ffifiSSSBS

IiON'DOJN

WALTON AND MABERLY,

UPPER OOWER STREET X IVY LANE.
Price Eighteenpence.

EDITED BY

DIONYS1US LARDNER, D.C.L.,

Formerly Professor of Natural Philosophy and Astronomy in University College, London.

ILLUSTRATED BY ENGRAVINGS ON WOOD.

VOL. XL

LONDON:
WALTON AND MABERLY,

UPPER GOWER STREET, AND IVY LANE, PATERNOSTER ROW.
1856.

LOAN STACK

LONDON t
BRADBURY AND EVANS, PRINTERS, WTIITEFRI ARS.

L 37

CONTENTS'.

THE PRINTING-PRESS.

PAGE

CHAP. I. — 1. The improvement of the art not promoted by men
of letters and science. — 2. General signification of printing. — 3.
Printing by models in relief. — 4. Method of engraving the
block. — 5. Antiquity of this art. — 6. Invention of movable type.
— 7. Use of movable types for printing successive books or parts
of a book. — 8. Process of printing. — 9. Composition. — 10.
Quadrats. — 11. Use of the chase.— 12. Imposing.— 13. Reading
and correcting. — 14. Successive operations in printing. — 15.
Inking.— 16. Inking-rollers. — 17. Stanhope press. — 18. Printing-
machines. — 19. General description of them. — 20. Single
printing-machines. — 21. Double printing-machines. — 22. Per-
spective view and description of Applegath and Cowper's double
printing-machine .1

CHAP. II.— 23. Machine of The Times of 1814. — 24. Improvement
of this. — 25. Present printing machine of The Times. —26.
Marinoni's newspaper printing machine. — 27. Marinoni's book-
printing machine. — 28. Newspapers. — 29. Reporters. ; — 30.
Court newsman. — 31. Foreign correspondent. — 32. Newspaper
statistics ..... . 17

THE CRUST OF THE EARTH,
OR, FIRST NOTIONS OF GEOLOGY.

CHAP. I. — 1. The earth, a subject of long-continued observation and
investigation. — 2. Mathematical geography.— 3. Physical geo-
graphy.— 4. Phenomena of the oceans and seas included in it.
— 5. Hydrology, meteorology, and climatology. — 6. Political
geography. — 7. General subject of geography. — 8. Geology. —
9. Original fluidity of the eai-th inferred from its spheroidal
form. — 10. This form ascertained by observation and measure-
ment.— 11. The solid crust was formed while the earth was in a
state of rotation. — 12. Increase of temperature from surface
downwards. — 13. Within the crust the earth still in a state of

085

iv CONTENTS.

PAGE

fusion. — 14. Siibject of geology. — 15. How the structure oftlie
crust to a great depth is rendered manifest. — 16. Section of the
crust where no disturbance has taken place — strata occur in a
fixed order. — 17. Rocks in their geological sense. — 18. Their
classification in five principal divisions. — 19. Lowest bed, being
the foundation of th^ crust, consists of igneous rocks produced by
the superficial cooling of the molten materials of the globe. — 20.
Materials of which the igneous rocks are formed. — -21. Consti-
tuents of granite, feldspar, mica, quartz. — 22. These components
of igneous rocks agglomerated mechanically. — 23. But the com-
ponents themselves are chemical compounds. — 24. Varieties of
granite — porphyry. — 25. Gneiss. — 26. Secondary rocks. — 27.
Transition or metamorphic rocks. — 28. Stratification. — 29. Pro-
duced by aqueous deposition. — 30. Stratified rocks of aqueous
origin. — 31. Circumstances corroborating this inference. — 32.
Stratified and unstratifiedjrocks. — 33; Condition and materials of
transition rocks. — 34. Animal remains found in them. — 35. No
vegetable remains — probable reason. — 36. Fish and annelidans.
— 37. Stratified rocks in general. — 38. Secondary rocks. — 39.
Vast quantity of organic remains deposited in them. — 40. Ter-
tiary rocks. — 41. Diluvium, alluvium, and surface soil. — 42.
Subdivision of these principal groups of strata. — 43. General
inference as to the condition and ages of stratified rocks. — 44.
They constitute a chronological scale. — 45. Complexity and
defects of ; geological nomenclature ...... 33

CHAP. II. — 46. General section of the terrestrial crust tabulated. — 47.
Approximate thickness of. strata. —48. Probable time necessary for
their deposition. — 49. Recapitulation of the physical history of the
globe. — 50. Deposition of organic forms. — 51. Alternate elevation
and depression of the crust. — 52. The strata are botanical and
zoological museums of past creations. — 53. The gradual increase
of forms of life. — 54. Creative power has always operated on the
same general plan. — 55. But has varied from period to period in
details. — 56. Animals created gradually ; the more perfect being
the more recent. — 57. Tabular view of the progress of the ani-
malisation of the earth. — 58. Great increase of vertebrates in the
tertiary period — No human fossil — Man characteristic of the
present period. — 59. Temporary existence of certain extinct genera
and species. — 60. Geological use of characteristic genera and
species. — 61. Examples — Trilobites characteristic of the Silurian
strata. — 62. Description of them. — 63. Dr. Buckland's reflections
on them. — 64. Species characteristic of the lias — Ichthyosaurus.
—65. Characteristics of the Wealden— Hylseosaurus — Iguanodon.
— 66. Characteristics of the chalk. — 67. Ammonites, their dis-
tribution between the Silurian and chalk.— 68. Fossil cephalopocles
— Nautilus — Danians. — 69. Fossil gasteropodes — Bigranulosa mur-
chisonia — Cyprsea elegans — Voluta elongata — Pterocera oceani. . 49

CHAP. III. — 70. Spondylus.— 71. Pentamerus. — 72. Reticulipora. —
73. Ceratites. — 74. Enormous masses of animal remains forming
entire islands and continents — Ehrenberg's discoveries. — 75. Dr.
Mantell's table of organic strata. — 76. Forms of life in the Silu-

CONTEXTS. v

VACE

rian period. — 77. Sir R. Murchison's observations on the changes
of the forms of life from period to period. — 78. Stratification in
undisturbed plains horizontal. — 79. Strata thrown into oblique
positions by disruption of igneous rocks. — 80. Formation of moun-
tains.— 81. Arrangement of strata on their flanks. — 82. Strata
sometimes upheaved without being disrupted. — 83. Sometimes
disrupted. — 84. Sedimentary strata deposited subsequently to dis-
ruption— discordant stratification. — 85. How these supply data
for determining the epoch of the disruption. — 86. Determination
of the relative ages of mountains — Cumbrians and Grampians
much older than the Alps. — 87. How inclined strata have enabled
geologists to analyse the structure of the terrestrial crust to the
level of the igneous rocks. — 88. Erosion of stratification by the
action of water, and the subsequent deposition of other strata. —
89. Basalts — their character and composition. — 90. Various forms
of basaltic rocks. — 91. Their extensive diffusion over all parts of
the earth. — 92. Their columnar structure — Giant's Causeway. —
93. Basaltic causeway of the Volant — Dykes and colonnade of
Chenavari. — 94. Veins of basalt. — 95. Basalts in mounds. — 96.
Basaltic grottoes — Kase grotto of Bertrich-Baden — Fingal's Cave.
—97. Trachytic rocks.— 98. Trachytic mountains.— 99. Their
origin igneous . . . . . . . . .65

CHAP. IV. — 100. Elie De Beaumont's explanation of the formation
of mountain chains. — 101. Effects of the earthquake of 1838 in
South America. — 102. Inference as to the probable effects of
the vast earthquakes which produced the great mountain ranges.
— 103. Dislocations in parallel directions produced parallel
chains. — 104. Origin of mineral veins explained. — 105. Veins
are found in groups — generally parallel. — 106. Deposition of
rock-salt in cavities of Muschelkalk. — 107. Natural agencies
still manifested are sufficient to explain all geological phenomena.
— 108. Internal fluidity of the earth. — 109. Effects of internal
heat on the surface. — Why the climates of the higher latitudes
at former epochs were similar to those of the tropics at present
— explanation of the presence of tropical fossils in polar latitudes.
— 110. The undulations and disruptions assumed by geologists
as physical causes still proceed, though with less energy. — 111.
Effects of earthquakes— that of Calabria, 1783. — 112. Effects
in Sicily. — 113. Earthquakes at Chili. — 114. Earthquake of
1819 in India. — 115. Like phenomena recorded in all ages and
countries. — 116. Similar phenomena traditional — island of
Atlantis. — 117. Permanency of the sea-level proves the undula-
tions of the land. — 118. Undulations of the Swedish peninsula.
— 119. Similar changes in Greenland, and in the Indian archi-
pelago.— 120. General sinking of South America. — 121. Singular
example of a submerged forest on the western coast of America. —
122. Temple of Jupiter Serapis. — 123. Historical researches
of Professor Forbes. — 124. Tradition of a submerged city under
Lough Neagh. — 125. Why these undulations of the earth's crust
might be expected. — 126. Effects of disruption of the crust. —
127. Volcanic eruptions of 1808 in the Azores.— 128. The Monte
Nuovo . 81

CONTENTS.

CHAP. V.— 129. Volcanoes of the Andes— Pichincha, Cotopaxi, and
Tunguragua. — 130. Mexican volcanoes — Orizaba, Popocatapetl,
Jorullo, and Colima. — 181. Singular circumstances attending the
elevation of Jorullo. — 132. Ancient state of Vesuvius, according to
Strabo — Eruption of 76 A. D. — Destruction of Herculaneum and
Pompeii. — 133. ^olcano of Kirauea in Owhyhee — Mr. Ellis's
account of it. — 134. Visit of Mr. Stewart and Lord Byron to it
in 1825— Mr. Stewart's account of it. — 135. Illustrations of the
ejection of lava through the strata. — 136. .Effect of obstruction
— Formation of lateral craters. — 137. Submarine eruptions —
Formation of volcanic islands.— 138. Volcanic islands off Santorin
in the Grecian Archipelago. — 139. Phenomena . attending the
formation of such islands. — 140. Craters of elevation. — 141.
Stratification around these craters — Island of Palma. — 142.
Formation of islands not necessarily preceded by .eruptions. —
143. Mount Etna. — 144. Historical accounts of its eruptions. —
145. Eruption of 1832.— 146. The Valde Bove.— 147. Section
and plan of Etna.- — 148. Volcanic lakes.— 149. Crescent form
of volcanic islands. — 150. Craters of elevation, temporary and
permanent. — 151. Barren island in the Bay of Bengal. — 152.
Traditional volcanic origin of Santorin. "... . . . .97

CHAP". VI.: — 153. Form of craters. — 154. Stromboli — Formation of
internal' adventitious cones and craters. — 155. Strata of lava —
how formed. ; — 156. Formation of lava in dykes. — 157. Salses, or
mud volcanoes. — 158. Fumarolles. — 159. Geysers. — 160. Valleys
of .elevation. — -161. . Formation of parallel ridges. — 362. Slow
operation of air, water, and heat. — 163. Atmospheric effects upon
rocks.— 164. Effects of water on rocks, — 165. Effects of .the sea
on cliffs.— 166. Effects of waterfalls. — 167. Effects of the sea and
of icebergs on the forms of rocks. — 168. Geological phenomena
explicable by natural causes still in operation. — -169. Portland
dirt-bed — -Its organic deposits.— 170. Climate of England tropical
at the epoch of these deposits. — 171. Example of coal deposits. — -
172. Northumberland coal mines. — 173. Colliery near Wolver-
hampton. — 174. Deposits in Treuille mine at St. Etienne. — 175.
The character of the waters shown by the organic deposits —
Fossil shells . .113

CHAP. VII.— 176. Fossil foraminifera. — 177. Fossil infusoria. —
Researches of Ehrenberg. — 178. . Marine deposits at great heights
produced by former undulations of the land.— 179. Organic de-
posits show that the same parts of the land have undergone a
series of alternate elevations and depressions. — 1 80. Example of
the Portland beds.— 181. Fossil footsteps. — 182. Such, traces
common in the new red sandstone, at Hesseburg, and near
Liverpool. — 183. Footprints of birds. — 184. Fossil rain-drops
and ripple-marks. — 185. Conclusion ..... 129

THE STEREOSCOPE.

1. Surprising effect of the instrument explained. — 2. Causes of visual
perspective and relief. — 3. Effects of binocular parallax. — 4.

CONTENTS. vu

Example of the bust of Cuvier. — 5. Principle of the stereoscope.

— 6. Origin of the name. — 7. Wheatstone's reflecting stereoscope.

— 8. Sir David Brewster's lenticular stereoscope. — 9. Method of
obtaining stereoscopic pictures. — 10. How the effects of relief are
produced. — 11: Natural relief greatly exaggerated. . . 137

COMETS.

CHAP. I. — I. COM ET ART ORBITS : 1. Prescience of the astronomer. —
2. Strikingly illustrated by cometary discovery. — 3. Motion of
comets explained by gravitation. — 4. Conditions imposed on the
orbits of bodies which are'Subject to the attraction of gravitation.
— 5. Elliptic orbit. — 6. Parabolic orbits. — 7. Hyperbolic orbits.
— 8. Planets observe in their motions order not exacted by the
law of gravitation. — ;9. Comets observe no such order in their
motions. — 10. They move in conic sections, with the sun for the
focus. — 11. Difficulty of ascertaining in what species of conic
section a comet moves.— 12. Hyperbolic and parabolic comets
not periodic. — 13. Elliptic comets periodic like the planets. —
14. Difficulties attending the analysis of cometary motions. — 15.
Periodicity alone proves the elliptic character. — 16. Periodicity
combined with the identity of the paths while visible establishes
identity. — 17. Many comets recorded— few observed. — 18. Classi-
fication of the cometary orbits. — II. ELLIPTIC COMETS REVOLVING
WITHIN THE ORBIT OF -'SATURN: 19. Encke's comet. — 20. Table of
the elements of the orbit. — 21. Indications of the effects of a
resisting medium. — 22. The luminiferous ether would produce such
an effect. — 23. Comets would ultimately fall into the sun . .145

CHAP. II. — 24. Why like effects are not manifested in the motion of
the planets.— 25. Corrected estimate of the mass of Mercury. — 26.
Biela's comet. — 27. Possibility of the collision of Biela's comet with
the earth. — 28. Resolution of Biela's comet into two. — 29. Changes
of appearance attending the separation. — 30. Faye's comet. — 31.
Reappearance in 1850-1 calculated by M. Le Verrier.— 32. De
Vice's comet. — 33. Brorsen's comet. — 34. D'Arrest's comet. —
35. Elliptic comet of 1743.— 36. Elliptic comet of 1766.— 37.
Lexell's comet. — 38. Analysis of Laplace applied to Lexell's comet.
—39. Its orbit before 1767 and after 1770 calculated by his
formulae. — 40. Revision of these researches by M. Le Verrier. —
41. Process by which the identification of periodic comets may
be decided. — 42. Application of this process by M. Le Verrier to
the comets of Faye, De Vico, and Brorsen, and that of Lexell
— their diversity proved. — 43. Probable identity of De Vice's
comet with the comet of 1678.— 44. Blainplan's comet of 1819.
— 45. Pons's comet of 1819. — 46. Pigott's comet of 1783. — 47.
Peters's comet of 1846. — 48. Tabular synopsis of the orbits of
the comets which revolve within Saturn's orbit. — 49. Diagram of
the orbits.— 50. Planetary character of their orbits. — III. ELLIP-
TIC COMETS, WHOSE MEAN DISTANCES ARE NEARLY EQUAL TO THAT
OF URANUS : 51. Comets of long periods first recognised as periodic.

viii CONTENTS.

PAGE

— 52. Newton's conjectures as to the existence of comets of long
periods. — 53. Halley's researches. — 54. Halley predicts its re-
appearance in 1758-9 161

CHAP. III. — Halley's prediction (continued). — 55. Great advance of
mathematical and physical sciences between 1682 and 1759. — 56.
Exact path of the coiiet on its return, and time of its perihelion,
calculated and predicted by Clairaut and Lalande. — 57. Remark-
able anticipation of the discovery of Uranus. — 58. Prediction of
Halley and Clairaut fulfilled by reappearance of the comet in
1758-9. — 59. Disturbing action of a planet on a comet explained. —
60. Effect of the perturbing action of Jupiter and Saturn on Hal-
ley's comet between 1682 and 1758. — 61. Calculations of its return
in 1835-36. — 62. Predictions fulfilled.— 63. Elements of the orbit
of Halley's comet. — 64. Pons's comet of 1812. — 65. Olbers's
comet of 1815.— 66. De Vice's comet of 1846.— 67. Brorsen's
comet of 1847. — 68. Westphal's comet of 1852.— 69. Data
necessary to determine the motions of these six comets. — 70.
Diagram of their orbits. — 71. Planetary characters are nearly
effaced in these orbits. — IV. ELLIPTIC COMETS, WHOSE MEAN

DISTANCES EXCEED THE LIMITS OP THE SOLAR SYSTEM : 72. Syn-

opsis of twenty-one elliptic comets of great eccentricity and long
period. — 73. Plan of the form and relative magnitude of the orbits.
— V. 74. HYPERBOLIC COMETS : VI. 75. PARABOLIC COMETS. —
VII. PHYSICAL CONSTITUTION OP COMETS : 76. Apparent form
— head and tail. — 77. Nucleus. — 78. Coma. — 79. Origin of the
name. — 80. Magnitude of the head. — 81. Magnitude of the
nucleus. — 82. The tail. — 83. Mass, density, and volume of
comets 177

CHAP. IV. — 84. Light of comets. — 85. Enlargement of magnitude on de-
parting from the sun. — 86. Professor Struve's drawings of Encke's
Comet. — 87. Remarkable physical phenomena manifested by
Halley's comet. — 88. Struve's drawings of the comet approaching
the sun in 1835. — 89. Its appearance on the 29th of Sept. — 90.
Appearance on Oct. 3. — 91. Appearance on Oct. 6. — 92. Ap-
pearance on Oct. 9. — 93. Appearance on Oct. 10. — 94. Appear-
ance on Oct. 12. — 95. Appearance on Oct. 14. — 96. Appearance
on Oct. 29.— 97. Appearance on Nov. 5.— 98. Sir J. Herschel's
deductions from these phenomena. — 99. Appearance of the comet
after perihelion. — 100. Observations and drawings of MM. Maclear
and Smith. — 101. Appearance on Jan. 24. — 102. Appearance on
Jan. 25. — 103. Appearance on Jan. 26. — 104. Appearance on
Jan. 27. — 105. Appearance on Jan. 28. — 106. Appearance on
Jan. 30. — 107. Appearance on Feb. 1. — 108. Appearance on
Feb. 7. — 109. Appearance on Feb. 10. — 110. Appearance on
Feb. 16 and 23. — 111. Number of comets. — 112. Duration of
the appearance of comets. — 113. ^Near approach of comets to the
earth , 193

ERRATA.

lu Crust of the Earth, page 59, lines 1 to 19 inclusive, for "species" read
"genera" or "geuus" everywhere without exception.

Fig. 2.

THE PRINTING PRESS.

CHAPTER I.

1. The improvement of the art not promoted by men of letters and
science. — 2. General signification of printing. — 3. Printing by models
in relief. — 4. Method of engraving the block. — 5. Antiquity of this
art. — 6. Invention of movable type. — 7. Use of movable types for
printing successive books or parts of a book.— 8. Process of printing.
— 9. Composition. — 10. Quadrats. — 11. Use of the chase. — 12.
Imposing. — 13. Reading and correcting. — 14. Successive operations
in printing. — 15. Inking. — 16. Inking-rollers. — 17. Stanhope press.
— 18. Printing-machines. — 19. General description of them. — 20.
Single printing-machines. — 21. Double printing-machines. — 22. Per-
spective view and description of Applegath and Cowper's double
printing-machine.

1. IT is a remarkable fact, that printing, which has so far
transcended all other arts in the influence it has exerted in the
advancement of knowledge and the progress of the human race,
owes almost nothing to the class devoted professionally to letters
and the sciences, and on whom it has, nevertheless, bestowed the
largest measure of advantage.

LARDNER'S MUSEUM OF SCIENCE. B 1

No. 131.

THE PRINTING PRESS.

