7
1718
27
i 7 6o
7
6
49
1802
4
2
38
8
1 844
22
IO
i3
6
1719
32
i 7 6i
12
5
7 5
i8o3
6
5
5o
9
i845
18
22
33
7
1720
28
1762"
18
7
5i
1804
6
4
70
8
1 846
39
3o
47
8
1721
'9
i 7 63
4
6
37
i8o5
4
4
5o
9
1847
38
22
79
9
1722
46
1764
9
12
34
1806
3
4
3o
1 848
38
53
IOO
ii
1728
3o
i 7 65
8
7
23
1807
o
2
10
1849
42
20
96
IO
1724
26
i 7 66
o
o
17
1808
i
O
2
i85o
25
3o
64
10
1726
3o
1767
5
4
34
1809
o
2
I
i85i
J 7
ai
62
8
1726
46
1768
2
7
52
1810
o
O
i85 2
45
42
52
8
1727
67
1769
10
18
86
1811
o
I
i853
26
22
38
7
1728
86
1770
i3
i4
79
1812
o
5
i854
36
15
*9
7
1729
65
1771
29
i5
73
i8i3
O
74
7
i855
20
7
6
1780
116
1772
21
7
49
1814
4
3
20
8
i856
20
4
6
i 7 3i
57
i 77 3
3i
'7
4o
i8i5
o
I
35
8
i85 7
i5
22
7
1732
IOO
i 77 4
48
20
48
1816
I
o
45
i858
34
5 1
7
I 7 33
27
i 77 5
21
5
27
1817
I
44
9
i85 9
46
9 6
10
1734
38
i 77 6
12
4
35
1818
2
4
34
1860
33
99
IO
1735
5i
i 777
26
i5
63
1819
3
6
22
8
1861
35
77
9
1736
43
i 77 8
3o
18
9 5
1820
2
2
9
8
1862
33
5 9
8
i 7 3 7
4o
i 77 9
37
4
99
1821
2
O
4
9
i863
36
44
8
1738
9
1780
2O
25
73
1822
I
3
i864
4 7
46
8
I 7 3 9
27
1781
2 9
25
68
1823
O
O
i
8
:865
98
3o
7
TABLE XXXV. CATALOGUE OF LARGEST IRON METEORS. 283
TABLE XXXV.
CATALOGUE OF THE LARGEST IRON METEORS.
" Locality.
Year
found.
Pounds'
Weight.
Spec.
Grav.
Remarks.
Duranffo Mexico .
l8ll
1784
1869
1784
1866
35,ooo
33,000
22,000
i7,3oo
sev'l tons
7.88
7.60
7-7-3
7.82
Specimens at Berlin, Vienna, etc.
( Specimen of 1400 Ibs. belongs to
| British Museum.
Specimens at Vienna, Boston, etc.
Specimens at Munich, London, etc.
Spec, belongs to Prof. C. U. Shepard.
Otumpa, Buenos Ayres. .
Rogue River, Oregon . . .
Bemdego River, Brazil . .
Bonanza, Mexico
Near Melbourne, Aus. . .
Sierra Blanca, Mexico . .
Bitbero* Prussia
1861
1784
1802
1861
1792
8.287
4,OOO
3,4oo
2,800
2,000
7 .5i
6.5o
6.33
7 .5i
7 .5o
Belongs to British Museum.
Specimen at Berlin.
Specimens at Vienna, Berlin, etc.
Belongs to Colonial Government.
Specimens in Br. Museum, Berlin, etc.
Near Melbourne, Aus. . .
Zacatecas Mexico . . .
Cocke Co., Tennessee. . .
Santa Rosas, New Gran.
Jenisey River, Siberia
Red River Texas
i84o
1810
1772
1808
1735
2 ; 000
1,700
I 680
1,635
i,4oo
7.26
7 .3o
6.48
7.70
Belongs to British Museum.
Specimens at Vienna, Paris, etc.
Belongs to Imp. Mus., St. Petersburg.
Belongs to Yale College.
Belongs to Smithsonian Institute.
Tucson, Arizona, U. S. . .
La Caille France
1828
i863
1 846
1866
i854
1,100
632
542
436
368
7-64
7.29
5. 97
7.69
Belongs to Jardin des Plantes, Paris.
Belongs ;to the City of San Francisco.
Specimens at Vienna, London, etc.
Belongs to Prof. C. U. Shepard.
Geological Cabinet at Montreal.
Tucson, Arizona, U. S. . .
Tula Russia
Bear Creek, Colorado . .
Madoc, Upper Canada . .
Orange River, S. Africa .
Cape of Good Hope ....
Atacama Bolivia
i856
i 79 3
1827
i85o
1 846
326
3oo
3oo
292
280
7-4o
7-44
7 .38
Belongs to Prof. C. U. Shepard.
Belongs to Haarlem Cabinet, Holland.
Belongs mostly to British Museum.
Specs, belong to Prof. Silliman, et al.
A large spec, belongs to Brit. Museum.
Pittsburg, Pennsylvania .
Carthage, Tennessee. . . .
Coahuila, Mexico
i855
1847
1784
1810
1814
262
218
218
200
194
7.81
7.70
7.38
6.20
7 . 7 5
Belongs to Smithsonian Institute.
Belongs partly to British Museum.
150 Ibs. belong to Prof. C. U. Shepard.
Belongs to University at Kiew.
Belongs to Museum of Pesth.
Seelasgen Silesia
Toluca Mexico
Brahin, Russia
Lenarto Hungary
Elbogen, Bohemia
1811
i853
i832
i856
1819
I 9 I
I 7 2
i65
161
i5o
7.74
7.60
7.26
75o
Chiefly in the Cabinet at Vienna.
Belongs to Prof. C. U. Shepard.
Half belongs to British Museum.
Specimens in Berlin, London, etc.
Spec, belongs to Prof. C. U. Shepard.
Lion River, South Africa
Walker Co., Alabama . .
Nelson Co., Kentucky . .
Burlington, New York . .
Ruff's Mountain, South *
Carolina . . /
i85o
1860
1829
i 7 5i
i84?
116
112
io3
87
72
7.10
7.89
7.60
7.82
7.71
Mostly belongs to Prof. C. U. Shepard.
Mostly belongs to Prof. C. U. Shepard.
Belongs to Museum of Prague.
Belongs chiefly to Vienna Cabinet.
Specimens in Vienna, Berlin, etc.
Lagrange, Oldham Co., )
Kentucky /
Bohumilitz, Bohemia . . .
Agram Croatia
Braunau, Silesia
Putnam Co., Georgia. . .
jTazewell, Claiborne Co., j
* Tennessee . . /
1839
i853
i85o
1866
i834
70
55
43
4o
4o
7.69
7.88
7-77
7-67
6.5o
Belongs partly to Prof. C. U. Shepard.