2. Printing, in its most general sense, is the name given to all
processes in the arts, by which the same forms or characters are
indefinitely multiplied, by the impressions of the surface upon
which they are formed repeatedly and successively, upon the
surface to which they are to be transferred. Thus, in calico-
printing, the same figures are transferred in rapid succession to
different pieces of cloth, or to different parts of the same piece, by
means of blocks or rollers, upon which, the figures are produced,
either in relief or intaglio, that is, either in raised or sunken
characters. In such cases the blocks, or rollers, are pressed in
succession on the surface of the cloth to be printed, being however
previously smeared with colouring matter. In the printing of
copper or steel engravings, the design is produced by the engraver
in lines sunk in the surface of the copper or steel by the engraving
tool. These lines being filled with the printing-ink, and all
other parts being carefully cleaned, the plate is impressed on the
paper with an intense pressure by means of a press specially
adapted to that purpose. The paper being previously moistened,
absorbs the ink from the lines in which it is deposited, and
exhibits after the impression a perfect copy of the engraving, —
the sunken lines of the engraving being represented on the paper
by corresponding lines in ink.

3. In wood- engravings, and in ordinary book- printing, the
original from which the impressions are taken is in relief. A
model in relief of the page to be printed is formed in metal called
type-metal, consisting of lead rendered hard by being alloyed
with a small proportion of antimony. The surface being smeared
with the colouring-matter called printing-ink, is impressed upon
the paper, and ufac-simile of the original relief is thus produced.

4. In the earliest and rudest attempts at printing, a manu-
script page was attached to the surface of a block of wood,
which was carved into relief corresponding with the characters,
of the manuscript. The impression produced by this means was
necessarily a fac-simile, more or less accurate, of the manuscript
itself.

5. The method of printing by means of blocks of wood, or
metal, carved in relief, is the earliest example on record of the
practical use of the art which, in its more improved state, has
exercised so important an influence upon civilisation. According
to some antiquarian authorities, the art of producing characters in
this way may be traced as far back as the building of Babylon.
The characters found upon bricks, taken from the supposed site of
that city, having been undoubtedly printed by the method here
described. "We are in possession of metal stamps, with words
engraved in relief, which the Romans made use of to mark their

FIRST ATTEMPTS.

various articles. If the modern art of making paper had been
known in those remote times, it is very probable that the art of
printing books would have existed at a much earlier date than
that of its actual commencement, for with the same kind
of stamps precisely, as those by which the Roman tradesmen
marked their wares, books might have been printed, and the same
engravings which adorned the shields and pateras of ancient
times might, by the aid of paper, have spread the intelligence of
Greece and Italy over the world.

According to Du Halde and certain missionaries, the art of
printing from blocks carved in relief was practised in China fifty
years before the Christian era ; and, from the early commercial
intercourse of the Venetians with that country, there is reason to
believe that the knowledge of this art, in its application to the
multiplying of books, was originally derived from thence, for
Venice is the first place in Europe in which it is recorded to have
been practised.

Its first application was to the production of playing cards and
religious prints, and when the art was first extended to books
they were printed by carving each page upon a separate block.
This process of carving the characters in relief, which was probably
executed by attaching the manuscript to the face of the block
and engraving from the manuscript, will afford an easy and
obvious explanation of the diversity of characters found in ancient
books printed from such blocks, and will explain the great simi-
larity which exists between books thus printed and manuscripts.
This similarity was increased by their being printed on one side
of the paper only, the indentation produced by the impression
being removed by burnishing the back. Two leaves were then
pasted together, making such a perfect fac-simile of the manu-
script, as to require, even at the present day, great discrimination
and much chemical skill to distinguish such books from real manu-
scripts ; and as they have no printers' names, dates, or places affixed
to them, it is impossible to ascertain by whom, or when, or where
they were executed. The fabrication of these pseudo-manuscripts
involved the first introduction of the art of printing.

6. Movable Types. — About the middle of the fifteenth century
the art of printing in this rude manner acquired considerable
extension ; but as each separate work required to have separate
blocks for each page, and since the blocks for one work were
altogether useless for another, the printers soon began to feel the
inconvenience arising from the storage of such numerous collections
of blocks, to say nothing of the expense of carving them. They
were, therefore, stimulated to seek for some method less costly
and cumbrous, by which the models in relief of the pages could be

B 2 3

THE PRINTING PRESS.

produced, and so that the materials of the model of any one page
might be afterwards useful, in the formation of the models of
other pages. The discovery of the means of accomplishing this
object by movable types constitutes the most important epoch in
the history of printing, and is sometimes even regarded, in all
essential respects, as the"Snvention of printing itself. After it had
undergone some successive improvements, it resolved itself into
the production of models in relief of the letters of the alphabet,
formed upon the extremities of small bars of metal, which
being properly selected and placed in juxtaposition, formed the
words and letters of a page. Such are the types of the modern
printer.

The honour of the invention of movable types has been disputed
by two cities, Haarlem and Mentz. The claims of Haarlem rest
chiefly upon a statement of Hadrien Junius, who gave it upon the
testimony of Cornelius, alleged to be a servant of Lawrence
Coster, for whom the invention is claimed. The claims of Mentz,
which appear to be more conclusive, are in favour of Peter
Schseffer, the assistant and son-in-law of John Faust, better
known as Dr. Faustus. The first edition of the Speculum humance
salvationis was printed by Coster at Haarlem, about the year
1440, and is one of the earliest productions of the press of which
the printer is known. The celebrated Bible, commonly known as
the Mentz Bible, without date, is the first important specimen of
printing with movable metal types. This was executed by Guten-
berg and Faust, or Fust, as it is sometimes spelt, between the years
1450 and 1455. The secret of the method then becoming known,
presses were speedily established in all parts of Europe, so that
before the year 1500 there were printing-offices in upwards of
220 different places in Austria, Bavaria, Bohemia, Calabria, the
Cremonese, Denmark, England, Flanders, France, Franconia,
Frioul, Geneva, Genoa, Germany, Holland, Hungary, Italy,
Lombardy, Mecklenberg, Moravia, Naples, the Palatinate,
Piedmont, Poland, Portugal, Rome, Sardinia, Upper and Lower
Saxony, Sicily, Silesia, Spain, Suabia, Switzerland, Thessalonica,
Turkey, Tuscany, the Tyrol, Yenice, Yerona, Westphalia,
Wurtemberg, &c.

This vast and rapid extension of the art, combined with the
skill which the earlier printers displayed in it, seems to be totally
incompatible with the date assigned to the invention, and it is
more probable, that the art having been long practised in private
under continued attempts at secrecy, it at length broke into
publicity after it had already attained a considerable degree of
perfection.

7. When the page of a book is formed by properly combining
4

PROCESS OF PRINTING — COMPOSITION.

the types, and a sufficient number of copies of it produced by the
process of printing, the types which form it are disengaged and
separated and used to form other pages of the same or other
books. Thus, while in the first attempts at printing, each model
of letter in relief never did any other duty than the printing of
the very book for which it was formed, the type of each letter in
printing with movable types is transferred from page to page,
and is used successively in the printing of an indefinite number of
pages of the same or different works.

8. The process of printing, then, consists in a certain suc-
cession of operations, the first of which is putting together the
types so as to form lines and pages ; the second, putting together
these pages in such a manner, that when impressed upon sheets,
and the sheets folded, they will succeed each other in the proper
order. The subsequent operations of folding, stitching, and
combining these sheets together, so as to form a volume, is the
business of the bookbinder.

9. Composition. — The process of putting the types together is
called composing, and the person who performs this operation is
called a compositor. He stands before an inclined desk, as shown
in the view of the composing-room (fig. 1), which is divided into
a number of compartments of different sizes, A, B, in each of which
are placed a certain number of types of a particular letter. By
practice he learns without hesitation to direct his hand to the
compartment which contains the letter he wants, without removing
his eye from the manuscript which lies before him. He holds in
his left hand an instrument called a composing-stick, which is so
formed as to receive the types in successive juxtaposition, until
the requisite number have been placed to form a complete line,
after which another line is composed in like manner, and thus
line after line is composed until a complete page has been formed.
The spaces between words are made by the insertion of small
bars called quadrats, similar to those of type, but having no
letter cast upon their ends, and the spaces between line and line
are produced by the insertion of thin plates of metal called
leads. When the lines are considerably separated from one
another, they are accordingly said to be " leaded." "When a
page has been composed the compositor ties a cord round it,
called a page-cord, to hold the types of which it consists pro-
visionally together, and placing it apart, proceeds to form another
page, and so on.

10. Since every line of the same page must necessarily have the
same length, whether the types which compose it fill out that
length or not, any deficient space is filled by quadrats placed at
the most convenient points between the words.

5

IMPOSING READING — CORRECTING

In like manner the blank parts of last lines of paragraphs
are filled with quadrats.

11. When the pages of one side of a sheet have been thus com-
posed, they are placed in the divisions of an iron form called a chase,
c, D, several of which appear in the view of the composing-room,
fig. 1. These chases, of course, vary in their form and mode of
division according to the size and number of the pages which form
a sheet. We have here supposed the pages which are composed
and arranged in the chase to be those which are required to be
printed on one side of the sheet. A similar number are composed
and similarly put together in another chase, being those to be
printed on the other side of the sheet.

12. Imposing. — The process of arranging the pages in the
chase is called "imposing."

13. Readers and Correctors. — When the pages composing
each side of the same sheet have been thus " imposed" and the
types securely fastened in the chase by proper wedges, the chases are
brought to a printing-press, which will be presently described,
where a single impression is taken from them, called the first
proof, which, being properly folded, is taken to a person called
the " reader" who has always a boy capable of reading the
manuscript to assist him. While the boy reads the manuscript,
the "reader" follows him upon the proof, which he carefully
examines, and upon which he marks the errors of the compositor.
The proof is then returned to the compositor, who corrects the
errors indicated by the reader, and a second impression is then
taken with more care, and generally on better paper. This is
called a clean proof, and is again examined by the reader to
ascertain whether the compositor has corrected all the errors
previously indicated ; and if there are none uncorrected, the
proof is then sent to the author. In good printing-offices there are
few or no press errors found in the author's proof, those corrected
by him being in general errors which had been overlooked in his
own manuscript, or corrections of language suggested to him in the
revision of the sheet.

14. After the sheet has received the correction of the author,
the form to be printed is laid upon a horizontal table, with the
faces of the types uppermost, and the following operations are
executed:— 1st. Printing-ink is applied to the faces of the type,
so evenly that there shall be no blotting or inequalities in the
printing ; 2nd. The sheet of paper to be printed is laid upon the
form so as to receive the impression of the type in its proper
position, and in the centre of it ; 3rd. This paper is urged upon
the type by a sufficient pressure to enable it to receive the printed
characters, such pressure, however, not being so great as to cause

7

THE PRINTING PRESS.

the type to penetrate or deface the paper ; 4th. The paper is, in
fine, when thus printed, withdrawn from the type and laid upon
a table, where the printed sheets are collected.

15. Inking. — In the old process these operations were per-
formed by two men, one of whom was employed to ink the types,
and the other to print. {The former was armed with two bulky
inking-balls, consisting of a soft black substance of leathery
appearance, spherical form, and about twelve inches in diameter.
He nourished these with dexterity, dabbed them upon a plate
smeared with ink, and then with both hands applied them to the
faces of the types until the latter were completely charged with
ink. This accomplished, the other functionary — the pressman —
having prepared the sheet of paper while the type was being
inked, turned it down upon the type, drew it under the press,,
and with a severe pull of the lever gave the necessary pressure by
which the paper took the impression of the type. A contrary
motion of the apparatus withdrew the type from under the press,
and the pressman, removing the paper now printed, deposited it
upon a table placed near him to receive it. The same series of
operations was then repeated for producing the impression of another
sheet and so on. In this manner two men in ordinary book- work
usually printed at the rate of 250 sheets per hour on one side.

16. Xnking-rollers. — One of the first improvements which,
took place upon this apparatus consisted in the substitution of a
cylindrical roller for the inking-balls. This roller was mounted
with handles, so that the man employed to ink the type first
rolled it upon a flat surface smeared with ink (fig. 2), and having
thus charged it, applied it to the type form, upon which he rolled
it in a similar manner, thus transferring the ink from the roller
to the faces of the type. The substitution of these inking-rollers
for the inking-balls constituted one of the most important steps-
in the modern improvement of the art of printing. The rollers-
were composed of a combination of treacle and glue, and closely
resembled caoutchouc in their appearance and qualities.

17. Stanhope Press. — The process by which these operations-
were executed, assumed in the course of years a great variety
of improved forms, and one of the most celebrated and most uni-
versally adopted having been supplied by the inventive genius
of Earl Stanhope, has accordingly retained his name, and is-
known as the Stanhope press. This machine, which, resembling all
other improved presses in its general features, and serving, there-
fore, as an example of hand-presses in general, is shown in fig. 3.
The two principal parts of the machine arejirst, that which pro-
duces the pressure, and the second, that which supports the paper.

The former is a massive frame of cast iron, formed in a single
8

THE FEINTING PKESS.

Fig. 4.

piece, in the upper part of which, is a nut, in which a screw
moves — the point of which acts upon the upper end of a slider,
which is fixed into a dovetailed groove formed between two
vertical bars of the frame. This slider has attached to its lower
end a square plate, called the platten, which rises and falls
according as the screw is^ turned in the one direction or the other.
The weight of the platten fig. 3, A, and slider, which is considerable,
is counterpoised by a heavy weight c suspended by a lever behind
the press. The flat table, called the carriage, upon which the form
of types is laid, is moved backwards and forwards by means of, a
winch, which appears in fig. 3. By means of this winch, and
the rack or band with which it is connected, the carriage with
the type form can be moved by the pressman alternately back-
wards and forwards, so that it can be brought under the platten,
and after having received the pressure, can be removed back to
its first position. The platten is made to impart the pressure by
means of a lever called the knee lever, an expedient which- is
much used in the arts in cases where any intense pressure is
required to be produced through a very limited space. The
mechanical effect of this species of lever will be understood by a
reference to fig. 4, where A B is a metal rod, having a fixed point

of support A on which it works ;
another bar G c is jointed to it
at c, a point intermediate be-
tween A and B. This bar c G
is jointed at G to a plate such
as R, or any other object to
which it is desired to trans-
mit any intense force acting
through a very limited space,
as, for example, in the present
case, where the paper is pressed
upon the type by a plate which
is driven upon it by a sudden
and severe force. The handle
B of the lever being pressed in
the direction of the arrow, ex-
erts a corresponding pressure
on the point c, which is driven
in the direction c D perpendicular to A B. This motion c D is re-
solved into two by the parallelogram of forces, one in the direction
c E, and the other in the direction c F ; the latter exerts pressure
on the fixed point A, and the other acts upon the plate R by means
of the joint G forcing it downwards. As the joint c advances, the
angle A c G becomes more and more obtuse, and the component c E
10

STANHOPE PRESS.

of the force acting at B, bears a rapidly increasing proportion to
the force itself, so that when the levers A c and c G come nearly
into a right line, the pressure exerted at B is augmented at G in an
almost infinite proportion.

In working the press, the pressman places a sheet of paper to
be printed upon the frame F, called the tympan, where it is held
in its place by turning over it the frame G, which is supplied
with rods or cords corresponding to the spaces between the pages
of the form. While the pressman is thus employed, his assistant
is engaged in inking the types with a roller, as shown in fig. 3.
When this has been accomplished, the pressman turns down the
tympan carrying the paper upon the types, and then, by turning
the handle, moves the carriage with the type form upon it under
the platten, and applying his hand to the handle above him
presses the platten down upon the tympan carrying the paper, and
by means of the knee lever produces a sudden and severe pressure
by which the paper receives the impression of the type. The
handle is then moved in a contrary direction, the platten being
raised from the type form, and by turning the other handle the
carriage with the type form is removed from under the platten.
The pressman then raises the tympan from the types, and taking
the printed sheet off, replaces it by a fresh blank sheet, over which
he turns the frame as before ; and while he is performing this
• operation, his assistant is occupied in inking the types, after
which the same operations are performed, and another sheet is
printed, and so on.

18. Printing Machines. — The printing presses which served
the purposes of publication for some hundred years, during which
they received no other improvements than such as might be
regarded merely as modifications in the detail of their mechanism,
have been almost entirely superseded by engines of vastly increased
power and improved principles of construction. Although these
-admirable machines differ one from another in the details of their

mechanism, according to the circumstances under which they are
applied, and the power they are expected to exert, they are
nevertheless characterised by certain common features.

19. The form to be printed is laid in the usual manner upon a
perfectly horizontal table, with the faces of the types uppermost ;
and upon the same table, in juxtaposition with the form, and
level with the faces of the types, or nearly so, is placed a slab
upon which a thin and perfectly regular stratum of printing-ink
is diffused ; the table thus carrying the form and inking-slab is
moved by proper machinery right and left horizontally, with a
reciprocating rectilinear motion through a space a little greater
than the length of the form.

11

THE PRINTING TRESS.

Above the form and slab are mounted, also in juxtaposition, a
large cylinder or drum, which carries upon it the sheet of paper
to be printed, and three or four inking-rollers similar to that
already described. There are also three or four other rollers in
juxtaposition with the latter, one of which supplies ink to ^the
others, which severally spread it in a uniform stratum upon the
slab. The paper-cylinder and the inking and diffusing rollers are
so mounted, that when the horizontal table, carrying the form
and inking slab, moves alternately backwards and forwards under
them, they roll upon it.

In this way, when the table is moved towards the rollers, the
form, passing under the inking-rollers right and left, receives
from them the ink upon the face of the type ; and at the same time
the slab, moving backwards and forwards under the diffusing
rollers, receives from them, upon its surface, the proper stratum
of ink to supply the place of that which was taken from it by the
inking-rollers.

20. Single Printing Machines — When the table is moved
alternately towards the other side, the form, with the types
already inked, passes under the cylinder carrying the paper, the
motion of which is so regulated as to correspond exactly with the
rectilinear motion of the table carrying the form. The cylinder
is urged upon the type with a regulated force, sufficient and not
more than sufficient, to impress the type upon the paper.

The sheets of paper are supplied in succession to the cylinder,
and held evenly upon it by bands of tape while they pass in
contact with the type. After receiving the impression of the
type, the tapes which bound them are separated, and the printed
sheets discharged.

Such is the general principle of single printing-machines.

21. Double Printing Machines. — In these the table which
is moved alternately right and left, carries two forms, one corre-
sponding to the pages to be printed on one side of the sheet, and
the other to those to be printed on the other side. There are also
two inking-slabs, one to the left of the left-hand form, and the
other to the right of the right-hand form. There are also two
paper cylinders, and two sets of inking and diffusing-rollers.
Each sheet of paper to be printed, being held between tapes, as
already described, is carried successively round the two cylinders,
being so conducted, in passing from one to the other, that one
side of it passes in contact with, and is printed by, one form, and
the opposite side by the other form. The proportion of the
motions is so nicely regulated, that the impression of each page or
column made on one side of the paper, corresponds exactly with
that of the corresponding page or column on the other side.

12

PRINTING MACHINES.

This general description will be more clearly understood by
reference to the following illustrative diagrams : —

Fig. 5 illustrates the operation of a single printing-machine. The form
A and the inking-slab B, are placed on a horizontal table ; above them is
the paper-cylinder D, the inking-rollers i i i, the diffusing-rollers c c, and
the rollers c, which supply the ink to the diffusing-rollers. The first of
these, o, is called the ductor roller. When the table x Y is moved towards
the left, from Y to x, the form A passes under the inking-rollers i i i, and
receives ink from them on the faces of the type ; at the same time the slab
B passes under the diffusing-rollers c c, and receives from them a supply of
ink to replace what it has just given to the inking-rollers.

When the table is moved in the contrary direction, from x towards Y, the
form once more passes under the inking-rollers, and afterwards under the
paper-cylinder, which being pressed upon it, while it moves in exact accord-
ance with it, the types discharge upon the paper the ink they have just
received from the rollers ; and the printing of the paper being thus effected
on one side, the sheet is discharged from the tapes. The table is then
again moved to the left, and the types are again inked, and the same effects
ensue as have already been described.

In this manner sheet after sheet is printed.

The inking and diffusing-rollers rest upon the slab and
types by their weight, the axes projecting from their ends being
inserted in slits formed in upright supports, attached to opposite
sides of the frame which supports the moving table. The two
upright pieces in which the axes of each roller are inserted, are
not placed in exact opposition to each other ; the consequence of
which is, that the rollers are placed with their axes slightly
inclined to the sides of the table. This arrangement is attended
with a very important effect : for, in consequence of the friction
or adhesion of the rollers with the slab, they are moved alternately
in contrary directions with the longitudinal motion across the
table. This motion, combined with their rolling motion upon the

13

THE PRINTING PRESS.

slab, aids materially in diffusing the ink in a perfectly uniform
stratum.

An illustrative diagram of a double printing-machine is shown in fig. 6,
where » and D' are the two paper-cylinders ; A and A', the two forms ;,
i i i and i' i' i', the inking-rollers ; c c and c' c, the diffusing-rollers ; and
c and c' the ductor-rollers. The pile of paper placed on the table E, is

Fig. 6.

supplied sheet by sheet to the tapes between which it is held ; and being
passed over the roller R, and under the cylinder D', it receives the impres-
sion of the types of the form A' ; it then passes successively over the roller
T', under T, and round the cylinder D, at the lower point of which its
imprinted side comes into contact with the types of the form A, by which
it is printed ; after which, the tapes opening, it is thrown out by the cen-
trifugal force upon the receiving table p.