Specimens in London, Berlin, etc.
Chiefly in Berlin.
In Geological Cabinet at Austin.
Spec, belongs to C. T. Jackson, Boston.
Schwetz Prussia
Denton Co Texas
Claiborne, Clarke Co., \
Alabama J
284 TABLE XXXVI. AEROLITES FALLEN IN UNITED STATES.
TABLE XXXVI.
AEROLITES FALLEN EN THE UNITED STATES.
Locality.
Date of FalL
Weight in
Pounds.
Specific
Gravity.
Present Owners.
Weston, Conn. . . .
Caswell Co., N. C. .
Nobleborough, Me.
Nanjemoy, Md. . . .
Sumner Co., Tenn.
Richmond, Va. . . .
1807, Dec. i4.
1810, Jan. 3o.
1 823, Aug. 7.
1825, Feb. 10.
1827, May 9.
1828, June 4-
3oo
3
5
16
ii
4
3.58
3.5?
3.09
3.66
3.55
3.34
Yale College, et al.
Unknown.
C. U. Shepard, et al. [al.
Yale Coll.; C. U. Shepard, et
C. U. Shepard; Leyden Cab.,
C. U. Shepard, et al. [et al.
Forsyth, Monroe )
Co Ga f
1 829, May 8.
36
3.46
Yale Coll. ; C. U. Shepard, et
Deal N J ....
1829 Aug 1 5
JL.
3.25
C. U Shepard, et al t al *
Dickson Co., Tenn.
Little Piney, Mo. .
Bishopville, S. C. .
Linn Co., Iowa . . .
i835,July3i.
i83 9 ,Feb.i3.
i843,Mar. 2 5.
1 847, Feb. 25.
1
si
i3
75
7.76
3.5
3.o4
3.58
Mobile Cabinet, et al.
C. U. Shepard, et al.
C. U. Shepard, et al. [al.
Yale Coll. ; C. U. Shepard, et
Castine, Me
1 848, May 20
1
3.45
Bowdoin College, et al
Cabarrus Co., N. C.
Petersburg, Tenn. .
Harrison Co., Ind. .
Bethlehem, N. Y. .
New Concord, O. .
1 849, Oct. 3i.
i855 r Aug. 5.
1 859, Mar. 28.
1859, Aug. ii.
1 860, May i.
18*
4
2
}
7OO
3.63
3.20
3.46
3.56
3.54
Yale Coll. ; C. U. Shepard, et
C. U. Shepard, et al. [al.
C. U. Shepard, et al.
Albany Cabinet, et al. [al.
Marietta Coll. ; Yale Coll., et
EXPLANATION OP THE TABLES.
Table I., page 251, contains a comparison of French millimetres
with English inches, and will be found convenient for reducing
French measures into English. It is deduced from the assump-
tion that the French metre at the freezing point is equal to
39.37079 English inches at the temperature of 62 Fahrenheit,
the standard temperature of the French scale being 32 Fahren-
heit,- and that of the English scale being 62 Fahrenheit. This
is the result given by Captain Kater in the Philosophical Trans-
actions for 1818, page 109. The table of proportional parts in
the last column gives the value of tenths of a millimetre in En-
glish inches, and will serve for hundredths by removing the deci-
mal point one place to the left.
Table II., page 252, enables us to convert French metres into
English feet, and is derived from the same data as the preceding
table ; that is, the French metre is equal to 3.2808992 English
feet. The table of proportional parts in the last column may be
used in the same manner as described in Table I.
Table III., page 253, enables us to convert French kilometres
into English miles, and is derived from the same data as Table I. ;
that is, the French kilometre is equal to 0.6213824 English mile.
The table of proportional parts in the last column may be used
in the same manner as described in Table I.
Table IV., page 254, enables us to convert French feet into En-
glish feet. The old legal standard of France was the Toise de
PeroUj so called from its being used by the French academicians
in their measurement of an arc of the meridian in Peru. It is
formed of iron, and was made in 1735. According to Base du
Systeme Metrique, t. iii., p. 237, the metre is equal to 0.513074 toise,
286 METEOROLOGY.
or 3.078444 French feet, which is equal to 3.2808992 English feet.
Hence one French foot is equal to 1.065765 English feet.
The arrangement of Table IV. is similar to that of the preced-
ing tables. The same table will serve equally well for converting
French inches into English inches.
Table V., page 255, enables us to convert degrees of the cen-
tesimal thermometer into degrees of Fahrenheit. It is founded
g
on the equation x centesimal = (32 + -=x) Fahrenheit.
o
Table VI, page 256, enables us to convert degrees of Keau-
mur's thermometer into degrees of Fahrenheit. It is founded on
9 *
the equation x Keaumur=:(32 + jx) Fahrenheit.
Table VII., page 257, gives the height of a column of air cor-
responding to a tenth of an inch in the barometer for different
temperatures from 40 to 90, and may be used for reducing ba-
rometrical observations to the level of the sea, or to any other
level.
Example. At Cambridge, Massachusetts, at 70 feet above the
sea, the mean height of the barometer is 29.940 inches, and the
mean temperature 48 ; what would be the height at the level of
the sea?
From Table VII. we find for barometer 29.94, and temperature
48, the number 90.8.
70
Then the required correction equals -- = 0.075.
And 29.940 + .075 =30.015 inches, the height of the barometer
at the level of the sea.
This table is derived from Guyot's Meteorological Tables, pub-
lished by the Smithsonian Institution, D. 92.
Table VIII., pages 258-9, gives the correction to be applied to
English barometers with brass scales for reducing the observa-
tions to 32 Fahrenheit, and is the same as adopted by the Eoyal
Society of London. From 29 up the correction must be sub-
tracted from the observed height, while from 28 down it must be
added.
EXPLANATION OF THE TABLES. 287
Example 1. Observed height of barometer, 29.876 ; attached
thermometer, 73 Fahrenheit.
On page 259, in the column headed 30 inches, on the horizon-
tal line corresponding with 73 in the first vertical column, we
find the correction .119. Hence the barometer, reduced to 32
Fahrenheit, will be 29.876 .119=29.757 inches.
Example 2. Observed height of barometer, 29.854 ; attached
thermometer, 17 Fahrenheit.
On page 258, under 30 inches and opposite to 17, we find the
correction +.031. Hence the barometer, reduced to 32 Fahren-
heit, will be 29.854 + .031 =29.885 inches.
If we wish the correction for a fraction of a degree,, we must
take a proportional part of the difference between the corrections
for the nearest whole degrees in the table.