It will be apparent by the figure, that while the sheet is printed on one
side by the form A', the form A is passing between the inking-rollers i i /,
and the slab B ; and on the contrary, while the paper is printed on the other
side by the form A, the form A' is passing between the inking-rollers i' i' i'
and the slab B'.

In this manner, by each alternate motion from right to left, and
from left to right, a sheet is printed on both sides.

22. A perspective view of a double-acting printing machine, as
constructed by Messrs. Applegath and Cowper, is shown in fig. 7.

A boy, called the layer-on, E, standing at an elevated desk,
pushes the paper, sheet by sheet, towards the tapes, which, closing
upon it, carry it over a roller R, passing under which it is carried
to the right of the cylinder D, under which it passes, and being
carried up to the left of it, passes successively aver the roller T,
under the roller T', over the cylinder D, and drawn along its left
side, after which it passes under it, and is flung into the hands of
a boy, F, called the taker-off, seated before a table placed between
the two cylinders D and D', upon which he disposes the sheets as
he receives them.

In this manner the layer-on feeds the machine in constant
succession with blank sheets, which, being carried under the
cylinder D, are printed on one side, and afterwards under the
cylinder D', are printed on the other, when they are received by
the taker-off.
14

APPLEGATH AND COWPER S MACHINE.

THE PRINTING PRESS.

The ductor-rollers, c and c', are kept in revolution by endless
bands, carried over rollers at the lower parts of the frame, and
then over grooved wheels fixed on the axes of the cylinders. The
table carrying the forms and slabs is moved alternately right and
left by means of a pair of bevelled wheels, w, under the frame,
and a double rack and pinion above ; one of the bevelled wheels,
having a horizontal axis, receives motion from a steam-engine
or other moving power ; it imparts motion to the other bevelled
wheel, having a vertical axis. This latter axis has a pinion fixed
at its upper end, which works in a double rack attached to the
movable type-table, which is so constructed that the continuous
rotation of the pinion imparts an alternate rectilinear motion
right and left to the rack and to the table attached to it.

The manner in which the tapes lay hold of and conduct the
paper successively round the cylinders, and finally discharge it
upon the table of the taker-off, will be easily understood by re-
ference to fig. 8, page 177.

c and D are two grooved rollers, surrounded by an endless band
which pushes the paper from the table of the layer on towards the
tapes. The two endless tapes, between which the paper is held,
are represented in the diagram by the continuous and dotted
lines, and the direction of their motion round the rollers and
cylinders is represented by the arrows. It will be perceived that
opposite the table of the layer-on, the tapes converge, from two
small rollers d and A, and come into contact at the top of the
roller E. The edges of the sheets of paper, being advanced from
the table of the layer-on, are caught between the tapes im-
mediately above the roller E.

It must be understood that there are two or three pairs of tapes
parallel to each other, which correspond to the margins of the
pages or columns ; but only one pair is shown in the figure.

The paper being thus seized between the tapes above the roller
E, is carried successively, as shown in the figure, still held
between the tapes, under and over F H I and G, until it arrives at
i, where the tapes separate, that which is indicated by the con-
tinuous line being carried to the roller a, and that by the dotted
line, over the roller *, to the roller k. The tapes being thus se-
parated the printed sheet is discharged at «, upon the table of the
taker-off ; meanwhile, the tape indicated by the continuous line
is carried successively over the roller a, under b, under c, outside
d, and is finally returned to the roller E.

In the same manner the tape indicated by the dotted line is
carried successively under the roller k and m, outside n, over v
and A, from which it returns to E, where it again joins the other
tape proceeding from d.
16

THE PRINTING PRESS.

CHAPTER II.

23. Machine of The Times of 1814. — 24. Improvement of this. — 25.
Present printing machine of The Times. — 26. Marinoni's news-
paper printing machine. — 27. Marinoni's book-printing machine. —
28. Newspapers. — 29. Reporters. — 30. Court newsman. — 31. Foreign
correspondent. — 32. Newspaper statistics.

23. THE first machines constructed upon this improved prin-
ciple for printing newspapers, were erected at the printing office
of The Times newspaper, and it was announced in that journal,
on the 28th of November, 1814, that the sheet which was then
placed in the hands of the reader, was the first printed by steam-
impelled machinery.

By this, with some improvements which the apparatus received
soon afterwards, the effective power of the printing-press was
augmented in a very high proportion. With the hand-presses
previously in use, not more, as we have seen, than 250 sheets
could be printed on one side in an hour. Each of the two
machines erected at The Times office produced 1800 impressions
per hour.

24. Further Improvement. — The power of the printing-
machine constructed upon this principle was soon after aug-
mented, by increasing the number of printing- cylinders to

LARDNKR'S MUSEUM OF SCIENCE. c 17

No. 133.

THE PRINTING PRESS.

four, the principle of the machine, however, remaining the
same.

The manner in which this was accomplished will be easily
understood by the aid of the illustrative diagram, fig. 9, where 1,
2, 3, and 4, are the printing-cylinders : p p r' r', are the tables
of the four layers-on, and o o o' o', lead to those of the four
takers-off. The course followed by the sheets of paper, in passing
to and from the cylinders, are indicated by the arrows. Inking
rollers are in this case placed at R, between the printing-
cylinders ; the two type-forms are inked twice, while they move
from right to left, and twice again while they move from left to
right. The printing- cylinders are alternately let down upon the
type and raised from them in pairs ; while the type-table moves
from left to right, the cylinders 1 and 3 are in contact with the
table, the cylinders 2 and 4 being raised from it, and, on the
contrary, when it moves from right to left, the cylinders 2 and
4 are in contact with it, 1 and 3 being raised from it.

By this improvement, which was adopted in The Times office
in 1827, the proprietors of that journal obtained the power, then
unprecedented, of printing from 4000 to 5000 sheets per hour on
one side of the paper. By this means they were enabled to
satisfy the demands of a circulation amounting to 28000.

In reference to newspaper-printing, it must be here observed
that the great object to be attained, is to increase the celerity by
which printing on one side only of the sheet can be augmented.
It is found convenient so to arrange the letter-press that the
matter appropriated to one side of the sheet shall be ready for
press at an early hour, and may be printed before the contents of
the other side, in which the most recent intelligence is included,
can be prepared. Hence the advantage of using machines
adapted to print one side only with the most extreme celerity for
newspapers.

25. " Times " Printing-machine. — This machine continued to
serve the purposes of The Times newspaper until a later epoch,
when again the exigencies of the press exceeded even its immense
powers, and another appeal was made to the inventive genius of
Mr. Applegath. It was, in short, necessary to provide a machine
by which at least 10000 sheets an hour could be worked off from
a single form !

In considering the means of solving this problem, it is neces-
sary to observe, that whatever expedient may be used, the sheets
of paper to be printed must be delivered one by one to the
machine by an attendant. After they once enter the machine
they are carried through it and printed by self-acting machinery.
But in the case of sheets so large as those of the newspapers, it is
18

Fig. 9.

THE PRINTING PRESS.

found that they cannot be delivered with the necessary precision
by manipulation at a more rapid rate than two in five seconds, or
twenty-five per minute, being at the rate of 1500 sheets per hour.
Now, in this manner, to print at the rate of 10000 per hour,
would require seven cylinders, to place which so as to be acted
upon by a type- form moving alternately in a horizontal frame,
in the manner already described, would present insurmountable
difficulties.

In the face of these difficulties, Mr. Applegath, to whom the
world is indebted for the invention of The Times printing-
machine, decided on abandoning the reciprocating motion of the
type-form, arranging the apparatus so as to render the motion
continuous. This necessarily involved circular motion, and
accordingly he resolved upon attaching the columns of type to
the sides of a large drum or cylinder, placed with its axis ver-
tical, instead of the horizontal frame which had been hitherto
used. A large central drum is erected, capable of being turned
round its axis. Upon the sides of this drum are placed verti-
cally the columns of type. These columns, strictly speaking,
form the sides of a polygon, the centre of which coincides with
the axis of the drum, but the breadth of the columns is so small
compared with the diameter of the drum, that their surfaces
depart very little from the regular cylindrical form. On another
part of this drum is fixed the inking-table. The circumference
of this drum in The Times printing-machine measures 200 inches,
and it is consequently 64 inches in diameter.

The general form and arrangement of the machine are repre-
sented in fig. 10, where D is the great central drum which carries
the type and inking tables.

This drum in The Times machine is surrounded by eight
cylinders, n, E, &c., also placed with their axes vertical, upon
which the paper is carried by tapes in the usual manner. Each
of these cylinders is connected with the drum by toothed-wheels,
in such a manner that their surfaces respectively must neces-
sarily move at exactly the same velocity as the surface of the
drum. And if we imagine the drum, thus in contact with these
eight cylinders, to be put in motion, and to make a complete
revolution, the type-form will be pressed successively against
each of the eight cylinders, and if the type were previously
inked, and each of the eight cylinders supplied with paper, eight
sheets of paper would be printed in one revolution of the drum.

It remains, therefore, to explain, first, how the type is eight
times inked in each revolution ; and, secondly, how each of
the eight cylinders is supplied with paper to receive their
impression.
20

TIMES PRINTING MACHINE,

Beside the eight paper- cylinders are placed eight sets of inking
rollers ; near these are placed two ductor rollers. These ductor
rollers receive a coating of ink from reservoirs placed above

THE PRINTING PRESS.

them* As the inking-table attached to the revolving drum
passes each of these ductor-rollers, it receives from them a coating
of ink.. It next encounters the inking rollers, to which it
delivers over this coating. The types next, by the continued
revolution of the drum, encounter these inking rollers, and
receive from them a coating of ink, after which they meet the
paper-cylinders, upon which tli^- are impressed, and the printing
is completed.

Thus in a single revolution of the great central drum the
inking-table receives a supply eight times successively from the
duetor-rollers, and delivers over that supply eight times succes-
sively to the inking rollers, which, in their turn, deliver it eight
times successively to the faces of the type, from which it is con-
veyed finally to the eight sheets of paper held upon the eight
cylinders by the tapes.

Let us now explain how the eight cylinders are supplied with
paper; Over each of them is erected a sloping desk, A, A, &c.,
upon which a stock of unprinted paper is deposited. Beside this
desk stands the layer-on, who pushes forward the" paper, sheet
by sheet, towards the tapes.

These tapes, seizing upon it, first draw it down in a vertical
direction between tapes in the eight vertical frames, until its
edges correspond with the position of the form of type on the
printing-cylinder. Arrived at this position, its downward motion
is stopped by a- self-acting apparatus provided in the machine,
and it is then impelled by vertical rollers towards the printing
cylinder, these rollers having upon them marginal tapes which
carry the paper round the cylinder, from which it receives the
impression of the types. After this the central and lower marginal
tapes dismiss the sheet of paper, which the upper ones only become
charged with, and carry it to its proper place, where the tapes are
stopped with the paper suspended between them, until the taker-
off draws the sheets down, ranging them upon his table. These
movements are continually repeated ; the moment that one sheet
passes from the hands of the layer-on, he supplies another, and
in this manner he delivers to the machine at the average rate of
two- sheets every five seconds ; and the same delivery taking place
at each of the eight cylinders, there are sixteen sheets delivered
and printed every five seconds.

It is found that by this machine in ordinary work between
10000 and 11000 per hour can be printed ; but with very expert
men to deliver the sheets, a still greater speed can be attained.
Indeed the velocity is limited, not by any conditions affecting
the machine,, but by the power of the men to deliver the sheets
toit.
22:

TIMES PRINTING MACHINE.

In case of any misdelivery a sheet is spoilt, and, consequently,
the effective performance of the machine is impaired. If, how-
ever, a still greater speed of printing were required, the same
description of machine, without changing its principle, would
be sufficient for the exigency ; it would be necessary that the
types should be surrounded with a greater number of printing
cylinders.

It may be right to observe, that the cylinders and rollers are
not uniformly distributed round the great central drum ; they are
so arranged as to leave on one side of that drum an open space
equal to the width of the type form. This is necessary in order
to give access to the type form so as to adjust it.

One of the practical difficulties which Mr. Applegath had to
encounter in the solution of the problem, which he has so success-
fully effected, arose from the shock produced to the machinery by
reversing the motion of the horizontal frame, which, in the old
machine, carried the type-form and inking-table, a moving mass
which weighed twenty-five hundred weight. This frame had a
motion of 88 inches in each direction, and it was found that
such a weight could not be driven through such a space with
safety, at a greater rate than about forty -five strokes per
minute, which limited its maximum producing power to 5000
sheets per hour.

Another difficulty in the construction of this vast piece of
machinery was so to regulate the self-acting mechanism that the
impression of the type-form should always be made in the centre
of the page, and so that the space upon the paper occupied by the
printed matter on one side may coincide exactly with that occu-
pied by the printed matter on the other side.

The type-form fixed on the central drum moves at the rate of
about 80 inches per second, and the paper is moved in contact
with it of course at exactly the same rate. Now, if by any error
in the delivery or motion of a sheet of paper, it arrive at the
printing-cylinder l-80th part of a second too soon or too late, the
relative position of the columns will vary by l-80th part of 80
inches — that is to say, by one inch. In that case the edge of the
printed matter on one side would be an inch nearer to the edge of
the paper than on the other side.

This is an incident which rarely happens, but when it does, a
sheet, of course, is spoilt. In fact, the waste from that cause is
considerably less in the present vertical machine than in the
former less powerful horizontal one.

The vertical position of the inking-rollers, in which the type is
only touched on its extreme surface, is more conducive to the
goodness of the work than the horizontal machine, where the

THE PRINTING PRESS.

inking-rollers act by gravity ; also any dust shaken out of the
paper, which formerly was deposited upon the inking-rollers, now
falls upon the floor.

With this machine 50000 impressions have been taken without
stopping to brush the form or table.

26. Marinoni's Newspaper Printing-press. — Messrs. Marinoni
and Co., of Paris, have, within the last few years, constructed
improved printing presses for newspapers of large circulation,
several of which have been erected and brought into operation in
the printing-office of the Paris journal La Presse. This printing-
machine, which is capable of working off 6000 copies per hour,
printed on both sides of the paper, is represented in fig. 11. It
will be perceived that eight men are employed in the process, four
layers-on and four takers-off. The machine is double, the parts
at each side of a vertical line drawn through the axis of the fly-
wheel being perfectly similar. The manner in* which the sheets
pass to and from the printing-rollers will be more readily under-
stood by fig. 12, where A is the upper and A' the lower delivering-
board, and B the upper and B' the lower receiving-board on the
right, the two delivering and receiving-boards on the left being
similarly placed. The motion of the sheets, as they are conducted
to and from the rollers by the tapes, is indicated by arrows, and
the course followed by each sheet from the moment it leaves the
delivering-board until it arrives at the receiving-board, is indi-
cated by the numbers 1, 2, 3, 4, &c. Thus, the sheet delivered
from the board A is taken by the tapes which pass round the
roller M, and carried from 1 to 2. Arriving at the lower roller, it
passes, as shown by the arrow, between the rollers, 3, and is
carried from 4 under the printing-roller I, where it is printed on
one side, after which it is carried up between the tapes to 5, from
whence it is discharged between the tapes of 6, and carried up
over the roller B, at 7, from which it is carried down between the
tapes 8, and thrown, as shown by the arrow, to the tapes 9, by
which it is again carried under the roller and printed on the other
side ; after which it is carried up successively between the tapes
10 and 11 to 12, and finally discharged from 13 at 14 upon the
receiving-board B.

The sheet delivered from the lower receiving-board A', follows
a course precisely similar, entering at 1' and passing round the
printing-roller at 2' 3', from which it passes between the tapes 4'
round the roller n' at 5' 6', and thence from the tapes 1' 8' round
the printing-roller i' at 9', by which it is printed on the other
side ; after which it is carried by 10', 1 1', 12' to the lower receiving-
board B' at 13'.

The inking-rollers are shown at E P D and D', and are arranged
24

MABIKONI S PRINTING MACHINE.

THE PRINTING PRESS.

26

MARINONI'S FEINTING MACHINE.

in the usual manner, at T, T', T", T'", and T"", to spread the ink on
the types.

It will be perceived that the power of this press is equal to that
of The Times, the difference being that The Times prints 12000
sheets on one side only, while this prints 6000 on both sides. The
Times machine requires eight layers-on and eight takers-off, being
double the number required by Marinoni's press. It must, how-
ever, be observed that, in the practical management of newspaper
printing, as conducted in The Times office, the power of Marinoni's
press, though in a certain sense equal to that of The Times, would
be altogether insufficient; for it is indispensably necessary for
that journal to print 60000 copies on one side of the paper during
the last five hours of the morning. The matter allotted to the
other side of the paper is so selected that it can be composed and
printed in the earlier part of the night, or even of the previous
day; the pressure falling exclusively on the matter which occupies
the other side of the paper, consisting chiefly of the latest intelli-
gence and Parliamentary reports.

It may be asked, therefore, how the journal of La Presse, of
which the circulation, though inferior to that of The Times, is
still very large, can be printed with the necessary celerity ? The
answer is, that La Presse does not contain as much as the tenth
part of the letter-press of a copy of The Times, and that therefore,
it is found practicable to compose the matter in type twice or
oftener, so as to produce two or more distinct forms, as they are
called, which are put to work at as many different presses. The
expense of composition is further economised at the printing office
of La Presse by stereotyping the matter, which is composed at a
sufficiently early hour to admit of that process, the stereotype
plates being melted down the next day. By this expedient double
or triple composition is only necessary for the intelligence which
comes too late to allow of being stereotyped.

27. Marinoni's Book-printing Machine. — A convenient form
of printing-engine for books, constructed by the same engineers,
is shown in fig. 13, by which, however, the sheets are printed
on one side only. The layer-on delivers the sheets upon the
board M (fig. 14), from which they pass round the printing
roller i, and are discharged as indicated by the arrow upon the
receiving-board n. The rollers for delivering and spreading the
ink on the types are arranged in the usual way.

28. Newspapers. — Of all the applications of printing to the
uses of life, that which has conduced most to the advancement and
improvement of the art has been the printing of newspapers.
These organs of public opinion and intelligence combine the con-
ditions which require from the printing press the greatest con-

27

THE PRINTING PRESS.

NEWSPAPERS.

Fig. 14.

ceivable degree of
perfection. Ex-
treme rapidity of
composition com-
bined with the
greatest attain-
able celerity of
press- work are
above all things
indispensable,
and how far these
objects have been
attained by the
modern printing-
machine, will be
understood by
what has been
stated in the
preceding para-
graphs. On the
other hand the
improvement in
the machinery of
printing, which
the exigencies of
journalism have
thus produced,
have themselves
reacted on jour-
nalism in the
most surprising
manner, so that
the feats of art
now accomplish-
ed in the esta-
blishment of the
London daily pa-
pers, j ustly excite
the admiration
and the wonder
of all well-in-
formed persons.
These newspapers
are remarkable
for the vast mass

THE FEINTING PRESS.

and variety of intelligence which, they contain, the celerity with
which they are printed and circulated, and the accuracy and
copiousness of the reports which they afford of the proceedings of
all public bodies. These results are obtained by an enormous ex-
penditure of money and a minute and judicious division of labour.
A corps of able and intelligent reporters is maintained, whose duty
it is to attend the House* of Parliament, the Courts of Law, Police
Offices, and all public meetings. These relieve each other at short
intervals of from half to three-quarters of an hour, when they return
successively to the office of the journal and there write out in long-
hand the substance of their short-hand notes. These are imme-
diately delivered over to the compositor, who proceeds to set them
up in type, and when a column has thus been composed it is handed
over to the reader, after receiving whose correction it is returned to
the compositor, who introduces the corrections into the columns of
type. A proof being taken it is laid before the editor who decides
the part of the paper it is to occupy, and whether it may need
alteration or abridgment. It happens thus in the case of long
debates, and sometimes in the case of long speeches, that a part
will be actually set up in type and printed in proof before the
remainder has been yet spoken.

29. Reporters. — In appreciating the functions of reporters, it
is a great though very common mistake, to suppose that their duty
consists merely in reproducing verbally the speeches delivered.
If this were to be done no journal, however great its magnitude,
would be sufficient to contain even a small fraction of the matter
reported. The reports are therefore necessarily abridgments, with
the exception of certain passages of striking importance, occurring
occasionally in the speeches of the most eminent public men, and
these can always be distinguished in the reports by the use of the
first person instead of the third. The reporter takes therefore
not verbatim notes but merely abridged memoranda of what is
said, and as he remains in attendance only for the brief interval
of half an hour or a little more, his memory by practice enables
him to supply the lacunae, so that when he arrives at the office of
the journal he is enabled to write out a good abridged report of
what he has heard. It is in this admirable capacity for judicious
abridgment that the skill of the reporter, and more especially of
the parliamentary reporter, is shown.

It will easily be understood from what has been here stated, that
a well-conducted journal in London is obliged to maintain a large
corps of reporters. They are generally classified according to
their particular abilities and fitness. The highest class being
parliamentary reporters, who are understood to be paid at the rate
of about 51. per week during the session of parliament. The law
30

NEWSPAPER STATISTICS.

reporters are a peculiar class requiring special qualifications, and
are generally barristers who have not yet obtained sufficient
practice to occupy their time. Police reporters form another
distinct and peculiar class, who supply that part of the journals
to which are consigned the proceedings of the police offices.

In fine, there is another class of agents for the supply of
general intelligence, whose business it is to collect information on
all subjects in all parts of the town.

30. The Court Newsman is not to be overlooked. This per-
sonage supplies daily to all the journals, those paragraphs in which
are found recorded the movements of the Sovereign and the Royal
Family ; who was invited to dinner at Windsor or Buckingham
Palace ; what music was performed at and after dinner, and by
what band, and so forth. The same functionary is entrusted to
supply accounts of the various parties and entertainments given by
the aristocracy.

31. Foreign Correspondents. — Among the staff of daily
London journals the foreign correspondent holds a conspicuous
place. The principal journals maintain such a correspondent in
all the principal foreign capitals, and, in case of war, such a cor-
respondent accompanies the army and the fleet. The foreign cor-
respondents maintained in the principal European capitals usually
keep bureaux, and have assistants, who collect news and supply
reports. A despatch is forwarded to London always once, and
often twice, a day, the telegraph being resorted to when news of
considerable importance is required to be transmitted with more
promptitude.

32. The rapidity with which the circulation in newspapers has
increased in the United Kingdom during the last century, but
more especially during the latter half of it, may be judged from
the following facts.

In the annexed table is given the total number of newspapers
circulated in this country during the years expressed in the first
column : —

Years.

Number
circulated.

Average annual
increase.

1751
1801
1821
1831
1841
1849

7,41*575
16,085085
24,862186
35,198160
59,936897
78,792934

173450
438855
i»o33597

2,473873
2,357oo5

Thus it appears, that while the average annual increase of the
circulation of journals, in the latter half of the last century,

31

THE FEINTING PEESS.

was limited to 173000, the average increase during the first
twenty years of the present century was 439000 ; the next ten
years this rate of increase was more than doubled, and in the
succeeding period it was augmented in a sixfold ratio. The
total circulation in 1849 was more than ten times the circulation
in 1751.

By comparing the circulation of journals with the population,
an estimate may be obtained, if not of the diffusion of know-
ledge in general, at least of that description of information of
which journalism is the vehicle. In the following table are
given the amount of the population at the epochs above men-
tioned, and the number of journals circulated per head of the
population.

Years.

Population.

Number of
journals per head.

1751
1801
1821
1831
1841

6,377574
10,942646
14,391631

i6,5393i8

18,720394

I "I

I'5

i*7

2'I

3*«

Thus, relatively to the population,, the circulation of journals
had increased in a twofold proportion in 1841 , as compared with
1821, and in a threefold ratio as compared with 1751. Taking
the population of Great Britain in round numbers in 1849, the
ratio of journals to the populations would be 4 to 1, being an
increase in the same ratio on the circulation in 1751.

So far, therefore, as the circulation of journals can be regarded
as an exponent of the diffusion of knowledge, a greater amount
of general information prevails now than prevailed a century ago-
in a fourfold proportion.

Since the dates of these returns, a vast change has been made
in the circulation of journals, by the abolition of the newspaper
stamp and reduction of the advertisement duty. Before the
abolition of the stamp, the amount of the daily circulation of
each journal was known by the Stamp-office returns. The
average daily circulation of The Times was then about 40000, or
about double the aggregate circulation of all the other morning
journals. Since the abolition of the stamp duty, and the conse-
quent reduction of the price of the journals, a considerable
increase of circulation has taken place as well in The Times as in
the other journals, and we shall not perhaps overrate the present
circulation of the London morning newspapers if we put them
down in the aggregate at 100000.

32

Fig. 31. — BASALTIC CAUSEWAY OF THE RIVER VOLANT. (PEP. ARDECHE, FRAKCE.)

THE CRUST OF THE EARTH;

OR, FIRST NOTIONS OF GEOLOGY.
CHAPTEE I.

1. The earth, a subject of long-continued observation and investigation. —
2. Mathematical geography. — 3. Physical geography. — 4. Phenomena
of the oceans and seas included in it. — 5. Hydrology, meteorology,
and climatology. — 6. Political geography. — 7. General subject of
geography. — 8. Geology. — 9. Original fluidity of the eai'th inferred
from its sphei-oidal form. — 10. This form ascertained by observation
and measurement. — 11. The solid crust was formed while the earth was
in a state of rotation. — 12. Increase of temperature from surface down-
wards.— 13. Within the crust the earth still in a state of fusion. — 14.
Subject of geology. — 15. How the structure of the crust to a great depth
is rendered manifest. — 16. Section of the crust where no disturbance
has taken place — strata occur in a fixed order. — 17. Rocks in their
geological sense. — 18. Their classification in five principal divisions. —
19. Lowest bed, being the foundation of the crust, consists of igneous
rocks produced by the superficial cooling of the molten materials of the
globe. — 20. Materials of which the igneous rocks are formed.— 21.
Constituents of granite, feldspar, mica, quartz. — 22. These components
of igneous rocks agglomerated mechanically. — 23. But the components
themselves are chemical compounds. — 24. Varieties of granite —
porphyry. — 25. Gneiss. — 26. Secondary rocks. — 27. Transition or
metainorphic rocks. — 28. Stratification. — 29. Produced by aqueous

LARDNER'S MUSEUM OF SCIENCE. D 33

No. 132.

THE CRUST OF THE EARTH.

deposition. — 30. Stratified rocks of aqueous origin. — 31. Circum-
stances corroborating this inference.— 32. Stratified and unstratified
rocks. — 33. Condition andjnaterials of transition rocks. — 34. Animal
remains found in them. — 35. No vegetable remains — probable reason.
— 36. Fish and annelidans. — 37. Stratified rocks in general. — 38.
Secondary rocks. — 39. Vast quantity of organic remains deposited in
them. — 40. Tertiary rocks. — 41. Diluvium, alluvium, and surface
soil. — 42. Subdivision .of these principal groups of strata. — 43.
General inference as to the condition and ages of stratified rocks. —
44. They constitute a chronological scale. — 45. Complexity and
defects of geological nomenclature.

1 . IT cannot be matter of surprise that of all the great bodies of
the universe the earth, which has been assigned by the Creator as
the habitation of the human race, has received the largest share
of attention on the part of those who have devoted their faculties
to the observation and investigation of nature. Regarded in
different points of view it has formed the exclusive subject of
several branches of science.

2. Taken in its entire mass, and considered in relation to the
other bodies of the solar system, it constitutes the subject of
MATHEMATICAL GEOGRAPHY, which includes the solution of such
problems as the determination of the magnitude of the earth —
its exact form — its relation to the other bodies of the solar system
— its annual motion round the sun which produces the apparent
movement of that luminary through the signs of the Zodiac — its
diurnal rotation which produces those apparent motions of the
firmament which cause the vicissitudes of day and night — the
peculiar position of its axis which produces the succession of
seasons — the division of the globe into zones and climates, and
the system of imaginary circles of latitude and longitude which
supply the means of expressing the position of all places on the
globe, relatively to each other, and to the equator and poles.

3. The earth, viewed in regard to the various physical states of
its surface, constitutes the subject of PHYSICAL GEOGEAPHY,
which includes a description of the distribution of land and
water — the extent and configuration of continents and islands —
the elevation and prevailing direction of mountain chains — the
form, extent, and direction of plains and valleys — the general
altitude of the surface of the land above the common level of the
sea — the effects of soil and climate, and the local distribution of
animal and vegetable productions.

4. This division of geographical science also includes the
various phenomena of the ocean and seas, their depth, salt-
ness, and temperature, the prevailing direction and velocity of
ocean currents, the extent of the polar ice and circumstances
incidental to it.

34

SUBDIVISIONS OF GEOGRAPHY.

o. The subdivisions of physical geography are sometimes
denominated HYDROLOGY, which includes all that relates to the
ocean and seas; METEOROLOGY, which includes the investigation
of atmospheric phenomena ; and CLIMATOLOGY, which involves
the consideration of the mean temperature of different countries,
the altitude of the line above which there is perpetual snow, the
prevailing winds, the barometric pressure, the annual quantity of
rain, and so on.

6. The earth, considered as the abode of mankind, distributed
into different nations and placed under different forms of govern-
ment, constitutes the subject of POLITICAL GEOGRAPHY, which
therefore includes the consideration of the moral and social con-
dition of different peoples, their language, religion, forms of
government, and civilisation, and the population, resources and
local relations of different countries.

7. It appears, therefore, that these several divisions of geo-
graphical science are limited to the appearances and phenomena
developed upon the surface of the earth, in the waters which par-
tially cover it, and in the atmosphere by which it is surrounded.

8. Another, and not less important department of science con-
cerning the earth, relates to the condition and structure of those
parts of the globe which lie between its surface and its centre, and
which constitute the subject of GEOLOGY.

9. In mathematical geography and astronomy, certain physical
circumstances attending the interior of the earth have been
developed. Thus it is proved that the density of the globe
gradually increases from the surface to the centre ; and from its
peculiar form it is inferred that, at the epoch of its formation,
the materials composing it must have been in a fluid state. It
has been already shown in our Tract on the "Earth," that the
form of the globe is what in geometry is called an oblate spheroid,
a figure somewhat resembling an orange or a turnip. By reason
of this figure, the earth is flattened at the poles and bulged out at
the equator. Now it is proved in mathematical physics that if a
fluid globe have a motion of rotation on one of its diameters as
an axis, the centrifugal force of the matter composing it will cause
it to bulge out at the equator and to flatten at the poles, so that all
sections of it made by planes passing through the axis are ellipses,
and the degree of the elliptic form of these sections will be so
much the greater as the velocity of rotation is more rapid ; and so
closely connected is this degree of ellipticity with the velocity of
rotation, that mathematicians are able to assign the exact form
of the elliptic section which must correspond to each particular
velocity of rotation.

10. Before the exact form of the earth had been determined by

D 2 35

THE CRUST OF THE EARTH.

direct observation and measurement, mathematicians had already
ascertained by computation what ought to be its form consequent
upon its rotation upon its polar axis, and the time in which it is
actually known to rotate, and the result of their computations was
afterwards found to agree with the utmost possible numerical
precision with the actual form which the earth was proved to have
by immediate observatiofl and measurement.

11. It is therefore inferred from this that the earth, in its
original state, and before it assumed its present condition, was a
fluid mass, and that while in this fluid state, it received the
diurnal rotation by which it is now affected, and which produces
the vicissitudes of day and night. It was, therefore, after having
assumed this spheroidal form, that its superficial parts hardened
and solidified, and after undergoing a certain succession of changes
assumed their present condition.

12. It has been also shown in our Tracts on " Terrestrial Heat,"
and on " Earthquakes and Volcanoes," that when we penetrate
into the crust of the earth by mines, boring, or other artificial
means, the temperature is found to undergo a gradual and regular
augmentation, and this augmentation being continuous, so far as
direct observation has been carried, may be assumed by analogy
to increase to a still greater depth in the same proportion — a con-
clusion which also is verified by the phenomenon of hot springs
rising from a considerable depth, and by various volcanic effects.
Assuming this gradual increase of temperature to go on indefinitely
in descending towards the centre, it has been shown that at a
certain depth the temperature must be such that even the most
refractory constituents of the globe would be reduced by it to a
state of fusion.

13. The increase of temperature being at the rate of about a
hundred thermometric degrees per mile, it would follow that at
the depth of about 40 miles, or about the 100th part of the entire
distance from the surface to the centre, we should arrive at a
temperature of 4000°, at which it is quite certain that no part of
the matter composing the earth could remain solid. It is therefore
inferred, without the necessity for extreme numerical precision,
that the earth is a spherical shell, the superficial part only being
solid — all the central part being in a state of igneous fusion — the
thickness of the solid crust being, as just stated, about the 200th
part of the entire diameter. A section of it would be such as is
represented in fig. 1.

If, therefore, as we have already stated in our Tract on
11 Earthquakes and Volcanoes," the egg of a fowl be imagined to
represent the earth, its shell would be much too thick to represent
its solid crust.
36

INTERNAL FLUIDITY OF THE EARTH.

It is no rhetorical exaggeration that the globe we live on is a
stupendous, but very thin spherical shell, charged with liquid fire ;
and if such be the case it may naturally be asked how it happens,
that so thin a crust supported on a fluid so mobile can maintain that

Fig. i.

general state of stability which characterises it so strongly, that it
is referred to in times ancient and modern as the type of all that
is most solid and most durable. An answer to this reasonable
question will be collected from all we shall have to explain in the
present Tract.*

14. The investigation of the structure and condition of this
solid crust of the terrestrial spheroid, extending from the fluid

* According to Mr. Hopkins, the thickness of the crust is subject to
much local variation, being very unequal on its inner surface. He con-
siders that it is probably cavernous, and that masses of fluid mineral
matter may be distributed through its cavities. According to him the
thickness in some parts may be as great as 800 or 1000 miles. See
Memoirs on the State of the Interior of the Earth, in the Philosophical
Transactions from 1839 to 1842.

37

THE CRUST OF THE EARTH.

nucleus upon which it is supported to the surface, constitutes the
subject of geology.

15. Considering that however thin may be the crust of the earth
as compared with its diameter, its absolute thickness being at least
from 30 to 40 miles, and that this very far exceeds any depth
which is accessible to c|jrect observation, it might be imagined
that all attempts at an analysis of its structure would be vain
and futile. A circumstance nevertheless which at first view
would seem to throw difficulties insurmountable in the way of the
solution of this problem has, on the contrary, happily facilitated
it. This circumstance consists in certain local irregularities in
the actual state of the solid crust. What man could not do, the
accidental effects of internal forces has done for him. The solid
crust has been locally disrupted, so that its section in some cases
for many miles of depth has been turned upwards and exposed
to direct observation. It will be sufficient here to allude briefly
to this as one of the principal means, by which the structure of
the terrestrial crust has been ascertained. The point will be more
fully developed hereafter.

16. To explain the condition and structure of the terrestrial
crust, we will suppose in the first instance a part of the earth's
surface to be selected, which throughout a considerable extent is
plane and level, and which has been subject to no derangement
of structure by internal convulsion, so that the materials which
form it, extending from the surface to the internal igneous fluid on
which it rests, have remained in their normal and original order
and state. If we imagine a vertical section of the crust in such a
situation to be made extending indefinitely downwards, it will be
found to consist of a series of strata superposed in a certain fixed
order, each stratum, taken in succession, consisting generally of
the same constituents in the same state.

17. The matter composing these strata is called by geologists
BOCKS, a term used in this science in a sense somewhat different
from its common popular signification. ROCKS in the geological
sense does not necessarily imply masses of stone. It signifies any
agglomeration whatever of matter which may be found to enter
into the composition of the crust of the earth. In this sense clay
and sand come under the name of ROCKS as well as granite and
marble.

18. Taking then the term rocks in this extended sense, the
successive strata composing the shell of the earth is found to
consist of different layers of rock, each layer being characterised
by rocks existing in a peculiar state of aggregation.

The strata thus superposed, and extending from the funda-
mental layer which rests immediately upon the matter in igneous

STRATIFICATION — IGNEOUS ROCKS.

fusion within the terrestrial shell upwards to the surface, are very
numerous.

They have, however, been reduced to the five following classes
proceeding upwards.

1° The igneous rocks.

2° The transition or metamorphic rocks.

3° The secondary rocks.

4° The tertiary rocks.

5° The diluvial and alluvial layers of matter upon which the
superficial soil is spread.

We shall first briefly notice these five great layers, each of
which, however, as will hereafter appear, consists of numerous
subordinate courses or strata.

19. The lowest or fundamental layer, called igneous rocks,
consists exclusively of agglomerations of mineral masses in a
state of crystallisation. This is the condition which matter
would necessarily assume, and which it could only acquire by
having been gradually cooled and solidified, after being brought
to a state of fusion by a great elevation of temperature. Under
such circumstances its chemical constituents would group them-
selves according to their mutual affinities, and would assume the
various crystallised forms proper to them. After cooling and
solidifying, the materials would present the appearance of an
agglomeration of crystals thrown arbitrarily together and without
regularity or order. Now this being exactly the appearance pre-
sented by the rocks which form the foundation of the crust of
the globe, it is inferred that they were originally in a state of
igneous fusion, and that by the gradual loss of heat by radiation,
they were superficially cooled and solidified. The parts of the
primitive rocks which have been brought under the observation
of geologists are considered as forming the external parts of this
solid layer.

These rocks are from circumstances here explained often
denominated PLUTONIC or IGNEOUS HOCKS, or BOCKS OF IGNEOUS
ORIGIN.

20. Those primitive layers, which may be regarded as the
original materials of which the entire crust of the globe is formed,
consist chiefly of that rock familiar to all observers of mountainous
countries called GRANITE, the most imperishable of all stones, and
therefore the most precious for the purposes of construction. This
granite is mixed in the fundamental layer in smaller proportions
with the minerals called AMPHIBOLE, PYROXENE, and PERIDOTE.

21. Granite is an agglomeration of the crystals of three
minerals, called FELDSPAR, MICA, and QUARTZ. Feldspar is the
soft grey part of the granite, which is easily scratched. The

39

THE CRUST OF THE EARTH.

oxides of iron and manganese are occasionally mixed with this
constituent of granite, and though present in extremely minute
quantity, produce nevertheless a very striking appearance, render-
ing the rock white, cream-coloured, or red, according to their
varying proportions.

MICA is the shining glossy particles of the stone, which reflect
light like bits of glass or metal. The name is derived from
"MICANS," glittering. Mica may be seen in many other stones,
as also in sand.

QUARTZ, which appears in granite in the form of white crystals,
is the substance known as SILISX or SILICA, or the EARTH OF
FLINTS, and is one of the hardest and most abundant of mineral
substances, entering largely into the composition of many other
mineral masses. It is from this constituent, chiefly, that granite
receives its hardness.

Silica is familiarly known as ROCK CRYSTAL.

22. It must be remembered that the several materials of which
the igneous rocks are thus composed, are not combined together
chemically. They are not combined for example in the same
manner as are sulphur and oxygen, when these constituents pro-
duce vitriol. They are on the contrary merely agglomerated and
brought into mechanical juxtaposition, forming a solid mass by
the mere cohesion of crystal to crystal, so that by the action of
mechanical force it would be possible to resolve the rock into its
component parts.

23. Each of these constituents is,' however, itself a compound.
Thus feldspar is a compound of the silicates of several chemical
substances, such as alumina, lime, and potash, or soda — that is, it
is a combination of these severally with silicic acid.

Mica is composed of like silicates, with the addition of silicate
of iron.

Quartz is in fact silicic acid itself. It will appear, then, from
this statement, how important a part silex, or earth of flints,
plays in the formation of the globe.

24. If the constituents of the igneous rocks were combined,
one with another, chemically, instead of being mechanically
juxta-posed, they would, according to a general law of nature,
always be found to prevail in the same rocks in one invariable
numerical proportion; but being, as explained above, merely
agglomerated by cohesion, without any chemical union, they may
exist in any proportion whatever, and hence have arisen a cor-
responding variety of granites. In some specimens the quartz
and mica are altogether absent, and then the granite, consisting
of feldspar only, in the pasty and crystallised state, takes the
name of PORPHYRY.

40

CONSTITUENTS OF GRANITE.

In other specimens, the proportion of feldspar being large, and
that of mica and quartz small, the rock is called POEPHYEOUS

GEANITE.

25. In general, the little laminae of mica are distributed irre-
gularly through the granite, their faces being turned in all
conceivable directions. In certain specimens, however, they are
observed to be placed parallel to each other, so as to give the rock
a lamellated, slaty, or schistous texture. The granite, in such
cases, takes the name of GNEISS, from the Danish " ynister."

26. Having thus briefly described the composition and condition
of the fundamental layer of matter, upon which the solid shell of
the earth is based, and indicated the circumstances and characters
which are evidence of its igneous origin, we shall now proceed to
explain the condition and character of the superincumbent strata,
which, as will presently appear, have had an origin of a very
different kind, and dates of incomparably more recent formation.