Table IX., pages 260-1, enables us to compute the difference in
the heights of two places by means of the barometer. The con-
struction of the table is fully explained in my Introduction to
Practical Astronomy, page 480.
Method of Computation.
Take from Part I., page 260, the two numbers corresponding
to the observed barometric heights h and h f . From their differ-
ence subtract the correction found in Part II., with the difference
T T' of the thermometers attached to the barometers. We thus
obtain an approximate altitude, a.
We then calculate the correction ~-^r a for the tempera-
ture of the air by multiplying the nine hundredth part of a by
the sum of the temperatures t and t f diminished by 64. This
correction is of the same sign as t-\-t f 64. We thus obtain a
second approximate altitude, A.
With A and the latitude of the place, we seek in Part III. the
correction arising from the variation of gravity with the latitude.
With A we also seek in Part IV. the correction arising from the
diminution of gravity on a vertical. Also, when the height of the
lower station is considerable, another small correction is found in
Part Y. The last? two corrections are always additive.
Example. The following observations were made at Geneva and
on Mount Blanc, 3.3 feet below the summit of the mountain.'
288
METEOROLOGY.
Mount Blanc, A'=16.695 inches, T'=24.4 Pah., *' = 18;3 Fah.
Geneva, h =28.727 " "T =65 .5 " Z =66 .7 "
26476.8
12297.3
14179.5
-96.2
( for h =28.727 inches,
Part I. gives { forV=1&695
Difference,
Part II. gives for T T'=41.l
Approximate altitude, a= 14083.3
m=^
Second approximate altitude, A =14411.9
Part III. gives for lat. 46, 1.4
Part IY. gives for 14412, +46.0
Part Y. gives for bar. 28.7, + 1.5
Sum,
Height of Geneva above the sea,
Barometer below summit of Ml Blanc,
14458.0
1335.4
3.3
Height of Mount Blanc above the sea, 15796.7 feet.
Table X., page 262, furnishes the mean height of the barometer"
at nine stations upon the American continent ; also at nine sta-
tions in the western part of the Eastern continent ; and at nine
stations in the eastern part of that continent. The precise locali-
ty of these stations is shown in the following table :
Georgetown, Br. Guiana
Havana Cuba
Latitude.
6 50'
23 9
Longitude.
58 12'
82 23
Greenwich, England..
St Petersburg Russia
Latitude.
51 28'
59 56
Longitude.
0'
- 30 18
Natchez Miss
31 34:
91 24
Archangel Russia
64 32
- 40 33
38 37
90 15
Hammerfest, Lapland
70 39
- 23 42
Philadelphia, Penn
Boston Mass .
39 58
42 21
75 10
71 3
Singapore, Malacca...
Madras, Hindostan ...
1 17
13 4
-103 50
- 80 19
Toronto Canada
43 40
79 22
Bombay Hindostan
18 56
72 54
Port Bo wen Arc Reg
73 14
88 56
Canton China . . .
23 8
-113 16
Van Rensselaer Harbor.
Christiansborg Africa
78 37
5 24
70 53
- 16
Benares, Hindostan..
Pekin China
25 18
39 54
- 82 56
116 26
Aden Arabia
12 50
-45 6
Tiflis Georgia
41 41
- 45 17
30 2
-31 15
Nertschinsk Russia
51 18
119 20
Constantinople, Turkey.
Paris. France ...
41
48 50
-29-
- 2 20
Jakutsk, Siberia
62 1
-129 44
A portion of the numbers in this table was derived from an
article by Professor Dove, published in the Monatsberichte der
Akademie zu Berlin, 1860, pages 644-692 ;. the remainder was
derived from a variety of sources.
EXPLANATION OF THE TABLES. 289
Table XL, page 263, furnishes the mean height of the barome-
ter for all hours of the day at nine stations from the equator to
latitude 78. Most of these places are included in the preceding
list. A portion of these numbers was derived from Kamtz's
Lehrbuch der Meteorologie, vol. ii., pages 254-259 ; the others
were derived from various sources.
Table XII., page 263, furnishes the depression of mercury in
glass tubes on account of capillarity, according to several differ-
ent authorities.
Table XIII., page 264, gives the weight of a cubic foot of dry
air and of saturated air under a barometric pressure of 30 inches,
at temperatures between and 90 R The weight of a cubic
foot of dry air is assumed to be 563 grains troy at a temperature
of 32 F., and the coefficient of expansion is 0.002083 of its bulk
for 1 F.
The weight of a cubic foot of saturated air is found by adding
to the weight of a cubic foot of dry air the weight of a cubic foot
of vapor, and correcting this result for the enlargement of volume
resulting from the mixture. This table is derived from the Green-
wich Meteorological Observations for 1842, pages 46 and 51.
Table XI V., page 265, shows the height of the barometer cor-
responding to temperatures of boiling water from 188 to 213 F.
The temperature at which water boils in the open air depends
upon the weight of the atmospheric column above it, and under
a diminished barometric pressure the water will boil at a lower
temperature. Since the weight of the atmosphere decreases with
the elevation, it is evident that, in ascending a mountain, the high-
er the station, the lower the temperature at which water boils.
Hence, if we know the height of the barometer corresponding to
the temperature of boiling water, we can measure the altitude of
a mountain by observing the temperature at which water boils.
This table is copied from my Practical Astronomy, page 398.
Table XY., page 266, gives the corrections to be applied to the
means of the hours of observation to obtain the true mean tem-
perature at New Haven. These numbers are the differences,
with opposite signs, between the hourly temperatures and the
T
290 METEOROLOGY.
true mean temperature of each month and also of the year.
Thus, at New Haven, the mean temperature of January is 26.5 ;
the mean temperature at midnight in January is 24.2 ; the dif-
ference is 2.3, which is the quantity which must be added to
midnight observations to obtain the mean temperature of that
month, and so for the other hours and months of the table.
At the bottom of the table is given a comparison of some of
the different modes which have been proposed for deducing the
mean temperature from a limited number of observations. Thus,
if we have observations at 7 A.M. and 1 P.M. in January, the
former require a correction of -f 4 A and the latter of 6.l ; the
mean of the two will require a correction of 0.8, as given in
line 26th of the table.
If we have observations at 6 A.M., 2 and 6 P.M. in January,
the corrections for these three hours will be +4.3, 6. 3, and
1.4. The mean correction is l.l, which is the number given
in line 36th of the table.
If we have observations at 7 A.M., 2 and 9 P.M , and if we add
twice the nine o'clock observation to the surn of the other two
observations, and divide the result by 4, the error of the result
for the separate months in only one instance exceeds a quarter of
a degree. This table is copied from the Transactions of the Con-
necticut Academy of Arts and Sciences, vol. i., p. 231.
Table XVL, page 267, is constructed for Greenwich, England,
in the same manner as the preceding, and is taken from the Green-
wich Meteorological Observations.
Table XVII., pages 268-9, gives the mean temperature of 45
places on the American continent for each month of the year.
Some of these numbers are derived from Dove's Tables in the
Eeport of the British Association for 1847, page 376 ; others are
derived from the Army Meteorological Register, 1855, and some
from other sources.
Table XVIII., page 270, furnishes a list of places whose mean
temperature is above 80 F. The materials are derived chiefly
from Dove's Tables.
Table XIX., page 270, furnishes a list of places whose mean
EXPLANATION OF THE TABLES. 291
temperature is below 18 F. This is also derived chiefly, but not
exclusively, from Dove's Tables.
Table XX., page 271, furnishes a list of places where the mean
temperature of the hottest month differs less than six degrees
from that of the coldest month, and is chiefly derived from Dove's
Tables.
Table XXI., page 271, furnishes a list of places where the mean
temperature of the hottest month differs more than sixty-six de-
grees from that of the coldest month. The materials are derived
partly from Dove's Tables, partly from Kupffer's Annales, and
partly from other sources.
Table XXII., page 272, furnishes a list of places where the an-
nual range of temperature is less than 40. The materials were
derived partly from Arago's Works, vol. viii., pages 184-646, but
many of the numbers were obtained by an extensive comparison
of Meteorological Journals.
Table XXIII., page 272, furnishes a list of places where the
annual range of temperature is greater than 130. The materials
were derived partly from Arago, vol. viii., but many of the num-
bers were obtained by an extensive comparison of Meteorological
Journals, particularly Kupffsr's Annales, the Army Meteorolog-
ical Eegister, and the New York Meteorological Observations.
Table XXIV., page 273, shows the height of the line of per-
petual snow above the level of the sea for a variety of latitudes.
The works chiefly depended upon in preparing this table are the
Encyclopaedia Metropolitana, Art. Meteorology, page 84 ; Miiller's
Lehrbuch der Kosmischen Physik, page 353 ; and Kaemtz's Me-
teorology.
Table XX V., page 273, contains the factors by which the dif-
ference of readings of the dry -bulb and wet-bulb thermometers
must be multiplied in order to produce the difference between
the readings of the dry -bulb and dew-ppint thermometers. These
factors are derived from a long series of observations made at the
Greenwich Observatory, and enable us to convert observations
292 METEOROLOGY.
made with the wet-bulb thermometer into observations made
with Daniell's hygrometer.
Example. The temperature of the air being 44.5, and that of the
wet-bulb being 38. 7, it is required to determine the dew-point.
The difference between the dry and wet bulb thermometer is
5.8, which, multipled by 2.17, gives 12. 6, which is the difference
between the dry-bulb and dew-point thermometers. Hence the
dew-point was at 31.9.
Table XXVL, pages 274-5, shows the relative humidity of the
air at temperatures from 6 to 95, and for a difference of tem-
perature of air and of the dew-point from to 24. The rela-
tive humidity is the ratio of the quantity of vapor actually con-
tained in the air to the quantity it could contain if fully satu-
rated, Art. 105. This humidity is deduced from Table XXVII.
Example. Suppose the temperature of the air is 90 F., and
that of the dew-point is 80 F., the difference being 10 F. Ac-
cording to Table XXVIL, the elastic force of vapor at these two
temperatures is 1.410 and 1.023 ; their ratio is .73, which is the
relative humidity, and is the number given in the table for a tem-
perature of 90, and a dew-point 10 below the temperature of the
air. Making the point of saturation 100, all the numbers in the
table are to be regarded as integers. This table is abridged from
one given in the Smithsonian Meteorological Tables, B. 75.
Table XXVIL, page 276, gives the elastic force of aqueous va-
por for temperatures from 30 to 101 F., according to the ex-
periments of Eegnault. The table is abridged from the Smith-
sonian Tables., B. 43.
Table XXVIII, page 277, is designed to furnish a comparison
between the pressure and velocity of the wind. It is derived
from the Meteorological Papers of the British Board of Trade,
third number, page 99, and was computed by Colonel James, as-
suming that the square of the velocity in miles per hour, multi-
plied by 0.005, gives the pressure in pounds per square foot.
These numbers differ slightly from those given on page 70, but
neither table can be regaled as perfectly reliable. More numer-
ous experiments are needed for determining the pressure of the
wind at different velocities.
EXPLANATION OF THE TABLES. 293
Table XXIX., pages 278-9, gives the average amount of rain
for each month 'of the year at 45 stations on the American conti-
nent, extending from near the equator to the highest northern
latitude for which such observations could be found. A consid-
erable part of these results is taken from the Army Meteorologi-
cal Register, published in 1855 ; the remainder are chiefly de-
rived from Dove's Klimatologische Beitrage, and the Meteorolog-
ical Observations of the Smithsonian Institution, while a few have
been derived from other sources.
Table XXX., page 280, gives a list of stations at which the an-
nual fall of rain is less than ten inches. The table furnishes the
authorities for the results here given.
Table XXXI., page 280, gives a list of stations at which the
annual fall of rain exceeds twelve feet. These stations have gen-
erally considerable elevation above the sea, but in many of the
cases the heights, not being accurately known, could not be given
in the table.
Table XXXlL, page 281, shows the comparative radiating
power of different substances at night, according to the observa-
tions of Mr. G-laisher, made at Greenwich, England, and published
in the Philosophical Transactions for 1847, page 119. The num-
bers refer to long grass as the unit.
Table XXXIII., page 281, shows the fall of the barometer dur-
ing several remarkable hurricanes in the West Indies, the East In-
dies, and elsewhere. The table shows the fall in the number of
hours given in column fourth, but this is not generally the entire
fall of the barometer during the day of the hurricane, for the high-
est point of the barometer usually occurs some hours before the
rapid fall begins, or some hours after the most rapid rise at the
close of the storm.
Table XXXIV., page 282, gives a catalogue of auroras ob-
served in Europe since 1685, and in America since 1742, the lat-
ter being chiefly confined to Boston and New Haven. These num-
bers show clearly the unequal frequency of auroras in the different
years, and these inequalities indicate a period of ten or twelve
294 METEOROLOGY.
years, with a more decided period of about sixty years. The ta-
ble also shows the relative frequency of solar spots since 1749,
and the mean daily range of the magnetic needle as observed in
Europe since 1782. It will be noticed that the last two phenom-
ena show most decided periodic inequalities, and these periods
correspond remarkably with the periods of auroral abundance.
The table is abridged from several tables published in the Smith-
sdnian Keport for 1865, pages 225-243.