27. The strata which rest immediately upon the igneous rocks
have been denominated TRANSITION or METAMOEPHIC, inasmuch
as they partake partly of the character of the igneous rocks, and
partly of that of the rocks incumbent upon them. They partake,
in certain instances, of so much of the former and so little of the
latter, that in the earlier epochs of geological research, they were
classed with the igneous rocks, from which, nevertheless, they are
distinguished by sufficiently evident marks of incipient strati-
fication.

28. It has been explained that the materials composing the
igneous rocks are confusedly and irregularly agglomerated with-
out the least appearance of even an approach to any regularity
of structure, and it has been shown, that this is at once the con-
sequence and evidence of their igneous origin. Such, however,
is not the character of the superincumbent rocks. The materials
of which these are severally composed are found to be distributed,
one over another, in regular layers, bounded by parallel and
horizontal surfaces, resembling the courses of masonry. Now
such a distribution never could have resulted if these, like the
primary rocks, had, previously to their formation, been in a state
of fusion.

29. Such an arrangement, on the other hand, is precisely that
which would ensue, if the materials, composing the strata, having
been mixed with, and suspended in water, had, after the fluid
became tranquil, gradually subsided and settled at the bottom.
In such a case, the matter thus subsiding would be deposited in
a regular series of layers, one above the other, with level and
parallel surfaces. The lowest layer would be composed of the
heaviest part of the matter held in suspension by the water, that

41

THE CRUST OF THE EARTH.

being the most prompt to sink. Next would come layers of less-
ponderous matter, and then another lighter still, and thus layer
upon layer would be deposited until the whole of the suspended
matter would have subsided.

If from any cause after this subsidence the water retired, the
ground forming its bottom would be left bare, and the dry land
would, if it were excavated, be found to consist of the succession
of strata here described.

Now if the actual layers composing the successive strata, which
are superincumbent on the igneous rocks, did really in their
origin proceed from such a cause, it might be expected that they
would succeed each other in the order here indicated, those most
apt to subside holding the lower position, and such accordingly is-
found to be the case.

30. In accordance with these results of observation on the
strata forming the crust of the earth, and with concurrent
evidence deduced from other appearances, it has been inferred,,
with a degree of probability amounting to moral certainty, that
the stratification has resulted from such a series of physical,
causes as those above described. Each stratum consisting of a
series of parallel layers is assumed to have been a sedimentary
deposit precipitated from water, by which the surface of the solid
part of the globe has been at former epochs covered, and that
these waters having become quiescent before retiring, the matter
suspended in them was deposited in layers having more or less
regularity, their surfaces being parallel and level, or nearly so.

31. Among the many collateral circumstances which corro-
borate these conclusions may be mentioned two.

First, the frequent occurrence in the bounding surface of the-
layers of the form of the ripple of water, as it is observed in the
sands of the sea shore after the fall of the tide has laid them bare..

An example of such traces left upon the beds of carboniferous
limestone and Portland-stone, in the neighbourhood of Boulogne,,
is shown in fig. 2.

Fig. 2.

Secondly, by the remains of various aquatic animals and
plants, which are often preserved in their natural position, in
42

SEDIMENTARY DEPOSITS.

which they were tranquilly buried by the matter deposited upon
them. Numerous remains of Crustacea and mollusca are thus
found in perfect preservation in strata situate at great distances
from the shores of the sea, figs. 3, 4.
Ficr. 3.

These phenomena will be more fully explained hereafter.

32. These circumstances establish a marked distinction between
the igneous and the superincumbent rocks. The latter, con-
sisting of matter distributed in regular and horizontal layers
are called STBATIFIED ROCKS, while the former consisting of
materials agglomerated without any semblance of order are called

UNSTRATIFIED BOCKS.

As the unstratified rocks are called PLUTONIC or IGNEOUS
HOCKS, the stratified are denominated NEPTUNIAN or SEDI-
MENTARY ROCKS, and sometimes ROCKS OF AQUEOUS ORIGIN.

33. The transition or metamorphic rocks, which rest upon the
igneous rocks, show traces of stratification combined with such
partial crystallisation as may be inferred to have arisen from
their contact with the highly heated surface of the rocks below
them. The principal rocks composing the transition-system, are
the gneiss, already described, crystallised limestone, quartz,
hornblend, thick layers of the rock called the old-red sandstone,
and many varieties of slate and shale.

34. Independent of the existence of distinct stratification in
these, they are still more decidedly distinguished from those of
igneous origin by the deposits of animal remains found in them,
which, though neither numerous nor of a high order of organisa-
tion, are nevertheless present in sufficient quantity to put aside
in the most conclusive manner all other suppositions, than that of
sedimentary formation and aqueous origin.

3£. It has been assumed by many geologists, that although

43

THE CPcUST OF THE EARTH.

animal remains have been found in these transition-rocks, no
traces of vegetables were discovered there, from which it was
inferred that the existence of animal life upon the globe preceded
that of vegetation. M. d'Orbigny has shown, however, that
such is not the case. The remains of various marine plants
have been found by Mj;. Hall in the lower Silurian strata in the
State of New York. The coal-mines of Vallongo, in Portugal,
are in the same strata; and the richest coal deposits of Spain
are in the Devonian formation. It must, therefore, be considered
that animal and vegetable life were always co-existent, as indeed
is apparent, a priori, inasmuch as animals which do not feed on
each other must necessarily feed on vegetables.

Organic remains of animals have been found in the superior
layers of transition-rocks, which present singular interest as being
the earliest examples of life traceable in the growth of the earth.
It would seem, that after the external parts of the igneous matter
had been hardened by the process of cooling, the first sedimen-
tary layers deposited upon it became the habitation of certain
races of organised beings.

36. Among these the researches and observations of Professor
Phillips have brought to light various species of small fish, and there
have been found, near Llampeter in North Wales, in the same strata,
traces of a species which the late Mr. William Macleay, a profound

Fig. 5.

naturalist, pronounced to be a sea- worm of the class of Annelidans,
being the first in Cuvier's classification of articulated animals.
44

STRUCTURE OF THE CRUST.

The remains of one of these sea-worms, Nereites Cambrensis,
found at Llampeter, is shown in fig. 5. The body of this
creature consisted of about 120 joints.

. 37. In line, then, we find that upon the igneous rocks as a
foundation the superficial structure of the earth has been erected,
consisting of a series of layers or courses of natural masonry,
one placed above another — the formation of each of which has
been the work of countless ages ; that transition-rocks were the
first and earliest of these, while those which form the surface of
the earth, and are the habitation of the existing organised tribes,
were the last ; and that the epoch at which these latter tribes,
including the human race, were called into existence, remote as it
must appear, compared with all measures of time familiar to us,
is recent when referred to that system of chronology which is
written upon the crust of the globe.

38. Above the transition-rocks, which, as we have stated, were
first placed in the class of primitive rocks, succeed a series of
layers which have been denominated SECONDARY BOCKS. These
consist chiefly of chalk, clay, argillaceous slate, shale, red and
brown sandstone, limestone, iron and lead ore, and coal. They
abound in organic remains, animal and vegetable, in a high state of
preservation, the minutest parts being often perfectly observable.

39. The extent to which the earth was the theatre of organic
life, at the epochs of the deposition of these numerous strata, may
be conceived when it is stated that, in 1834, a German naturalist
and geologist counted no less than 9000 species, the remains of
which, at that date, had been found below the superior limits of
the stratified rocks, not one of which has ever existed since the
earth became the habitation of man.

Among the animal remains which abound in these secondary
strata may be mentioned corals, crinoides, mussels, trilobites,
fishes, reptiles, insects, marine and fresh-water shells, sponges,
and animalcules countless in number.

Of the reptiles, the most remarkable are various species of
lizard-shaped animals, constructed on a scale of colossal mag-
nitude, called Saurians, from the Greek word 2avpos (Sauros),
lizard. These have been variously denominated megalosaurus,
plesiosaurus, ichthyosaurus, and so on.

40. Upon the secondary rocks repose a series of strata of more
recent deposition, called on that account TERTIARY ROCKS. These
consist of a thick bed of clay, limestone, sand, pebbles, and white
sandstone. They abound in organic remains, which are dis-
tinguished from those of the lower and more ancient strata by
including a considerable proportion of the still living species.
Thus the lowest strata of the tertiary rocks contained 5 per cent.?

45

THE CKUST OF THE EARTH.

and the superior strata 10 per cent, of the species found among
the animal tribes which still continue upon the earth.

41. In fine, superposed upon these tertiary are several layers of
earthy matter, upon which the actual organised world is placed.
These are usually resolved into two beds, the lower of which,
denominated DILUVIAL, consists of deposits of gravel and clay,
with boulder-stones, rounded in different degrees by attrition,
giving indication of having been carried from a distance by the
extraordinary action of water, from which the general name
DRIFT has been given to them.

The superior bed consists of sand, clay, and gravel, upon which
the surface soil, which is the theatre of agriculture, rests. This
consists of decayed and decomposed vegetable matter, mixed with
more or less of the disintegrated matter of the inferior beds.
This uppermost layer is produced chiefly by the ordinary action
of water, and is denominated ALLUVIAL.

42. Such are the five principal groups of rocks, into which
geologists have divided the matter which forms the shell of the
globe. The transition, secondary, and tertiary groups, have
each been subdivided into several layers or strata, each of which
is distinguished by the peculiar sorts of mineral matter of which it
is composed, and the peculiar species of organic remains which it
contains. Geologists, however, are not agreed either as to the
limits of the five principal groups, or as to their distribution into
subordinate strata. Thus they are not agreed as to where each of
the principal groups ends and the next begins. The rocks,
which one calls primitive, another denominates transition or
metamorphic. Those which one assigns to the upper part of the
transition-system, another assigns to the lower part of the
secondary system ; and in like manner what one assigns as the
highest strata of the secondary, another gives as the lowest strata
of the tertiary. These discrepancies, however, arise more from
the nature of the things than from any deficiency of our
knowledge of them. Between one group and another there is
no essential distinction, and their classification into primary,
secondary, and tertiary, though convenient, is, like many other
classifications, to a certain extent arbitrary.

43. From what has been stated, respecting the strata constituting
the crust of the earth, the following consequences will follow : —

First. The unstratified and igneous rocks existed prior to the
stratified or sedimentary rocks.

Secondly. The stratified or sedimentary rocks were produced

in the chronological order in which they are found superposed,

the most ancient being the transition-system, and the others

being formed in the order of time in which they are superposed,

46

GEOLOGICAL CHRONOLOGY.

the oldest being the lowest strata of the secondary, and the most
modern the upper strata of the tertiary ; the intermediate strata
have intermediate dates in the order of their superposition.

Thirdly. The sedimentary strata in their original and natural
position were necessarily level; their hounding surfaces were
horizontal and parallel. Wherever, then, they may be found in
any other position, it must be assumed that they have undergone
derangement of position by some disturbing cause since their
original deposition. This derangement may arise either from the
strata being heaved upwards by a pressure from below, or by
their sinking downwards by their incumbent weight forcing them
into some inferior vacuity.

Fourthly. Although the succession of strata constituting the
series, from the primitive rocks upwards to the surface, is by no
means invariable, and is subject on the contrary to many local
variations, still its general character is such as has been described.
If the sedimentary strata, proceeding from the lowest upwards,
when complete, be expressed by the letters A, B, c, D, &c., it will
sometimes happen, from local causes, that the actual series may
be A, B, c, E, &c., or A, B, D, E, &c., or A, D, E, &c., but it can
never happen that the series shall be D, B, A, c, &c. In a word,
one or more strata of the series may be wanting, but their natural
order is never inverted.

44. It appears, therefore, that the character and order of the
sedimentary strata constitute a chronological scale indicative of
the history of their formation. It is true that the value of the
unit of this scale is not and cannot be known, inasmuch as the
absolute intervals of time, necessary for the deposition of the
strata severally, cannot be certainly determined. This, however,
does not prevent the geologist from pronouncing with perfect cer-
tainty upon the order of time of their deposition respectively.

45. Although the series of strata above described have been
deposited, subject to so many local disturbing causes, that there
is probably no point on the entire surface of the globe, where a
section would exhibit them complete, still by a careful and
judicious comparison of observations, made in different localities,
their normal arrangement and natural order of superposition
has be*en ascertained, and geologists have grouped and classified
them under a great variety of denominations. Owing to the
absence of any general convention, no single system of nomen-
clature has been adopted, as has been so happily effected in
chemistry, and though in a less degree, also in zoology. The
consequence is that geology is overlaid with a complicated, con-
fused, and discordant nomenclature, detrimental to the diffusion
and even to the progress and extension of the science. Those

47

THE CKUST OF THE EARTH.

who desire to obtain even the most superficial acquaintance
•with it, are therefore compelled to familiarise themselves with a
mass of most repulsive technicalities, in which the same thing is
called by many different names, according to the varying views,
tastes, and even personal caprices of the geological investigators
who have devoted their labours to the researches in which it finds
a place.

Names have been given to strata or groups of strata in some
cases from the localities in which they are found at the surface,
as for example the Jurassic, the Silurian, the Cambrian, and the
Devonian groups. In other cases, names are derived from the
prevalent materials constituting them, as the cretaceous, oolitic,
and carboniferous groups. Other names have been adopted from
the order of the deposits, as for example eocene, miocene,
pleiocene, and pleistocene, from Greek words signifying the first
dawn, or the earliest, less recent, more recent, and most recent,

Another set of names has been taken from the presence,
absence, or dates of the forms of life exhibited by the organic
remains found in the strata. Thus the strata, which are destitute
of all such remains, are called AZOIC, from a Greek compound
implying the absence of life. The term cainozoic is applied to the
most recent strata, including organic remains, mesozoic to the
middle strata, paloeozoic to the ancient strata, protozoic to the
first in which life appears, and hypozoic to those strata which
lie below the range of all organic remains.

43

Fig. 37.— CAVE OF FINGAL, STAFFA.

THE CRUST OF THE EA.RTH;

OR, FIRST NOTIONS OF GEOLOGY.
CHAPTER II.

46. General section of the terrestrial crust tabulated. — 47. Approximate
thickness of strata. — 48. Probable time necessary for their deposition.
— 49. Recapitulation of the physical history of the globe. — 50. Depo-
sition of organic forms. —51. Alternate elevation and depression of the
crust. — 52. The strata are botanical and zoological museums of past
creations. — 53. The gradual increase of forms of life. — 54. Creative
power has always operated on the same general plan. — 55. But has
varied from period to period in details. — 56. Animals created gradu-
ally ; the more perfect being the more recent. — 57. Tabular view of
the progress of the animalisation of the earth.— 58. Great increase
of vertebrates in the tertiary period — No human fossil — Man charac-
teristic of the present period. — 59. Temporary existence of certain
extinct genera and species. — 60. Geological use of characteristic genera
and species. — 61. Examples — Trilobites characteristic of the Silurian
strata. — 62. Description of them. — 63. Dr. Buckland's reflections on
them. — 64. Species characteristic of the lias — Ichthyosaurus. — 65.
Characteristics of the Wealden— Hylseosaurus — Iguanoclon. — 66. Cha-
racteristics of the chalk. — 67. Ammonites, their distribution between
the Silurian and chalk. — 68. Fossil cephalopodes — Nautilus — Damans.
— 69. Fossil gasteropodes — Bigranulosa murchisonia — Cyprsea elegans
— Yoluta elongata — Pterocera oceani.

46. A GENEEAL idea of the series of stratified rocks, proceeding
LARDNER'S MUSEUM OP SCIENCE. E 49

No. 134.

THE CRUST OF THE EARTH.

downwards from the superficial soil, which is the theatre of agri-
culture, through the alluvial and diluvial deposits upon which
it rests, and thence successively through the tertiary, secondary,
and primary stratified rocks to the igneous ones which form
the foundation of the terrestrial crust, may be formed from the
following tabular statement (page 51), which we have compiled
from the works of different geologists, and principally from that
of Sir H. de la Beche, representing the order of the strata in
"Western Europe, and which in its general character will be found
to correspond sufficiently with the condition of the terrestrial
crust in most parts of the world, especially as regards its major
divisions.

The strata, which in this table are denominated primary or
palaeozoic, are by some geologists included under the general
denomination of secondary rocks. These must not be confounded
with the igneous rocks, which are often called primitive rocks,
and which are not stratified at all. The hypozoic or lowest beds
of stratified roojpis, together with the lower groups of the primary
or palaeozoic, are those which in the preceding paragraphs have
been denominated transition or metamorphic rocks.

47. On contemplating the table, and on considering the
peculiar super- structure which it exhibits, combined with the
fact that each layer being a sedimentary deposit, must have been
the result of an interval of time of considerable duration, two
questions will naturally suggest themselves.

What are the dimensions of these several strata, and what the
total thickness of the whole structure from the granite foundation
on which it rests to the vegetable soil upon the surface, and
what has been the probable interval of time required to pro-
duce each stratum.

Although certain and definite answers cannot be given to
these questions, some degree of approximation may be made to
them. The thickness of the several strata and groups of strata is
subject to considerable local variation, nevertheless the indications
of the limits of these variations in certain parts of the earth,
which have been subject to geological survey of more or less
accuracy may be useful.

Thus, the following are the estimated thicknesses of several
strata, proceeding upwards from the transition-system to the
surface in Great Britain.

Gneiss system — a few miles.

Mica schist system — from a few yards to a few miles.
Cambrian system from one to five miles.
Llandeilo formation J 200 feet.

Caradoc formation . . . 2500 ,,

50

SECONDARY OR MESOZOIC. TERTIARY OR
CAINOZOIC.

Min
Plei
Plei
Mio
Eoc<

|

1

1

eral accumulations of historic period,
stocene,
scene,
sene,
me.

Chalk of Maestricht and Denmark,
Ordinary chalk with and without flints,
Upper green sand,
Gault,
Shanklin sands, Yecten Neocomian or lower green sand,
Wealden clay,
Hastings sands,
! Purbeck series.

&

'•o
^

Portland oolite or limestone,
Portland sands,
Kirnmeridge clay,
Coral rag with its grits,
Oxford clay with Kelloway's rock,
Cornbrash,
Forest marble and Bath oolite,
Fullers' earth, clay, and limestone,
Inferior oolite and its sands,
Lias upper and lower with its intermediate marlstone.

Triassic.

Variegated marbles,
Muschelkalk,
Red sand stone, gres bigarre, bunter sandstein.

PRIMARY OR PALEOZOIC.

1

Zechsteiu, dolomitic and magnesian limestone,
Lower new red, conglomerate and sandstones,
Coal measures.

feroxis
Limestone.

Carboniferous and mountain limestone with its coal, sand-
stone, and shale in some districts,
Carboniferous slates and yellow sandstones.

Devon-
ian.

Modifications of old red sandstone.

Silurian.

Upper Ludlow rocks, Wenlock shale and limestone, Wool-
hope limestone,
Middle Caradoc sandstone and conglomerate,
Lower Landeilo, Bala and Snowdon beds.

II

Barmouth sandstone, Penrhyn slates, Longmynd rocks, and
various rocks below the Silurian.

HYPOZOIC.

ll

g£

Beds of mica schist consisting of quartz and mica with or
without feldspar or garnets, chloritic schist, talc schist,
quartz rock, clay slate, limited beds of iron ore.

6

Beds of gneiss consisting of laminae of quartz, feldspar and
mica ; beds of mica schist, quartz rock, limestone,
hornblende schist.

PRIMITIVE OR

lONEOUS.

Syenite, 1 The relative position and age of these rocks
Porphyry, 1 is more or less uncertain, though it seems
Basalt, [ probable that they may stand in the
Porphyritic granite. J order here assigned to them,
f This system of igneous rocks descends to an un-
Grranite. < defined depth, and is assumed to rest upon
[ the internal liquid nucleus of the globe.

E2

51

THE CllUST OP THE EARTH.

Wenlock formation ..... 1 800 feet
Lucllow formation . ... 2000

Old red sandstone ..... 1000
Carboniferous or mountain limestone i 500 to 2500
Millstone grit . . . . 500 to 700
Coal formation . . . . . 3000

Lower new red Pontefract rock . . 100

Magnesian limestone . . . 300

New red sandstone 1000

Lias ........ 1000

Lower or Bath oolite . . . 400 to goo
Middle or coralliue oolite . . 300 to 800

Upper or Portland oolite . . 200 to 800

Green sand formation . . . .

Chalk formation

Lower tertiary or eocene, about . . . 1200 ,,

Miocene formation.

Pleiocene.

Pleistocene.

Rough as this approximation is, it may give some idea of the
general thickness, at least, of the stratified part of the crust of
the earth. It would appear from combining these results, that
the total thickness of the stratified crust, as far as these
observations go, varies from 10 to 20 miles. Of this thickness
the lower, or palaeozoic strata, constitute the principal part, as
will be seen by the annexed diagram given by Professor Phillips.

Tertiary or Upper Strata.

Secondary or Middle Strata.

Primary or Lower Strata.

52

AGE OF THE GLOBE.

48. To the question as to the lapse of time during which these
successive sedimentary strata have been formed, it is impossible
to give any answer even as definite as the estimates of their
thickness. All that can be said is, that the deposition from
turbid waters being generally a slow process, it may be imagined
that intervals of time of vast duration must have been required
for the formation of strata which measure many miles in
thickness.

But besides the mechanical deposition of matters suspended in
water, we find numerous traces of chemical decomposition, which
could only have been effected in long intervals of time. Thus
deposits of limestone lie in frequent alternation with sandstones
and clays. These indicate a series of changes in the mode of
action, by which the total stratified mass was produced, consisting
of successive cessations and renewals in chemical and mechanical
action.

In short all these effects combined with others, presently to be
mentioned, lead to the conclusion that the period during which
the human race and its contemporary tribes have existed upon
the earth, is but a brief interval compared with the immense
lapse of time occupied by the formation of the igneous rocks, by
the cooling down of the superficial part of the fused matter, and
the subsequent deposition of the stratified crust.

49. It may be useful here briefly to recapitulate the history of
the globe, as we find it inscribed upon its crust.

Originally a mass of fluid matter, in a state of igneous fusion,
it assumed the globular form in virtue of the mutual gravitation
of its parts. Launched by the Creator into space with a motion
of rotation round a certain diameter as an axis, it took the form
of an oblate-elliptical spheroid, flattened at the poles, in virtue of
the centrifugal force attending its rotation. The degree of
spheroidal ellipticity being of course precisely that which cor-
responded to its velocity of rotation.

In this state its extremely exalted temperature would not only
maintain the matter at its surface in a state of fusion, but would
also keep a certain portion of the solid matter in a state of
sublimation, and all the liquid matter in a state of vapour sus-
pended in and mixed with the surrounding atmosphere.

After a continuance of greater or less duration in this state,
the heat of the globe being continually radiated into the sur-
rounding space, the temperature of its surface would be gradually
diminished, and would, at length, fall below the point of fusion
of the matter composing its surface, and consequently the super-
ficial part would be solidified, and the globe would be coated, as
it were, by a thin skin or shell of solid matter, enclosing within

53

THE CRUST OF THE EARTH.

it the matter still remaining in fusion. By the continued effect
of radiation the temperature of the surface would continually
decrease, and consequently the thickness of the solidified shell
would be continually augmented. At length the superficial
temperature would fall to such a point that the sublimated
matter would be precipitated on the surface, and when the super-
ficial temperature, falling still lower, would descend below the
boiling point of water, a general condensation of the vapour sus-
tained in the atmosphere would ensue, and the entire surface of
the globe would be covered with an ocean of uniform depth.

If no disturbing force acted, this would have continued to be
the condition of the globe ; but the fused matter enclosed by the
solid crust being subject to effects more or less irregular, and
exercising unequal pressures, it was in some places protruded
upwards, and in others depressed. In this manner certain parts
of the solid crust were pushed above the level of the water, while
others may have suffered corresponding depressions. Instead,
therefore, of a universal ocean, the surface became diversified by
land and water.

The action of the water upon the subjacent solid crust of the
earth by erosion and disintegration and exposure to atmospheric
action, produced various changes in its condition ; and' the parts
thus washed off being subsequently deposited at the bottom of the
waters, produced the incipient stratification which has been above
described.

"When the temperature of the globe was reduced to such a point
as to be compatible with the existence of organised bodies, the
first forms of life were called by the Creator into existence, and
were such as were adapted to the then physical condition of the
globe, being, as might be expected, exclusively marine tribes.
When subsequently land emerged from the ocean, and by the
condensation and precipitation of vapour rivers and lakes were
formed, terrestrial, fiuviatile, and lacustrine tribes were called
into existence.

50. As each successive stratum was thus formed, the remains
of the animals and vegetables of the epoch were deposited in
them, and have accordingly been preserved to our times. Fiu-
viatile and land animals, in greater or less numbers, were swept
into the embouchures of rivers, and there deposited like the
others. Lacustrine tribes were, in like manner, deposited in the
bottoms of vast lakes or inland seas.

51. But, besides these, there are indications of other changes
either gradual or sudden, which would explain the deposition of
terrestrial organic remains in the strata. There are evidences
that the swellings upwards and subsidence downwards of the

54

ANIMALISATION OP THE EAETH.

crust, by the internal movements of the fluid nucleus of the
globe, caused various changes in the distribution of land and
water, so that parts of the globe which at one time were raised
above the waters, and inhabited by terrestrial tribes, were sub-
sequently submerged ; while other parts, being elevated, emerged
from the waters and formed new continents or islands. Indeed,
changes which are in actual progress, and which will be presently
noticed more fully, show that such phenomena are still produced,
though probably on a much smaller scale, than at the earlier
stages of the growth of the earth when its crust, having less
thickness and strength, offered less resistance to the internal
movements of its fluid nucleus.

52. Since the strata were deposited during a succession of
periods of long duration, each receiving the remains of the
organised tribes which inhabited the earth at the period of its
deposition, it follows that the organic contents of these successive
strata may be regarded as so many museums presenting to us
specimens of the zoology and botany of the globe at the successive
periods of their deposition. By examining, therefore, these re-
mains, we shall be able to compare with each other, and with the
existing tribes, the living inhabitants of the globe at the several
periods of the formation of these strata.

53. The first and most obvious inference suggested by such an
analysis is, that the number and variety of organised beings has
rapidly increased, from the period at which the earth became
habitable to the present epoch. This is rendered evident by a
comparison of the number of species found in a given thickness of
strata, proceeding downwards from the surface, which has been
estimated by Professor Phillips as follows : —

Number of Species in
1000 feet thickness.

Tertiary .... 1410
Cretaceous . . . . 707
Oolitic . . . .456
Triassic 1 g2

Permian j

Carboniferous ... 47
Silurian . . . . 27

These numbers represent the relative proportion of marine spe-
cies, by far the most numerous of the organic remains, more
especially in the lower strata. This calculation was based upon
the results of observations made about 1834, and consequently the
numbers given are considerably below what would be obtained
from more recent observation ; but their proportion, which is all
that concerns us here, would probably not be altered by subse-
quent results.

55

THE CRUST OF THE EAETH.

54. Since, therefore, it appears that the globe, through a long
series of periods, has been tenanted successively by various races
and tribes, both animal and vegetable, it is a question of profound
interest to determine whether creative power, in the production of
these organic beings, has operated upon the same principles which
are manifested in the structure of its actual inhabitants. It might,
for example, be imagined that the forms of life at those distant
epochs, existing probably under extremely different physical con-
ditions, might be totally different from, and utterly incomparable
with, those which now prevail. Or though they might agree in
certain general principles and conditions, they might be expected
to exhibit extreme differences in many important details. A
survey, nevertheless, of the existing tribes, and a comparison of
them with the remains found, in the terrestrial strata, lead to the
conclusion, that though the former inhabitants of the globe
differed from the present in many minute details of their struc-
ture, yet they agreed in all the more essential principles.

Naturalists have resolved the existing animal kingdom into four
primary divisions : the Vertebrates, the Articulated or Annulated,
the Mollusca, and the Zoophytes.

Quadrupeds, birds, and fishes, for example, are Yertebrated
animals ; insects, spiders, and certain shell-fish, such as crabs and
lobsters, present examples of Articulated animals; snails and
oysters are examples of Mollusca ; and star-fish, sea-blubber, and
corals, of the class of Zoophytes.

Xow a due examination of the organic remains deposited in the
terrestrial strata leads to the conclusion, that they admit of pre-
cisely the same general zoological division.

But the analogy between present and past creations is still
closer when those primary divisions are resolved into several
classes. Thus the living vertebrates are divided into mammifers,
birds, reptiles, amphibia, and fishes. In like manner the fossil
vertebrates admit of precisely the same classification. "We find
among them all these classes and no others.

Again, the living articulated animals are resolved into the
subordinate divisions of insects, myriapodes, arachnida, Crustacea,
and worms of various forms. A like subdivision is applicable to
fossil Articulata.

Fossil, like living, Mollusca are resolved into Cephalopodes,
Gasteropodes (snails), Acephala (oysters and mussels), Bryozoaria
(plumatella), and others. In fine, the Zoophytes, fossil as well as
living, are resolved into Echinodermata (sea-urchin, star-fish),
Polyparia (coral), Infusoria (monads), and Spongy aria (sponges),
all of which are found reproduced in the fossil state.

55. But when we descend to more minute distinctions we cease
5S

FOSSIL FORMS.

to find the same close correspondence. Genera or families of the
above-mentioned classes are found among existing species which
are altogether absent from the fossils ; and, on the other handr
numerous genera of fossil animals have no place in the existing
animal kingdom. Of about 1000 fossil genera, somewhere about
500 are identical with those of existing animals, the other 500
being extinct.

The difference between the present and past creations is, as
may be expected, still more remarkable when we come to compare
species with species. Thus of 10000 well- ascertained fossil species
there are not more than 200 or 300 which still survive.

56. Though the counterparts of all the principal divisions of the
animal kingdom are thus found in the fossil state, they are by no
means equally distributed through the strata. Nature, on the
contrary, seems to have called them successively into existence,
according to their increasing perfection of organisation, — the
Mammalia, the most perfect of all, being the most recent in date ;
and the Zoophytes and Mollusca, the lowest in their organisation,
the earliest.

Thus the first forms of life which appeared during the Silurian
period were chiefly confined to the Zoophytes and other classes of
the lowest organisation, the only Vertebrates then existing being
fishes, and those in very limited numbers. The same forms, for
the most part, prevailed upon the globe during the Devonian and
Carboniferous periods. During the Permian and Triassic periods
animated nature received no other increase than that of a few
reptiles. No other classes were added to the creation during the
long interval of the Oolitic period ; but the number of species of
reptiles, as of all the other classes just mentioned, were con-
siderably increased.

The first appearance of birds was manifested during the Creta-
ce'ous period, but they were very limited in number until the
Tertiary period.

It was not till the Tertiary period which immediately preceded
the present epoch that Mammalia were created. Birds, reptiles,
and fishes augmented in number and variety also during this
period, as did various others of the inferior classes, such as Gaste-
ropodes and Acephala.

It must be observed, however, that foot-prints of some Mam-
malia have been discovered in the Oolitic strata, and marks,
supposed to be those of birds, in the Triassic.

57. In the following table, compiled from the very extensive
tables of Paleontology, which accompany the work of Professor
d'Orbigny, we have exhibited the commencement, continuation,
and prevalence of the different classes of animals which inhabited

57

THE CRUST OF THE EARTH.

the earth during the successive periods from the Silurian to the
Human. The relative numbers of each class prevailing at the
different periods are indicated by the four following signs :

V least.

x more numerous.

A ttill more numerous.

O most numerous.

Classes.

Silurian.

Devonian.

Carboniferous.

Permian.

d

I
H

CRETA-
OOLITIC. CEOUS. TERTIARY.

Human Period.

_ 1
1

Middle.

|

1

I

cT £

3 i

£

0

2

Polyparia ....

X

X

Y

X

*.* i '•*

x

...

©

Crinoidea . . . .
Ophiuroidea, Asteroidea, "]
Echinoidea, Echinoder- >
mata . . . . J
Bryozoaria . . . .
Brachiopoda
Acephala . . . .

A

A

0

x

A
A
A
x

Ai x

0 x
A '.'

A A
x A

X

x

A

A
A
x

A

X

X

X

A
A
x

A

0
A

1

0

s-\

A

x

©

x

A

x

x

X

A
x

X

X

X

A
x

X

Marine Gasteropodes .
Fluviatile Acephala . .
Terrestrial and Fluviatile 1
Gasteropodes . . J
Acetabuliferous Cepbalopodes
Tentaculiferous Cephalopodes

©

A

A

X

*."

•

•

'.' A

-

• •

A

X

A Y

'

x

X

x

X
X

D:-©©0©

Reptiles . . . .
Birds ....
Mammalia . . . .

'•'

X

A

'•*

x

•

58. It appears that the creation of fishes, reptiles, birds, and
mammalia, the four vertebrated classes, underwent a sudden and
large augmentation at the epoch of transition from the tertiary to
the human period, and that, to crown all, man appeared for the
first time in the period to which he has given his name. Among
the infinite variety of fossils discovered in the strata of the earth,
there is no instance of human remains.

59. Although the various classes of animals and vegetables,
when once called into existence, have continued to prevail upon
the earth till the present time, the same is not true of the genera
constituting these classes, and still less of the species of these
genera, as already mentioned. Species appear in particular strata,
no trace of which is found in any other, superior or inferior. The
inference is, that such animals existed only during the period cor-
responding to the deposition of these particular strata.

58

CHARACTERISTIC GENERA AND SPECIES.

It happens sometimes that particular species prevail through
•certain groups of strata, superposed in regular order, but totally
disappear from all subsequent and antecedent. Such species
are accordingly characteristic, not indeed of particular strata,
but of those limited groups of strata in which they prevail,
and the inference is, that they had continued to exist upon the
•earth during the period corresponding with the deposition of the
groups, but did not exist there before or after.

60. The species which thus prevailed upon the earth during
geological periods more or less limited, and which ceased to exist
before the period marked by the presence of the human race, have
accordingly supplied to the geologist tests for the identification of
strata much more determinate than any which depend on their
mineral constituents. When the presence of a particular species
is strictly limited to particular strata, it becomes an unerring test
of the presence of that stratum wherever this species is found. If
it has prevailed through groups of strata, it is a test, though not of
particular strata, still of the group to which its presence is limited.

61. Numerous examples of these characteristic species may be
mentioned. A certain, family of Crustacea, called TEILOBITES,
are almost exclusively limited to the Silurian period, appearing
rarely in the lower bed of the carboniferous limestone, and never
above them.

62. The Trilobites consisted of an oblong body, divided trans-
versely into three parts, and also longitudinally into the same

Fig. 0.— (Trilobite) Ogygia Guettardi.

number of lobes. The comparison of the forms of these animals
with those of existing Crustacea, renders it probable that they
dwelt in the depths of the sea, far from coasts, floating on their
back, and never resting, inasmuch as their feet could not retain
them stationary, and movement was necessary for their respira-
tion. From a peculiarity of the mouth, it is inferred that they
were carnivorous, preying probably on naked Mollusca or

59

THE CRUST OF THE EARTH.

Annelida, with which their remains are found associated. But
one of the most interesting facts connected with them is the
structure of their eyes, which resemble those of insects described
in our Tract on " Microscopic Drawing." They consist of a vast
number of minute lenses of octagonal form, set in the ends of
tubes arranged side by si^e, so as to produce a radiating mass of
eyes, enabling the animal to look at the same time in every direc-
tion. As many as four hundred of these lenses have been found
set in a single cornea.

63. Such a structure proves, if proof were wanted, that the
properties of light, and of the transparent media constituting the
atmosphere and water, were, at the remote epochs when the earth
was tenanted by those creatures, what they now are. " With
respect to the waters," says Dr. Buckland, in reference to these
creatures, "we conclude that they must have been pure and
transparent enough to allow the passage of light to organs of
vision, the nature of which is so fully disclosed by the state of
perfection in which they are preserved. With regard to the
atmosphere, also, we infer, that had it differed materially from
its actual condition, it might have so far affected the rays of light
that a corresponding difference from the eyes of existing Crus-
taceans would have been found in the organs on which the
impressions of such rays were then received, Regarding light
itself, also, we learn, from the resemblance of these most ancient
organisations to existing eyes, that the mutual relations of light
to the eye, and of the eye to light, were the same at the time
when Crustaceans, endowed with the faculty of vision, were first
placed at the bottom of the primeval seas, as at the present
moment. Thus we find among the earliest organic remains, an
optical instrument of most curious construction, adapted to
produce vision of a peculiar kind, in the then existing repre-
sentatives of one great class in the articulated division of the
animal kingdom. We do not find this instrument passing onwards,
as it were, through a series of experimental changes, from the
more simple into more complex forms : it was created, at the very
first, in the fulness of perfect adaptation to the use and condition
of the class of creatures to which this kind of eye has ever been,
and is still, appropriate. If we should discover a microscope, or
telescope, in the hand of an Egyptian mummy, or beneath the
ruins of Herculaneum, it would be impossible to deny that a
knowledge of the principle of optics existed in the mind by which
such an instrument had been contrived. The same inference
follows, but with cumulative force, when we see nearly four
hundred microscopic lenses set side by side in the compound eye
of a fossil trilobite ; and the weight of the argument is multiplied
60

FOSSIL ANIMALS.

a thousand-fold, when we look to the infinite variety of adapta-
tions by which similar instruments have been modified, through
endless genera and species, from the long-lost Trilobites of the
transition strata, through the extinct Crustaceans, and the count-
less hosts of living insects. It appears impossible to resist the
conclusions as to unity of design in a common Author, which are
thus attested by such cumulative evidences of Creative Intelligence
and Power ; both as infinitely surpassing the most exalted faculties
of the human mind, as the mechanisms of the natural world,
when magnified by the highest microscopes, are found to transcend
the most perfect productions of human art."

64. In like manner the lias, which is the earliest deposit of the
Oolitic period, is characterised by various organic remains, as well
of reptiles as of mollusca and the lower divisions. Among the
latter may be mentioned a particular species of Ammonites, called
the Ammonites Bucklandi, and among the former the ichthyosaurus,

Fig. 7. — The Ichthyosaurus.

or fish-lizard, is an example of an extinct animal of this tribe, which
has the muzzle and general aspect of a porpoise, the head of a lizard,
the teeth of a crocodile, the vertebra) of a fish, the sternum or
breast-bone of an ornithorhynchus, and the fins of a whale. The
enormous magnitude of the eyeballs was one of the peculiarities
of this genus. The cavities in which they were lodged, in one of
the species, measured not less than fifteen inches in diameter. A
ring of bony plates surrounded the socket, which apparently
seemed to protrude more or less the globe of the eye, and vary the
convexity of the cornea, so as to adapt the organ for near or dis-
tant vision. This, combined with the great power of the fins or
propellers, must have conferred upon the reptile great promptitude
in perceiving and seizing its prey.

These reptiles were essentially aquatic, and the form of their
teeth proves them to have been carnivorous. Their coprolites,
or fossilised, excrements, show that their intestine was spirally
arranged, like that of certain fishes.

65. The Wealden strata, lying near the upper part of the oolitie
and the lower part of the cretaceous, is characterised by remains
of the Monocotyledonous * division of plants, by ferns, by various

* Having only one seed-lobe.

61

THE CRUST OF THE EARTH.

tribes of the lower animals, by insects, fishes of the genus Lapidotus,
and, among the colossal class of reptiles, by the Hylseosaiirus, and
among terrestrial quadrupeds, by the Iguanodon.

66. Among the numerous animals characteristic of the Cretaceous
period may be mentioned the Mososaurus of Hoffman, the Belem-
nites mucronatus, the Terebratula plicatilis.

67. In cases where a group of strata is characterised by the pre-
valence of a particular family of fossils, which first appear at its

Fig. 8. — Ammonites Humpriasiamus.

lowest, and finally disappear at its uppermost layer, the succeeding
strata are often distinguishable one from another, by the preva-
lence in them of a different species of this generic family. Thus

Fig. 11.

Nautilus Dauians.

for example, the Ammonites, so called from their spiral form, re-
sembling the horn sculptured on the head of Jupiter Ammon,
commence in the Silurian and finally disappear after the chalk.
But each stratum, from that in which the family first appears

FOSSIL SHELLS.

to that in which it disappears, is distinguished by the presence of
a peculiar species. Of 222 species of Ammonites 17 belong to the
oldest fossiliferous rocks, 7 to the carboniferous system, 15 to the
new red sandstone, 137 to the oolite, and 47 to the chalk.

68. Among the organic remains characteristic of strata or groups
of strata, the following examples may be mentioned : Fossil Cepha-
lopodes are exceedingly numerous in the paleozoic group ; but of
all the genera hitherto discovered, one only, that of the nautilus,
has come down to the present times.

These fossils appear in great numbers in the lower strata of the
secondary rocks, aie few in the lias and oolite groups, re-appear in
great numbers again in the cretaceous, and nearly disappear from
the tertiary rocks.

The examples are so numerous, and preserved in such per-
fection, that it is difficult to select any in preference to another,
as illustrations of their forms. The nautilus, the only surviving
genus of the Tentaculiferous Cephalopodes in the first periods of
animal life, had nearly the form which it still retains.