Table XXXV., page 283, gives a catalogue of the principal iron
meteors exceeding 40 pounds in weight. It is not claimed that
this catalogue is complete, for in the report of many meteors the
weight is not definitely stated. The number of iron meteors
whose weight is less than 40 pounds is nearly equal to the num-
ber embraced in this catalogue. This catalogue has been com-
piled from a great variety of sources, but chiefly from Buchner's
Meteoriten, 1863.
Table XXXVL, page 284, gives a list of the aerolites fallen
in the United States. Besides these, there are five or six other
cases in which aerolites have been claimed to have fallen, but as
those cases are not considered to be sufficiently well attested they
have been omitted.
EXPLANATION OF THE PLATES.
PLATE I. shows the prevalent winds at eight stations of the
American continent from near the equator to latitude 78 N.
Horizontal and vertical lines are drawn to represent the four
cardinal points, and diagonal lines are drawn for the intermediate
directions. On these eight lines are set off distances correspond-
ing to the relative frequency of the winds from these eight quar-
ters. The curve line passing through the eight points thus de-
termined may be regarded as showing the prevalent wind at that
station. It is thus seen that at Van Kensselaer Harbor and at
Godthaab the prevalent wind is from the N.B. ; at Norway
House it is from the north ; at St. Johns, New York, and Savannah
the prevalent wind is from the S. W. ; at Matanzas it is from the
N.E. ; and at Georgetown it is nearly from the east.
Plate II. represents the six varieties of cloud described on
pages 101 and 102, each variety being indicated by a symbol
shown at the bottom of the page.
Plate III. exhibits two outline maps of the United States, de-
signed to illustrate the phenomena of a storm described on pages
142 and 143.
WORKS ON METEOEOLOGY.
THE student who desires a more thorough knowledge of me-
teorology than can be obtained from this treatise, is referred to
the following works and memoirs :
Annales de 1'Observatoire Physique Central de Kussie. One
large quarto volume of observations annually.
Annuaire Magnetique et Meteorologique du Corps des Inge-
nieurs des Mines de Kussie.
Annuaire de la Societe Meteorologique de France. One large
octavo volume annually since 1849.
Apjohn. Theory of the Moist-bulb Hygrometer, Edinb. Philos.
Transact, xvii.
Arago. CEuvres Completes, 12 volumes, 8vo. Etat thermome-
trique du Globe terrestre. Influence de la Lune. La pluie. Le
tonnere. Puits Artesiens, etc.
Baddeley. On Dust Whirlwinds and Cyclones in India.
Biot. On Mirage and unusual Kefraction, Mem. de 1' Academic,
1809.
Birt. Reports to the British Assoc. on Atmospheric "Waves,
1844-8.
Blodget. Climatology of the United States. Philadelphia, 1857.
Boue. KatalogderKordlichterbisl856. Wien Acad., 74 pages.
Bravais and Martin. Comparaisons Barom. faites dans le Nord
de 1'Europe, Mem. Acad. Brux., xiv.
Brewster. On the Mean Temperature of the Globe, Edinb. Phil.
Transact., ix. Results of Thermometrical Observations at Leith
Fort, do., x.
Buchan. Handy Book of Meteorology. London, 1867.
Buchner. Meteoriten. Leipsig, 1863.
Bulletin de 1'Observatoire de Paris. One sheet daily, contain-
ing Meteorological Reports from every part of Europe.
Buist. Catalogue of Indian Hailstones and Meteors.
WORKS ON METEOROLOGY. 297
Buys Ballot. Sur }a marche annuelle du thermometre et du ba-
rometre en divers lieux de 1'Europe, 1849-59, Amsterd. Acad.,
1861, 116 pages.
Cordier. On Temperature of Interior of the Earth, Mem. Acad.
Sci., 1827.
Correspondence, Met. de 1'Obs. Central Physique de Kussie. A
thin quarto volume annually.
Cotte. Meteorologie, Paris, 1774.
Daguin. Traite de Physique, avec les applications a la Meteo-
rologie, 4 vols., Paris, 1861.
Dalton. On Eain and Dew, Manchester Mem., v. On the
Constitution of Mixed Gases, do. Met. Obs. and Essays, London.
On Constitution of the Atmosphere, Phil. Trans., 1826. On
Height of Aurora Borealis, Phil. Trans., 1828.
Daniell. Meteorological Essays, 2 vols. 8vo, London. On the
Constitution of the Atmosphere.
De Luc. On Hygrometry, Ph. Trans., 1791. On Evaporation,
do., 1792.
Dove. Tables of Mean Temperature, Eep. of Br. Assoc., 1847.
Verbreitung der Warme auf der Obernache der Erde, 4to. Kli-
matologische Beitrage, 1861. Das Gesetz der Stiirme, 2d edit.
Ermann. Ueber Boden und Quellen Temperatur. Ueber
einige Barom. Beob., Poggendorff, Ixxxviii.
Espy. Philosophy of Storms, Boston, 1841. Eeports on Me-
teorology of U. S.
Fitzroy. Weatherbook, a Manual of Practical Meteorology.
London, 1863.
Forbes. Eeport to Brit. Assoc. on Meteorology, 1832. Supple-
mentary Eeport, 1840. On the Climate of Edinburg for 56 years,
Tran|. E. S. Edinb., xxii., part 2.
Fritsch. Periodische Erscheinungen in Wolkenhimmel, E. Bo-
hem. Acad., v. Folge, Bd. iv.
Gallon. Meteorographica, or Maps of the "Weather, 4to. Lon-
don, 1863.
Q-ehler. Worterbuch, Arts. Meteorologie, Eegen, etc.
Olaisher. On Nocturnal Eadiation, Phil. Trans., 1847. On Cor-
rection of Monthly Means of Met. Obs., Phil. Trans., 1848.
Greg. On Aerolites, L., E., and Dub. Phil. Mag., 1854, p. 329.
Catalogue of Meteorites, Eep. Br. Assoc., 1860, p. 48.
Guyot. Meteorological Tables, 8vo, 1859.
298 WORKS ON METEOROLOGY.
Harvey. Art. Meteorology, Encyc. Metropolitana.
Heis. Ueber Sternschnuppen. Koln, 1849.
Herschel, J. F. W. Admiralty Manual of Scientific Inquiry. Lon-
don, 1851. Meteorology, 1862.
Hopldns. On Winds and Storms. London, 1860.
Hough. New York Meteorological Observations, 1826-50. Al-
bany, 1855.
Howard. Climate of London, 3 vols., 8vo. On a Met. Cycle of
18 years, Phil. Trans., 1841. Barometrographia.
Hudson. Hourly Obs. of the Barometer, Phil. Trans., 1832.