Fig. 13. — Cyprtea Elegans.

Fig. 12. — Murelnsonia Bigrauulosa.

69. Fossil Gasteropodes are, like the Cephalopodes, extremely
numerous. The terrestrial and fluvial genera have in general
appeared for the first time in the Tertiary period ; but marine

63

THE CRUST OF THE EAETH.

genera have been found in all the rocks from the Silurian
upwards, and in gradually- increasing numbers. They were,
therefore, among the earliest manifestations of animal life on the
globe; and what is remarkable is, that most of the genera,
including even those of the Silurian period, still survive.

The flose analogy of these ancient forms with the existing
species will be manifest, By some examples taken from among the
countless numbers of fossil shells collected by geologists.

A fossil shell from the Permian group is shown in fig. 12, and
one found in all the tertiary beds, in fig. 13.

One of the genera which first appears in the middle strata of

Fig. 14.— Voluta Egata.

the cretaceous group is shown in fig. 14, and one which begins in
the middle of the oolite, in fig. 15.

Fig. 15.— Pterocera Oceani.

Fig. 66. — BARREN ISLAND IN THE BAY f)F BENGAL.

THE CRUST OF THE EARTH;

OR, FIRST NOTIONS OF GEOLOGY.

CHAPTEE III.

. Spondylus. — 71. Pentamerus. — 72. Reticulipora. — 73. Ceratites. —
74. Enormous masses of animal remains forming entire islands and
continents — Ehrenberg's discoveries. — 75. Dr. Mantell's table of
organic strata. — 76. Forms of life in the Silurian period. — 77. Sir
R. Murchison's observations on the changes of the forms of life from
period to period. — 78. Stratification in undisturbed plains horizontal.
— 79. Strata thrown into oblique positions by disruption of igneous
rocks. — 80. Formation of mountains. — 81. Arrangement of strata
on their flanks. — 82. Strata sometimes upheaved without being
disrupted. — 83. Sometimes disrupted. — 84. Sedimentary strata de-
posited subsequently to disruption — discordant stratification. —
85. How these supply data for determining the epoch of the
disruption. — 86. Determination of the relative ages of mountains —
Cumbrians and Grampians much older than the Alps. — 87. How
inclined strata have enabled geologists to analyse the structure of the
terrestrial crust to the level of the igneous rocks. — 88. Erosion of
stratification by the action of water, and the subsequent deposition
of other strata. — 89. Basalts — their character and composition. —
90. Various forms of basaltic rocks. — 91. Their extensive diffusion
over all parts of the earth. — 92. Their columnar structure — Giant's
Causeway. — 93. Basaltic causeway of the Volant — Dykes and colonnade
of Cheuavari. — 94. Veins of basalt. — 95. Basalts in mounds. — 96.
Basaltic grottoes — Kase grotto of Bertrich-Baden — Fingal's Cave. —
97. Trachytic rocks. — 98. Trachytic mountains. — 99. Their origin
igneous.

LARDNER'S MUSEUM OF SCIENCE. F 65

No. 135.

THE CRUST OF THE EARTH.
70. THE Spondylus, figs. 16, 17, of which there are forty-five

Fig. 16. Fig. ir.

Spondylus Spinosus.

fossil species, first appears in the lowest stratum of the cretaceous
group, and presents an example of the fossil Lamellibranchise.

71. The Pentamerus, figs. 18, 19, of which there are twenty-
one fossil species, is an example of the Brachiopoda. This is

Fig. 19.

Peutamerus Kuightii.

first seen in the lowest strata of the silurian group, and becomes
extinct after the Devonian period, so that its existence was
limited to the earliest epochs of animalisation.

72. The Reticulipora, figs. 20, 21, 22, 23, an example of the
Bryozoares, is an extinct genus Retepora, of which there are five
fossil species known ; the first in the middle strata of the oolites,
and the others in the upper strata of the cretaceous group. In
this the meshes are formed of high vertical laminae, supplied
with cells by transverse lines on each side ; fig. 20 shows the
whole in its natural size; fig. 21, the external part magnified;
fig. 22, the internal part magnified ; fig. 23, the laminse as shown
with a still higher magnifying power.
66

FOSSIL ANIMALS.

73. The Ceratites belong to the family of the mollusca
called Bacculine, the only known species of them found in the
lowest strata of the cretaceous system.

Fig 20.

Fig. 22.

Fig. 23.

Reticulipora obliqua.

74. Among the most wonderful results of the animalisation of
the earth, in the remote geological periods, is the enormous extent
of matter which various species of animals elaborated from the
gaseous or liquid element around them, by vital action, and
which have remained as a perpetual record of their presence.
Whole islands, and even continents, have been produced by the
secretive functions and other vital agencies of countless myriads

F 2 67

THE CRUST OF THE EARTH.

of these living instruments. Ehrenberg, the celebrated Prussian

microscopist and naturalist, mentions a stratum in Germany, not
less than 14 feet in thickness, com-
posed exclusively of the shells of
animalcules, so minute that forty
thousand millions of them would
not fill a space greater than a cubic
inch. Mountains, hundreds and
even thousands of feet in height, are
found to be composed exclusively of
organic matter. The strata of ve-
getable origin are not less exten-
sive, consisting of forests engulfed
by the subsidence of vast tracts of
land, or embedded in the mud of
rivers and estuaries, of lignite
and brown coal in the tertiary de-
posits, of coals and shales in the

carboniferous strata, and of silicified and calcified trunks of

trees in the tertiary and secondary formations.

75. But the strata which consist wholly or principally of

animal remains are so numerous, and of such vast extent, that,

as Dr. Mantell observed, the exclamation of the poet may be

reiterated by the philosopher,

"Where is the dust that has not been alive ?"

for there is not an atom in the superior strata of the crust of the
globe that has not probably passed through the complex and mar-
vellous laboratory of vitality.

The various families of animals from the infusoria and
zoophytes, up to man himself, have then contributed more or less,
by their organic remains, to form the solid crust of the globe.
The following table, taken from the work of Dr. Mantell, pre-
sents a concise view of some of the most obvious examples of
these remarkable deposits.

Fig. 24.— Ceratites Nodosus.

ROCKS COMPOSED WHOLLY OR PARTLY OF ANIMAL REMAINS.

Strata.

Prevailing Organic Remains.

Formations.

Trilobite schist .

Dudley limestone . . •<
Shelley limestone
63

Trilobites

f Silurian
\ system.

n

Corals, crinoidea, crustaceans,
shells, &c. . . . .
Brachiopodous shells .

TABLE OF FOSSIL MINERALS.

EOCKS COMPOSED WHOLLY on PARTLY OF ANIMAL RKMAiNa

Strata.

Prevailing Organic Remains. .

Formations.

. f

Carboni-

Mountain limestone . .

Corals and shells . . . •{

ferous

L

system.

Encrinital marble .

Crinoidea and shells . . .

Mussel-band

Fresh water mussels

Ironstone nodules, . .

Trilobites, insects, and shells .

,

Lias-shales and clays .

Pentacrinites, reptiles, fishes .

Lias.

Limestone . . .

Terebratulae, and other shells

,

Lias conglomerates * .

Fishes, shells, corals . . .

Gryphite limestone

Shells, principally gryphites

>

Shelley limestone

Terebratulse, and other shells . .

f Inferior
I oolite.

Stonesfield slate .

Shells, reptiles, fishes, insects

Oolite.

Pappenheim schist . .

Crustacea, reptiles, fishes, insects

, ,

Bath stone . . {

Shells, corals, crinoidea, reptiles, 1
fishes . . J

?>

Ammonite limestone •<

Shells of cephalopoda, principally 1

I

ammonites j

5 y

Coral rag .

Corals, shells, echini, ammonites

Bradford limestone

Crinoidea, shells, corals, cephalopoda

5>

Portland oolite . . -1

Ammonites, trigoniaa, and other 1
shells , . ' . . . J

5 >

Purbeck and Sussex ~f
marble . . J

Fresh water-shells and Crustacea .

Wealden.

Wealden limestone -j

Cyclades and other fresh-water 1
shells, Crustacea, reptiles, fishes /

»

Tilgate grit (some beds)

Bones of reptiles, fishes, fresh- 1
water shells . . . . j

)»

Farringdon gravel .

Sponges, corals, echini, and shells

Green sand.

Jasper and chert
Green sand . . .

Shells ....

55

Fibrous zoophytes

Chalk . . 1

Polythalamia and other animal- J"
cules \

Cretaceous. ^

f

Corals, shells, ammonites, be- "j

Maastricht limestone <

lemnites, and other cephalo- I
poda, reptiles . . . J

it

Hippurite limestone .

Shells, principally hippurites . .

( ,

Hard chalk (some beds)

Echini and belemnites

? )

f

Sponges and other fibrous zoo- ~j

Flints . . <^

phytes ; infusoria, &c., echini, i-

>i

I

shells, corals, crinoidea . . J

Limestone

Fresh-water shells

Tertiary.

Nummulite rock

Nummulites

9 J

Septaria . . -1

Nautili, turritellte, and other 1
. shells J

»

Calcaire grossier

Shells and corals . . . .

»

Gypseous limestone -4

Bones of mammalia (Palaeotheria, "1
&c.), birds, reptiles, fishes . J

1
5 5

I

THE CRUST OF THE EARTH.

ROCKS COMPOSED WHOLLY OR PARTLY OF ANIMAL REMAINS.

Strata.

Prevailing Organic Remains.

Formations.

Lacustrine marl . . -j

Cyprides, phryganea, fresh-water "1
Shells /

Tertiary.

Monte Bolca limestone

Fishes

„

Bone-breccia . . . | Mammalia, and land shells

„

j Sub- Himalaya sand- ~[
stone . . . J

Bones of elephants, mastodons, 1
reptiles, &c. . . J

„

Tripoli . . .

Infusoria .....

Semiopal . . . j Infusoria

})

Richmond earth . .

Infusoria

•

Guadaloupe limestone -1

Human bones, land shells, and 1
corals . J

f Human
\ epoch.

Bermuda limestone

Corals, shells, serpulse, infusoria

Bermuda chalk

Comminuted corals, shells, &c.

y y

Bog iron ochre . .

Infusoria

Siliceous limestone . . '

Tertiary.

Even in this list, extensive as it is, numerous strata in which
animal remains largely predominate, have been omitted. In the
tertiary and more recent deposits, every order of existing
animated nature is found. The bones of man, however, are
confined to the superficial part, which has been formed since the
globe was peopled by the races which now inhabit it.

76. How completely changed the inhabitants of the earth have
been from one geological period to another, may be inferred from
the following observations of Sir R. Murchison. " Beginning,"
says he, "with the vertebrata, are not the fishes of the old red
sandstone as distinct from those of the carboniferous system, on
the one hand, as from those of the Silurian on the other ? M.
Agassiz has pronounced that they are so. Are any of the crus-
taceans, so numerous and well defined throughout the Silurian
rocks, found also in the carboniferous strata ? I venture to reply,
not one. Are not the remarkable Cephalopodus mollusca, the
Phragmoceras, and certain forms of Lituites, peculiar to the older
Silurian system ? Is there one species of the Crinoidea figured,
known in the carboniferous strata ? Has the Serpuloides longis-
simus, or have those singular bodies the Graptolites, or, in
short, any zoophytes of the Silurian system been detected in
the well-examined carboniferous rocks? And in regard to
the corals, which are so abundant, that they absolutely form
large reefs, is not Mr. Lonsdale, who has assiduously com-
pared multitudes of specimens from both systems, of opinion,
that there is not more than one species common to the two
epochs ? "
70

FOSSILS, THEIR VAST NUMBER.

77. These anticipations of Sir R. Murchison have been more
than realised by the subsequent researches of M. D'Orbigny,
founded upon his own observations, which extended over a large
portion of the New as well as of the Old world, and upon the entire
mass of facts connected with the analysis of the crust of the earth
collected by the observations of the most eminent geologists in all
parts of the world. It appears from these researches, that during
the long series of periods of geological time from the first appear-
ance of organised life upon the globe, to the period in which the
human race and the contemporaneous tribes were called into
existence, the world was peopled by a series of animal and vege-
table kingdoms, which were successively destroyed by violent
convulsions of the crust, which produced as many devastating
deluges. The remains of each of these ancient creations are de-
posited in a series of layers or strata one over the other ; and from
an examination of them it has been found that each successive
animal kingdom was composed of its own peculiar species which
did not appear in any posterior or succeeding creation, but that
genera once created were frequently revived in succeeding periods ;
that many of these genera, however, became extinct long before
the arrival of the human period ; that during the human period no
new genus was called into existence, except that of the human
race, which, however, according to the idea of the most eminent
naturalists, ought to be regarded as an order rather than a genus.

Each of the succeeding animal kingdoms which thus temporarily
inhabited the earth consisted of many hundred species. Thus it
has been ascertained byM. D'Orbigny, that there are deposited in
the Cambrian or Lower Silurian strata not less than 418 species of
the animals which inhabited the globe in the iirst period of its
animalisation, and that these included specimens of all the princi-
pal divisions of animals from the Vertebrata downwards.

It will be sufficient, however, for the present, thus briefly to
indicate these important discoveries, which we shall develop much
more fully in the next volume of these series.

78. As already explained, the strata, when originally deposited,
must in all cases have had a horizontal

position, and they must succeed each Fig- 25>

other in their normal order, whenever the fe^fr JL?,

part of the earth at which they lie has

undergone no disturbance subsequently

to the date of their deposition, which has

sometimes been the case with extensive

plain countries. In such cases, there-

fore, a section of the crust would exhibit them in parallel and

horizontal layers, as in fig. 25.

71

THE CRUST OF THE EARTH.

79. In undulating and mountainous countries it is found,
however, that, instead of being horizontal, they are variously

Fig. 26.

inclined (fig. 26), and sometimes bent into, or even beyond, the

vertical direction (fig. 27).

"When it is considered that at the time of their deposition the

strata must have
been horizontal, it
•will be evident that
the position shown
in these figures could
only have been given
to them by a force
acting from below,
by which they were
heaved upwards,

and by which the crust was broken, the igneous rocks having

forced themselves through it.

80. In such cases hills or mountains of greater or less elevation
are formed, at the summits of which the igneous rocks which
have been protruded through the stratified crust are at the
surface, and the edges of the strata thus broken are ranged along
the flanks, the lowest or most ancient being nearest the summit,
and the others succeeding each, other in their proper order in
descending towards the adjacent valley or plain.

81. It will be evident that the stratum whose edge is highest
on the mountain is that which lies the lowest on the plain, and
that whose edge is lowest on the mountain is that which is
highest or nearest the surface on the plain.

82. In the cases here exhibited the igneous rocks have split the
stratified crust and broken quite through it. This, however, is
not always the case. It often happens that the uplifting force
loses its energy before the superincumbent strata are cracked, in
which case they would cover the elevation preserving the
order of their superposition, but would be, as before, inclined
in accordance with the declivity of the hill produced by the
uplifting force.

72

DISRUPTION OF STRATA.

83. In other cases the superior strata, but not the inferior, are
broken through, as shown in fig. 28. The edges of the disrupted

Fig. 28.

strata are then exposed upon the flanks of the elevation, and are
ranged as above described ; while the inferior strata, not disrupted
retain their natural order upon the summit.
In other cases (fig. 29) the disruption is complete, and the

Fig. 29.

broken strata are ranged on each of the opposite declivities in the
same order as already described.

84. It happens often that after the cessation of the disturbing
force, by which the strata have been uplifted, the land having
been again submerged, new depositions take place, the strata of
which are of course horizontal and superposed upon those ren-
dered oblique by the previous disturbance. This sort of super-
position of strata is called by geologists discordant or unconform-
able stratification, and wherever it occurs it affords evidence of
the action of a disturbing force from below, the geological date of
which can be determined with more or less precision by a due
examination and comparison of the superposed strata.

Cases of this kind of discordant stratification are shown in fig.
28 and fig. 29, in both of which horizontal strata deposited upon
the oblique strata, are disposed along the slopes of the elevation.

85. It is evident that the epoch of the action of the disturbing
force must, in all cases, have been posterior to that of the deposition
of the inclined, that is, of the disturbed strata. It is equally
apparent that the disturbing action must have ceased before the

73

THE CRUST OF THE EARTH.

deposition of the lowest of the superincumbent horizontal strata.
The date of the disruption is therefore fixed geologically at some
period between those of the deposition of the two strata just
mentioned. Now whenever it so happens that the lowest, and,
therefore, the first deposited of the horizontal strata, stands the
next in order above th$ highest of the inclined strata in the
geological scale, the date of the disruption is geologically fixed,
being at the epoch between the deposition of two strata which
follow each other in immediate succession.

But if, as often happens, the lowest of the horizontal strata
hold a place in the geological scale separated from the highest of
the inclined strata by several intermediate layers, which are
locally absent, then the date of the catastrophe becomes more
uncertain, inasmuch as it may have taken place at any epoch
between those of the depositions of the highest of the inclined and
the lowest of the horizontal strata.

86. Nothing is more beautiful or conclusive than the reasoning
by which the geological dates of mountain-ranges have been
determined by these means. One of the most interesting con-
sequences resulting from the observation of such discordant
stratifications as are here described, is the means they afford
geologists of determining the relative ages of different ranges of
mountains. Thus, for example, it is easily demonstrated that the
mountains of Cumberland, Lammermuir, and the Grampians were
raised above the surface of the ocean long before the Alps had
burst the crust of the earth, and before even the continent of
Europe was dry land. An examination of the slopes cf these
British ranges shows that the strata dislocated and inclined are
those of the old slate and limestone, while the level strata super-
posed upon them in the adjacent plain are the more recent ones of
the red sandstone. It follows that these ranges were raised above
the waters posterior to the epoch of the deposition of the old slate
and limestone, but antecedently to that of the red sandstone ; and
since the red sandstone has been deposited horizontally along their
base, it follows that the land surrounding them was covered with
water subsequently to their elevation.

An examination of the Alps gives a very different result. On
the flanks of these mountains the tertiary strata are found
inclined, sloping downwards, until they become level upon the
general surface of Europe. It follows, therefore, that the date of
the disruption to which the Alps owe their elevation was poste-
rior to the deposition of the tertiary strata upon the European
continent, while the elevation of the British ranges above men-
tioned was anterior to the deposition of the red sandstone ; from
which it follows that the Grampian, Lammermuir, and Cumbrian
74

NATURAL SECTION OF THE CRUST.

mountains were dry land, while the greater part of the continent
of Europe was covered by the ancient ocean, and long before the
Alps were reared above its surface.

In a subsequent tract this subject of the relative ages of the
mountain system, will be more fully developed.

87. From what has been here explained, and from inspection
of figs. 26 to 29, the manner in which geologists have been
enabled to analyse the crust of the earth to depths far exceeding-
any which could be reached by direct excavation, mining, or
boring, will be easily understood. Nature herself, by these pro-
digious disruptions, has exposed to view the structure of the
sedimentary strata in all cases where no subsequent deposition
has taken place over them. By such disruptions it often happens
that the strata are inclined over a large extent of country, the
surface of which, intersecting their planes at a very oblique
angle, is necessarily formed by the section of the strata in the
order of their superposition, the breadth of the severa1 sections
being so much the greater, the more oblique the angle formed by
the horizontal plane with the planes of the strata.

In fig. 30 the strata are represented inclined very slightly from

Fig. 30.

the perpendicular, and consequently the breadth of each stratum
at the surface is very little greater than its actual thickness ; but
it will be easily understood that if the obliquity of the strata to a
level plain were greater, a very thin stratum would present at the
surface a considerable breadth. Supposing then the complete
series of strata in any tract of country to be inclined at a very
oblique angle to the level plain, it will be easily understood that
in travelling in a direction at right angles to the lines of intersec-
tion of the strata with the surface, the succession of strata may
be examined, and a much greater extent of their thickness sub-
mitted to observation, than could be accomplished by any artificial
sections of horizontal strata.

88. Among the indications afforded by discordant stratifications
of past convulsions to which the land has been subject, are some

75

THE CKUST OF THE EARTH.

which show that, by the violent agitation of the waters consequent
on sudden changes of level of their bed, and of the parts of con-
tinents over which they have swept, strata already formed have
been partially broken up and carried away by currents. Excava-
tions, such as that represented in fig. 32, are explained in this
manner.

Fig. 32.

In such cases it has often happened that the waters under
which such broken strata were submerged, having again become
tranquil, a new series of strata have been deposited horizontally,
filling up the excavation thus formed, as shown in fig. 33.

Fig. 33.

89. Closely allied with the matter ejected from volcanoes are the
Basaltic deposits, which form so grand a feature in the scenery of
many countries with which travellers are familiar.