Humboldt. On Isotherms, Mern. d'Arceueil, iii. On Inferior
Limit of Perpetual Snow, Ann. de Chim., xiv. Kosmos.
Jelinek. Tagliche Gang der Meteorolog. Elemente. "Wien.
Johnson. Met. Obs. at RadclifFe Observatory, Oxford.
Johnston, Keith. Physical Atlas of Natural Phenomena.
Kamtz. Lehrbuch der Meteorologie. Leipsig, 3 vols. On Iso-
barometric Lines. Meteorology translated C. V. Walker.
Kirkwood. Meteoric Astronomy. Philadelphia, 1867.
Roller. Gang der Warme in Oesterreich (Kremsmunster, 1841).
Kupffer. On Springs and Earth Temp., Poggendorff, xx.
Lamont: Beobachtungen auf d. Hohenpeissenberg, 1792-1850.
Miinchen, 1851. Annalen fur Meteorologie.
Lawson. Army Meteor. Eegister for 12 years, 1843-54. Wash-
ington, 1855.
Loomis. On two Storms which octurred in February, 1842,
Trans. Am. Phil. Soc., vol. ix. On the Storm of December, 1836,
Smith. Contrib., 1860. On the Aurora Borealis, Smith. Keport,
1865, p. 208. Mean Temperature of New Haven, Conn., Trans.
Conn. Acad., vol. i.
Mahlmann. Temperature auf der Oberflache der Erde (Cove's
Eepertorium, Bd. iv.).
Mairan. Traits' de 1' Aurore Boreale, 2d ed., Paris.
Maury. Storm and Eain Charts of the North and South At-
lantic.
Meech. Relative Intensity of the Heat and Light of the Sun
upon different Latitudes, Smith. Contr., 1855.
Meteorological Society (of London). Transactions and Council
Reports.
Muhry. Klimatologische Untersuchungen, 1858.
Muller. Lehrbuch du Kosmischen Physik.
WORKS ON METEOROLOGY. 299
Newton. On Shooting Stars, Mem. Nat. Acad. Sciences, vol. i.
Original Accounts of November Star Showers, Am. Jour. Science,
N. S., vol. xxxvii., p. 377. Contributions to Astro-Meteorology,
Journ. Sc., vol. xliii., p. 285, etc.
Olmsted. Secular Period of the Aurora Borealis, Smith. Contr.,
1855.
Partsch. Die Meteoriten. Wien, 1843.. .
Peltier. Sur les Trombes. Paris.
Phipson. Meteors, Aerolites, and Falling Stars. London, 1867.
Piddington. Nineteen Memoirs on Cyclones in the Indian and
China Seas. Sailor's Horn-Book.
Plantamour* Des Anomalies de la Temperature a Geneve, 1867.
Kesume' des Obs. Therm, et Bar. a Geneve.
Pouillet. Mem. sur la Chaleur Solaire, Comptes Kendus, 1838.
Quetelet. Sur le Climat de la Belgique. Catalogue des Appari-
tions des Etoiles Filantes, Acad. Brux., 1839. Variations Pe-
riodiques de la Tern., Acad. Brux., xxviii. Meteorologie de Belg.
Reid. Law of Storms. On Storms and Variable Winds.
Redfield. On the Courses of Whirlwinds. Am. Journ. Sc.,
xxxv., etc.
Robinson. Improved Anemometer, Koyal Irish Academy, xxii.
Saline. Eeport on Meteorology of Toronto, Br. Assoc., 1844.
Lunar Tide at St. Helena, Phil. Trans., 1847. Meteorology of
Bombay, Phil. Trans., 1853. Variations of Temperature at To-
ronto, Phil. Trans., 1853.
Saussure. Essais de 1'Hygrometrie. 1783.
Schlagintweit. Eesults of a Scientific Mission to India, 1854-8,
4 vols. 4to, Leipsig.
Schouw. Beitrage zu Vergleichenden Klimatologie, Bibl. U.,
xxxiv.
Schulkr. Atmospheric Electricity. Jahrbuch der Chem. und
Phys., 1829.
Secchi. Kesults of Met. Obs. at Home, Bibl. U., 1857.
Sykes. On Atmospheric Tides, Phil. Trans., 1835. Observations
in India, Phil. Trans., 1850.
Thomson. Introduction to Meteorology. London 1849.
Welsh. Account of four Balloon Ascents, Phil. Trans., 1856.
Wells. On Dew. London 1818.
Whewell. On a new Anemometer, Trans. Cam. Phil. Soc., vi.
Wollaston. On the finite Extent of the Atmosphere, Phil. Trans.
300 WORKS ON METEOROLOGY.
Coffin. Winds of the Nortnern Hemisphere, Smithsonian Con-
tributions, vol. vi.
Ferrel Motions of Fluids and Solids relative to the Earth's
Surface, Math. Monthly, vols. i. and ii.
Herrick. Kegister of the Aurora Borealis, Transactions of the
Connecticut Academy, vol. i.
Quetelet. Meteorologie.
Smithsonian. Meteorological Kesults, 1854-9.
INDEX.
Aerolite at Braunau, Bohemia
at Guernsey, Ohio
atOrgueil, France
atWeston, Conn
Aerolites, composition of.
conclusions
described
formed in our atmosphere
from lunar volcanoes
" terrestrial volcanoes.
in the United States
number of.
peculiarities of.
periodicity of.
Air exerting one tenth inch pressure.
Anemometer, Osier's
Robinson's
Whewell's
Woltmann's
Anemoscope described
" self-registering
Anomalous months, conclusions from
Arcs intersecting opposite the sun....
" touching halo of 46
Atlantic Ocean, two sides of.
Atmosphere, actual height of.
composition of.
heated, how
height deduced from twi-
light
limit of, estimated
regulates temperature..
upper half of.
" regions of.
Atmospheric circulation, system of...
August meteors described
meteors, elements of.
stream, dimensions of.
Aurora caused by nebulous matter...
colors of, described
" explained
conflicting estimates
height of
noise of.
polaris
varieties of.
Auroral arches, anomalous forms of..
Pag*
Auroral arches, anomalous position
of 196
Auroral arches, breadth of. 1 79
" described 174
" form of. 179
" movements of 180
' ' position described .... 1 79
" " explained... 196
" structure of. 181
beams, cause of. 196
" described 175
" explained 194
" motion of. 182
clouds 183
corona described 175
" explained 193
" position of. 182
exhibitions, terrestrial 191
flashes, cause of. 197
light is electric light 192
vapor 183
waves or flashes 176
annual inequality of. 198
" periodicity of. 189
dark segment of. 178
distribution described 186
" explained 199
diurnal inequality of. 198
" periodicity of 188
duration of. 177
geographical extent of. 177
in both hemispheres de-
scribed 188
in both hemispheres ex-
plained 200
in Southern hemisphere.... 188
recurring fits of. 177
secular inequality of. 198
" periodicity of. 189
solar spots, etc. , on 282
within the tropics 200
Page
242
242
243
241
244
249
241
247
247
247
284
243
244
246
259
68
67
66
66
65
66
158
224 Auroras,
223
37
11
9
27
210
11
43
76
10
78
235
236
236
191
177
193
185
184 Ball lightning 167
186 Banks, temperature of. 51
173 Barometer, accidental variations of... 21
173 affected by the moon 20
180 " air excluded, how 12
302
INDEX.
Barometer, Aneroid
" capillarity, correction of..
" column measured, how. ...
" construction of.
" corresponding to boiling
water
* * dependent upon height ....
" diurnal variation ex-
plained
" extreme fluctuations of...
" fall in hurricanes
" falls under a cloud
Hardy's self-registering . .
" height at all hours
" in different months
' ' heights measured by
" " " tables
high near lat. 32
* * Hough's printing
' ' hourly variation s of
influenced by the wind. ...
" influence of latitude on....
" low near lat. 64
" " the equator
" mean height of.
monthly means
" observations, how repre-
sented
reduced to freezing point.
4 ' self-registering
temperature, correction of
two daily maxima
Barometric depression, amount of....
Biela's comet, division of.
Breezes, land and sea
Pag
1
1
1
1
26
2
6
2
28
16
26;
262
22
26(
84 Desert
21
18
147
147
18
19
138
258
1
14
63
139
23
86
Cirro-cumulus clouds 102
Cirrus clouds 101
Climates, marine or continental 39
Climatology 23
Cloud formed by condensed vapor.... 137
Cloudiness, average amount of....
Clouds classified 101
defined 101
formation of. 104
height of. 103
how electrified 165
" sustained 105
indicate currents in the air. ... 106
mode of observing 102
negatively electrified 166
peculiar arrangement of. 106
shadows after sunset 107
shadows of. 106
vertical thickness of. 104
Cold which causes hail 133
Comet of 1866 234
Contact arches described 222
" variable 223
Corona?, cause of. 215
" colors of. 214
Coronas described 214
* ' produced artificially 215
Cumulus clouds 101
Current ascending near lat. 64 84
Cyanometer 207
Cyclone, premonitions of. 1 50
Cyclones defined 147
" diameter of. 150
" duration of. 151
gyratory movement of. 149
originate where 148
'* paths of. 148
" rate of motion of. 149
" season of. 148
Dalton's theory of the atmosphere. ... 10
Dark segment, cause of. 194
of Africa 120
Gobi 120
Deserts enumerated 119
Detonating meteors, examples of..... 238
" multiple nuclei.. 240
" number 239
" periodicity of.... 240
[)ew, amount of, determined 98
circumstances favorable to 91
during the day 92
origin of. 91
point deduced from wet bulb. ... 59
" defined 57
where unknown 93
Displacements by refraction 205
Earth, fluctuations of temperature. ... 44
frozen stratum. 46
increase of temperature 46
maximum and minimum tem-
perature 45
temperature at different depths 44
great depths. ... 45
Electric circulation, system of. 200
103 Electricity at considerable elevations 161
atmospheric, how observed 160
circulating about the earth 195
discharged to the earth... 166
diurnal change of. 164
" variation of. 161
due to evaporation 164
in cloudy weather 1 62
in dry houses 165
monthly change described 161
" explained 165
result of combustion 163
" condensed vapor 164
" friction 163
" unequal temper-
ature 163
" vegetation.: 163
variation with altitude 162
Electrometers 160
Elevation, influence upon humidity... 62
INDEX.
303
Evaporation, amount measured
" at all temperatures
" rate variable
Evening sky, redness of.
Factors for wet-bulb thermometer....
Fog bow described
" " explained
Fog, diameter of particles of.
" how sustained in air
Fogs constituted like clouds
" in spring and winter
" of polar regions
" over rivers
* ' where most prevalent
" where unknown
Force of wind, how measured
" " represented by a scale
French feet converted into English...
Frost in valleys
Gaseous atmosphere, annual variation
of
Gaseous atmosphere, diurnal varia-
tion of.
Gases, law of mixture
" proportions of, in atmosphere
Glaciers described
" geographical distribution of
Glow surrounding shadow
Gulf Stream, influence of.
Hail attended by two currents
" circumstances of.
" formation of.
" formed at what height
" geographical distribution of.
' ' how sustained in the air
" parallel bands of.
" preceded by a noise
" quantity of.
" rods
Hailstones, form <3f.
4 ' how long sustained
" sizejrf.
stnJRire of.
Hail-storms, track of.
Halo, how a circle is formed
" of 22 radius
" of 46 radius
" of 90 radius
" theory of.
Halos described
" produced artificially
Heat lightning
Heat, radiation of.
Hemispheres, northern and southern
Hoar-frost, crystalline structure of....
" how formed
Hot-springs, observations of
Hourly variations of barometer
Kilometres converted into miles 253
62
10
11
126
127
215
146
Humidity, extremes of 60
" of air, relative 274
" of the air denoted 60
Hygrometer denned 56
" Bache's 57
Darnell's 58
" Saussure's 56
Page
55
66
55
206
273
98
214
99 Ice, anchor described 53
[ce-houses, natural 49
99 Ice, polar 52
96 Indian summer described 100
97 Interval between flash and report 169
95 Iron meteors, catalogue of. 283
96 Isothermal lines 34
97
60
70
254 Lakes and rivers, temperature of. 53
94 Latent heat liberated 137
Light, absorption of. 206
Lightning caused by volcanoes 172
63 " color of. 168
different forms of. 167
duration of. 168
origin of. 166
tubes 172
Magnetic disturbances, cause of. 197
progress of... 190
needle, disturbance of 190
Mercury, depression in glass tubes.... 263
133 Meteoric orbits 230
129 " streams, origin of. 237
134 Meteorology defined 9
132 Meteors, detonating, defined 238
131 " of November 230
134 " of November, 1866 -. 231
135 " " 1867 232
133 Metres converted into feet 252
130 Millimetres converted into inches 251
135 Mirage at sea 204
130 " described 201
134 " experimentally illustrated 204
129 " lateral 205
131 " upon a desert 202
132 Mist, appearance of. 98
217 Monsoons, cause of. 85
216 " " of uniformity of..... 147
218 " described 85
219 Monthly means of barometer 19
216 Months, hottest and coldest
216 Moon, effect upon barometer 20
218 Morning twiligh t, colors of. 209
167 Motion, relative, from earth's rotation 82
89 Mountains enveloped in cloud 104
37
94 North pole, temperature of. 36
93 November meteors, conclusions from 235
47 " . " elements of. 233
19 " period of. 233
304
INDEX.
November meteors, procession of node
of
November stream, dimensions of.
Pacific Ocean, two sides of.
Parhelia of 22
" of 46
" of 120
Parhelic circle
Pillars of sand
Plants protected from frost
Pluviameter described
Pointed objects tipped with light
Polar winds, direction of.
Predictions founded on constant cli-
mate
Predictions founded on the laws of
storms .
Predictions founded on observations. .
* * of the weather possible. . . .
Prognostics from clouds, etc
" " twilight
Radiating power of different substan-
ces
Radiation from different substances..
' ' with partial exposure
Rain affected by elevation
latitude
mountains
" " proximity to the
ocean
" winds
" amount measured
" annual fall of.
" diameter of drops of.
' ' distribution over the earth
" for each month
' ' from clouds not in zenith
" " translucent clouds
" gauge, exposure of.
" great annual fall of.
" greatest fall of.
how caused
Hutton's theory of.
in the different months
maximum fall of.
origin of.
or snow in a storm
size of drops of.
small annual fall of.
without clouds
Rainbow, conditions of visibility
" described
* ' result of interferences
Rainbows, supernumerary
" theory of.
Rain-gauge, proper height of.
Rainy days, number of.
Rainy season and dry season
Redfield and Espy 's theories
Page
280
118
110
111
117
115
108
140
213
280
121
211
210
213
211
212
110
113
118
146
Page
Sea-breeze in temperate zones 87
232 Sea, currents of the 51
234 " surface, temperature of the 50
temperature at great depths of the 50
37 " temperature of the 49
221 Sheet lightning 167
221 Shooting-stars, altitude of. 226
222 described 225
220 " direction of motion of 227
153 length ofpath 227
94 light of 228
108 " magnitude of. 227
173 number at different
75 hours 225
" number for the globe 229
157 " in different
months 226
158 " periods of 237
158 " sound of. 228
157 telescopic 229
159 " visible train 228
210 Showers of toads, fishes, etc 156
' ' remarkable examples of. 119
Sky, blue color of. 207
281 " reflected light of. 207
90 Sleet defined 129
90 " origin of 135
114 Snow, annual amount of 123
113 " avalanches of. 128
115 Snow-balls, natural... 125
Snow-flakes, form of 124
116 " how formed 122
116 size of. 125
108 Snow from cloudless sky 122
117 Snow-line, height of. 273
108 Snow, perpetual limit of. 42
111 " phosphorescent 125
278 " red, in polar regions 126
121 " where unknown 123
121 Spaces, interplanetary, temperature of 43
109 Springs, ordinary temperature of. 47
Storm, American example of. 142
" defined 136
' ' European example of 141
" lull at the centre^. 144
" wind on the border of. 144
Storm's progress and direction 1 43
Storms, cause of 136
" parabolic course of. 151
" rate of progress of. 140
" rise and decline of. 140
" their course modified 145
Stratus clouds 102
Surface wind in middle latitudes 83
" " in polar regions 83
" winds defined 76
" " in equatorial regions.. 82
Telegraph wires affected by storms... 173
" " auroral influence de-
scribed 191
INDEX.
305
Telegraph-wires, auroral influence ex-
plained
Temperature, change from latitude...
" with elevation..
daily, defined
" decrease explained
different latitudes
for each month
great absolute range of.
" monthly range of.
highest observed
hourly variations
increase of, with height
Interplanetary, its
amount
invariable stratum of...
law of decrease with
height
lowest observed
maximum and mini-
mum
mean, above 80
" below 18
" from three hours
" " " two hours.
" of a place
monthly changes ex-
plained
monthly, determined. . . .
near the centre of a
storm
observations at single
hour
range of.
small absolute range of
" monthly range of
succeeding a storm
variation at Greenwich.
" New Haven
variations non-periodic.
" cause of
Thermometer described
exposure of.
graduated, how
hourly observations of.
maximum
Phillips's maximum...
photographic register..
requisites of
self-registering
wet-bulb
Thermometers, Centesimal and Fah-
renheit
Reaumur and Fah-
renheit
Thunder, cause of.
" clouds, height of.
" duration of
" rolling of.
Thunder-storms, distribution pf.
Page
197
32
40
29
41
34
268
272
271
39
29
90
270 Velocity of wind deduced from press-
270
31
30
33
32
31
145
30
40
272
271
145
267
266
33
27
23
27
23
28
25
26
26
24
25
59
255
256
168
171
169
170
172
Page
Thunder, succession of phenomena ... 170
Tornadoes, appearance of explosion.. 153
effects of 152
examples of 152
in the tropics 152
Trade winds described 74
Twilight curve 209
duration of. 208
Upper current in equatorial regions.. 82
" middle latitudes.... 78
polar regions 78
, annual variation of. 61
diurnal variation of. 61
elastic force of 276
how formed 54
how sustained in air 54
of atmosphere condensed 95
weight of, determined 60
Vapors distinguished from gases 10
69
Vertical columns through the sun 224
Vesicular theory of fog 98
Volcanic ashes, prevalence of 100
Volcanoes, information derived from. 47
show direction of wind. ... 76
Waterspouts 154
Waves, atmospheric 139
Weight of air, dry and saturated 264
Wells, temperature of. 48
Whirlwinds caused by fires 154
Widmannstaten figures 245
Wind, average velocity of. 70
direction, how determined 65
" indicated 64
effect upon the barometer 21
mean direction of 71
on different sides of a storm... 140
pressure and velocity of 277
temperature of. 87
Wind's direction observed 73
progress, how represented 72
Winds affected by rotation of the
earth 81
caused by unequal specific
gravity 80
causes of. 79
cold, from mountains 88
hot, from deserts 88
how propagated 81
influenced by the seasons 86
in the middle latitudes 74
on summits of mountains 77
propagated by aspiration 144
three systems of. 74
transport fine dust 77
u
PLATE I.
VAN RENSSELAER HARBOR.L4r.7fi'37' GODTHAAB, GREEN 'D. LAT.64-10
NORWAY HOUSE, H.B.TER. LAT. M'o'
IS
ST. JOEKS,ME^F(I^'D. LAT. 4-7* 25'
NEW YORK CITY.
1ST
SAVANNAH, GA. LAT. 32 6'
If
S
MATANZA5, GQBA*LAT.2.33'
IT
GEORGETOWN, B. G'A, L47.
IT
PLATE II.
Cirrus
~" v '" Cumulns
METEOROLOGICAL CHART
For 8 P.M. Dec. 20. 1836
METEOROLOGICAL CHART
For 8 A.M.Dec.21
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