All the circumstances and characters which attend these rocks
conspire to show that they have issued from openings in the crust
of the earth in a state of fusion much more complete than that of
volcanic lava, and in the process of cooling have, in many cases,
been crystallised, so as to assume those remarkable varieties of the
columnar form, so conspicuously developed in the north of Ireland,
the Scottish Islands, and many other parts of the world.

The Basaltic rocks are characterised by a dark colour and a
compact base of the mineral called Labradorite, including black
pyroxene, generally the magnetic oxide of iron, frequently
peridote, and sometimes crystallised feldspar, to which they owe
their porphyritic structure.
76

BASALTIC ROCKS.

. 90. The fluid basalt often assumes the form of prismatic columns
in the process of crystallisation, consequent upon slow cooling.
Mr. Gregory Watt imitated this artificially by reducing 7 cwt. of
Dudley basalt to fusion and causing it to cool slowly, when
globular masses were formed, which gradually enlarged and
pressed one against the other, until they forced themselves into
regular columns, resembling in all respects those of natural
basalt.

, In some places basalt forms vast plateaux of considerable thick-
ness, in others it is found in detached sheets of less extent, at
points of mountains, more or less distant one from the other, and
at the same level, as if it had originally been a single sheet and
had been disrupted, by the convulsions of which the mountains
have been the result.

In some cases the basalts form isolated masses or mounds rising
in the midst of plains, altogether removed from all similar forma-
tions. They are also often found in veins in the strata of the
earth, like those of minerals. They sometimes also present them-
selves as extensive walls, or in a series of separate mounds having
a common direction. When basaltic rocks are presented in the
form of sheets or mounds, the upper part is generally composed of
porous cellular scoriform matter, irregularly divided, and termi-
nated below by a plane surface, sensibly horizontal. When the
mass is composed of several layers, these layers are separated one
from another generally by thin beds of rapilli.*

91. Basaltic deposits are much more extensively scattered over
the surface of the globe than those of ordinary volcanic origin.
Unlike volcanic products, they are not confined to particular cen-
tres of action, but appear to have been produced wherever the
terrestrial crust, yielding to the pressure from below, was rent
so as to give issue to the fused matter. In the British Isles ba-
saltic products are found in various places, and more particularly
in the north of Ireland and Scotland. In France they are found
everywhere from the northern part of Auvergne to Montpellier,
and even to Toulon. On the borders of the Rhine they extend
from the Ardennes to Cassel, and are continued eastward into
Saxony, Bohemia, and the surrounding countries. They prevail
to a great extent in Iceland, are recognised in the West India
Islands and St. Helena, in the island of Ascension, and in almost
all the islands of the southern ocean.

92. The tendency of these rocks to form themselves into pris-
matic columns has more especially excited the attention of the
curious. In some cases all the prisms converge to the summit of

* Volcanic dust.

77

THE CRUST OF THE EARTH.

a mound, which thus assumes a sheaf-like structure. In others
they take the form of close columns with the most picturesque
aspect. In others again, these columns, cut off at a certain level, form
a sort of mosaic pavement, to which the name of causeway has
been given. Of this one of the most magnificent examples is pre-
sented in the case of ttye Giant's Causeway, in the north of
Ireland.

93. Examples of similar formations are presented in different
parts of Europe, and especially in the Vivarais, in the department
of Arddche, in France. A remarkable series of basaltic causeways is
there presented on the banks of the river Volant, between Yals and
Entraignes, a view of which is given in fig. 31, p. 33. The colon-
nades of Chenavari, near R,ochemaure, fig. 34, and the dykes which
are near the same place, fig. 35, present examples of other varieties
of basaltic forms.

94. Basaltic rocks, having all the prismatic characteristics above
described, are frequently presented in the form of mineral veins.
Examples of this are found in the central parts of France, and also
on the borders of the Rhine. Most commonly the matter composing
the vein is compact or divided by irregular cleavage, but it also
sometimes exhibits the prismatic form, the axis of the prisms being
horizontal, fig. 36.

95. When basalts take the form of a mound, the lower part
of the mass often presents a multitude of appendages which

Fig. 34. — Colonnade of Chenavari, Fig. 35. — Dykes of Chenavari.

near Rochemaure.

penetrate like roots into the subjacent earth ; showing that the
matter in a liquid state had flowed into the crevices, and
moulded itself there. The earth thus in contact with the
basaltic mass is often found calcined to a considerable depth,
and the vegetable remains which it includes are carbonised.
Examples of this are presented upon the cliffs of the plateau of
78

BASALTIC GROTTOES AND CAVES.

Mirrabel, in the Yivarais, descending towards St. Jean 'le JSoir,
fiar. 38.

Fig. 30.

Ficr. 3.9.

96. Grottoes, caves, and tunnels are often found in the midst of
these basaltic masses, and in those of trap rocks, which have a
close analogy to them. Examples of this may be seen in the Viva-
rais, on the borders of the Rhine, near Bertrich-Baden, between
Treves and Coblentz, where the columns forming the grotto con-
sist of rounded blocks superposed, resembling a pile of cheeses,
from whence the grotto has received the name of Kase Grotte, or
Cheese Grotto, fig. 39. But by far the most magnificent of these
basaltic grottoes is the celebrated Cave of Fingal, in the Island of
Staffa, fig. 37, p. 49.

Fig. 39.— The Kase Grotto of Bertrich-Baden.

97. Another eruptive product of the terrestrial crust, still more
extensive than the basalt, is the trachytic rocks, which form the
celebrated Puy-de-d6me, in Auvergne, the Mont D'or, the Cantal,

79

THE CRUST OF THE EARTH.

the Mezenc, and the Megal, upon the borders of the Velay and the
Vivarais. They prevail also on the right bank of the Rhine, and
the Siebengebirge. They form immense groups in Hungary and
Transylvania, in the Caucasus, in Greece, where their continuation
appears in the islands of Milo and Argentiera and extends to the
centre of Santorin. Th^y reappear in the Lipari Islands, in the
Campania, in the Euganean mountains, in the Azores, in the
Canaries, in South America, where the loftiest heights of the Cor-
dilleras are composed of tjiem, in Central Asia, and in many of the
islands extending along its coast to Kamschatka.

98. The trachytic formation presents itself not only in isolated
mounds, narrow bands, and sheets scattered over the surface of the
globe like those of the basalt, but also in vast mountains, gene-
rally assembled in large groups, forming the most elevated masses
and covered with terrific asperities. Their flanks are torn by
precipitous valleys and deep gorges.

99. The matter composing these vast accumulations was evi-
dently ejected from the bowels of the earth by the disruption of
the crust, issuing in a state of pasty fusion through the opening
thus made in the super jacent stratified rocks, fig. 40.

Fig. 40.

80

Fig. 52.

THE CRUST OF THE EARTH :

OR, FIRST NOTIONS OF GEOLOGY.

CHAPTEE IV.

100. Elie De Beaumont's explanation of the formation of mountain chains
—101. Effects of the earthquake of 1838 in South America. — 102.
Inference as to the probable effects of the vast earthquakes which
produced the great mountain ranges. — 103. Dislocations in paral-
lel directions produced parallel chains. — 104. Origin of mineral
veins explained. — 105. Veins are found in groups — generally parallel.
— 106. Deposition of rock-salt in cavities of Muschelkalk. — 107.
Natural agencies still manifested are sufficient to explain all geological
phenomena. — 108. Internal fluidity of the earth — 109. Effects of in-
ternal heat on»the surface. — Why the climates of the higher latitudes
at former tpochs were similar to those of the tropics at present— ex-
planation of the presence of tropical fossils in polar latitudes. — 110.
The undulations and disruptions assumed by geologists as physical
causes still proceed, though with less energy. — 111. Effects of earth-
quakes—that of Calabria, 1783.— 112. Effects in Sicily.— 113. Earth-
quakes at Chili. — 114. Earthquake of 1819 in India. — 115. Like
phenomena recorded in all ages and countries. — 116. Similar phenomena
traditional — island of Atlantis. — 117. Permanency of the sea-level
proves the undulations of the land. — 11 8. Undulations of the Swedish
peninsula. — 119. Similar changes in Greenland, and in the Indian
x archipelago. — 120. General sinking of South America. — 121. Singular

LAUDNER'S MUSEUM OF SCIENCE. a 81

No. 137.

THE CRUST OF THE EARTH.

example of a submerged forest on the western coast of America. — 122.
Temple of Jupiter Serapis. — 123. Historical researches of Professor
Forbes. — 124. Tradition of a submerged city under Lough Neagh. —

125. Why these undulations of the earth's crust might be expected. —

126. Effects of disruption of the crust. — 127. Volcanic eruptions of
1808 in the Azores.— 128. The Monte Nuovo.

100. THE formation of cracks and fissures in the crust of the
globe has been ingeniously explained by M. Elie de Beaumont, as
the natural and necessary consequence of the process of superficial
cooling taking place upon a globular mass of matter in a state of
fusion.

To render intelligible the reasoning and theory of this eminent
geologist and naturalist, let us suppose the globe, as we have for-
merly described it, to have been originally in a state of igneous-
fusion, and to undergo a gradual superficial cooling, by which a thin
solid shell would be formed upon it. The contraction of this shell,
taking place from its inner towards its outer surface, would leave
a vacant space between the central mass in fusion and the concave
interior surface of the solid shell. Supposing also, as we have
formerly explained, that after tho external surface had fallen below
the temperature which maintains water in a state of vapour, the
atmospheric vapours, being condensed, had fallen in rain ; the ex-
ternal surface of the terrestrial spheroid would then have been
covered with an ocean of uniform depth and would consequently
have been totally destitute of land.

An imaginary section of a part of the terrestrial crust in thi&
state is represented in fig. 41, where b is the solid crust, a the
liquid central mass, d the intermediate vacant space, and c the
ocean of uniform depth covering the entire surface.

Fig. 41.

« • a

But the state of equilibrium which would maintain this state
of things could not continue. The internal fluid matter would
press more or less upon the thin crust surrounding it, which being
unequal in its structure, would offer proportionately unequal re-
sistance, and consequently yielding at its weakest points would
be dislocated as shown in fig. 42, the fragments being thrown
into infinitely various positions. Thus the piece f being tilted
obliquely, would be raised at one end above the surface of the
ocean, depressed at the other. It would, therefore, form a chain of
82

FORMATION OF LAND AND WATER.

mountains with a gentle declivity on one side, and abrupt precipices
on the other, such as the Pyrenees and the Andes. In other places
the two parts fractured would form gentle declivities on both sides,

as at h. The matter in fusion within the crust forcing its passage
upwards in the opening between them would be solidified by the
process of cooling on arriving above them. Thus chains of moun-
tains would be formed of moderate declivities on both sides, having
igneous rocks at their summits.

In fine, some fragments, such as e, would remain nearly level, and
would be pushed above the surface, in which case they would form
extensive plains of dry land, or might remain below it, in which
case they would form the bottom of a shallow sea.

Thus we may understand in its most literal sense the brief
description of the formation of the earth in the sacred records :
"God divided the land from the water, and saw that it was
good."

Such dislocations of the terrestrial crust would not be confined
to a single catastrophe, but would from time to time be reproduced.
According as, by the continued process of cooling, the solid crust
of the earth became thicker and thicker, a vacant space would
still be produced between its inner surface and the central fluid
matter, and like consequences would from time to time ensue, so
,that the history of the earth would consist of a series of convul-
sions by which its crust would from age to age be disrupted, new
chains of mountains being formed, and new continents being raised
above the waters of the ocean and former ones submerged. Move-
ments of the waters would necessarily attend each such convul-
sion, compared with which the most violent oceanic commotion
with which we are familiar is tranquillity itself.

101. It is related that the earthquake of 1838, which took place
at Chili, in South America, although it did not change the level
of the continent by more than a few feet, produced effects at the
enormous distance of 4000 miles, extending even to the islands of
Oceania. The earthquake on the coast of Peru laid in ruins all
the towns along the shore. At the moment of the shock, the
waters of the ocean, raised with violence, were poured upon the
coast, carrying with them an immense mass of sand and shingles,
o2 83

THE CRUST OF THE EARTH.

and transporting vessels of the largest tonnage to a distance of
four miles inland.

102. When such effects arise from a change of the surface not
exceeding a few feet in elevation, it may be imagined what pro-
digious deluges must have been produced when the Alps and
Pyrenees were raised to £heir present altitude from the ordinary
level of the earth's surface, or when the chain of the Andes was
elevated by a dislocation, which mustjiave extended over nearly
3000 miles of the earth's surface.

It cannot then be doubted that the consequence of such con-
vulsions would be universal ; and some idea may be formed of the
extraordinary ravages which the frightful deluges consequent
upon them would occasion upon the surface of the earth, especially
at the moment when all levels of land and sea were changed in
consequence of the dislocation which caused them, and when a
considerable mass of sediment still in a movable state was trans-
ported by the torrents of the ocean. It will not be considered
extraordinary, that all the terrestrial animals should be at once
destroyed by the immediate action of the waters, while the marine
animals would suffer equal destruction by the violent transport
of the terrestrial matter swept among them.

103. M. Elie de Beaumont has shown that these movements of
terrestrial dislocation have never been partial, but that each of
them has been produced along lines, having one uniform direction,
as may be seen in the case of the Pyrenees, of certain ranges of
the Alps, and upon a still greater scale in the case of the Andes
and the Himalayas. "We shall show more fully hereafter that the
parallel lines of mountains have been raised at the same epoch,
and that the succession of convulsions by which the ranges of
mountains having different directions were produced, can be de-
termined, and their geological dates assigned with more or less
precision.

104. The circumstances which have been explained, attending
the past history of the earth, have also produced cracks and
fissures in its crust, through which the central liquid matter has
been forced, and in which it has been solidified, forming veins of
mineral matter different altogether from the strata which they
intersect. These veins often contain earthy matter, such as
carbonate of lime, sulphate of baryta, and quartz, in which case
they offer but little interest. They are, however, more frequently
filled, either wholly or partially, with metalliferous substances, in
which case they acquire great importance. These metalliferous
veins are generally found either in the igneous or in the most
ancient of the stratified rocks.

105. It is rare that a single vein is met with. Most commonly
84

MINERAL VEINS.

Fig. 43.

d/

Fig. 44.

great numbers prevail in the same system of strata, and generally
take a direction nearly parallel. Fig. 43 shows a transverse

section of such a sys-
tem. The similarity
of the mineral matter
which fills them de-
monstrates their com-
mon origin. It often
happens that one sys-
tem of such veins is
intersected by another,
presenting mineral
contents totally differ-
ent from the former.
These are called cross
veins. It is rare that
a vein is completely

filled with metalliferous matter. Most commonly, these sub-
stances form threads, «, 6, c, d, e, /, g, fig, 44, more or less
irregular, included in the middle of the stony crystalline matter
which fills the vein. The thickness of
the metalliferous thread varies at dif-
ferent parts of the vein ; at some points
it is considerable, and at others be-
comes very small, often vanishing
altogether.

106. Numerous cavities are often
formed in the midst of stratified rocks,
probably by the dissolving action of the
subterraneous waters. Such cavities,
which are met with in all the sedi-
mentary strata, are generally filled after
their formation with new substances
totally different from the surrounding
rock. It is thus that masses of rock-
salt are found in the Muschel-kalk
and in the Marnes Irisees, fig. 45.

Similar masses of carbonate of zinc,
as formed in the upper part of a stra-
tum of transition limestone, are shown
at c c, in fig. 30.

107. Since the memorable revolution
effected by Bacon in the conduct of phy-
sical inquiries, a maxim has been uni-
versally admitted and adopted, in virtue of which, in the formation

85

THE CRUST OF THE EARTH.

of hypotheses for the explanation of natural phenomena, no cause
can be admitted, except such as can he shown to have real
existence, and which, being admitted, shall appear to be sufficient
to produce the phenomena which it is put forward to explain. In
accordance with this rule, geologists are therefore required to show

•

Fig 45.

that the elevated internal temperature of the globe, those
upheavings and disruptions of land, those sedimentary deposits
from water, those ejections of fluid and pasty matter from clefts
and caverns, those abrasions of the solid crust by the corroding
action of water, and its modifications by the action of the atmos-
phere, are severally real agencies still in visible operation, and
producing effects similar in kind though different in degree
from those ascribed to them in geology, and also that even the
difference in degree, which must be admitted to be often very
considerable, admits of satisfactory explanation.

Before, therefore, proceeding further in the exposition of the
phenomena disclosed by the labours of geologists, we shall here
pause for the purpose of showing the reality of the various phy-
sical causes to which geologists have ascribed the phenomena.
We shall see that like phenomena have been, and still are,
developed on the surface of the earth ; and that the reasons why
these agencies were more energetic at remote epochs than at
present are sufficiently obvious.

108. It has been already very fully explained in our Tract on
4 'Terrestrial Heat," that in descending deeper and deeper through
the crust of the globe, the temperature continually rises ; so that
at a depth which forms a very insignificant fraction of the semi-
diameter of the globe, the materials which constitute it must
have a temperature altogether incompatible with their continuance
in the solid state, — a temperature, in short, which is above the
point of fusion of the most refractory of these materials.

109. The superficial heat of the earth may be considered,
therefore, to result from the combined effects of solar and internal
heat. In the present condition of the globe, the effects of the

86

EFFECTS OF EARTHQUAKES.

former are incomparably greater than of the latter ; but it may be
imagined that at earlier epochs, when the solid crust of the earth
was much less thick, the internal heat produced upon the sur-
face a much more powerful effect; so that the climates of all
parts of the globe must have been much more elevated than at
present. Numerous effects compatible with this reasoning have
been discovered by the researches of geologists. Thus, the
organic remains of animals and plants found in the sedimentary
strata deposited at earlier epochs in the growth of the globe, are
such only as could have lived in climates of a much more elevated
temperature, than those which now characterise the latitudes in
which they are found. Thus, the fauna and the flora (terms
adopted by naturalists to express the entire collection of animals
and plants existing in any place) prevailing in high latitudes at
remote epochs, were such as could 'exist at present only within
the tropics.

110. The alternate upheavings, depressions, disruptions, and
dislocations of the crust of the earth assumed by geologists in
their explanation of the phenomena are still exhibited, though

on a much smaller scale, in the phenomena attending earth-
quakes. These effects have been so fully explained in our Tract
on " Earthquakes and Volcanoes,'* that little need be added to
what has been there stated. As these phenomena are mani-
fested at present, they are most frequently more or less local,
though sometimes their effects are spread over little less than
an entire hemisphere. The earthquake which took place in the
island of Ischia, on the 2nd of February, 1828, was not felt
in the slightest degree either in the neighbouring isles or upon
the Continent ; while that of New Grenada, which took place on
the 17th of June, 1826, exercised its influence over many thou-
sand square miles. The earthquake which commenced in Lisbon
in 1755, and which has been fully described in a former Tract,
-extended in one direction to Lapland, and in the other to Mar-

87

THE CKTJST OF THE EARTH.

Unique. It was sensible, also, at right angles to this direction,
from Greenland to Africa, where the cities of Morocco, Fez, and
Mequinez were destroyed by it. Its effects were manifested in all
parts of Europe at the same moment.

Ill, These convulsions not only destroy entire cities and over-
turn the most solid edifices, but they are attended with important
modifications in the level* of the ground. Those of Calabria, in
1783, supply remarkable examples of them, and are so much the
more worthy of attention, as they were circumstantially described
by several eminent men who were witnesses of the phenomenon,
such as Vicencia, physician of the King of Naples, Grimaldi, Sir
W. Hamilton, and by a commission of the Royal Academy of Naples.
The whole extent of the country was convulsed, the beds of the
rivers were changed, houses were, some raised above the general
level, and others at short distances from them sunk below it ;
edifices of the greatest solidity were cracked in their walls, while
certain parts of them were raised above others, and their founda-
tions in many instances forced above the earth ; crevices were
formed in the ground, some of which measured five or six hundred
feet in width ; some were straight, some bifurcated ; sometimes a
single enormous cleft having a certain direction was intersected at
right angles by a number of others, fig. 46 ; others were developed
in clefts radiating from a centre, like a broken pane of glass, fig. 47.
Some crevices laid open at the moment of a shock, and into which
entire buildings had fallen, closed almost immediately again,
crushing between their sides all that they had thus engulfed.
In some cases the sides of the cleft were separated at the surface,
but brought into contact with each other at a certain depth, figs.
48-9. In other cases, the parts disrupted merely sunk below the
other without ceasing to be in contact, fig. 50.

Fig. 40.
Fig. 48.

In other cases, the force which burst the superior crust was
sufficient, not merely to split it into different clefts, but to pro-
duce in it a vast cavity, from the edges of which clefts diverged,
fig. 51.

In some cases a considerable tract of ground was suddenly
engulfed, carrying with it the plantations and buildings which
88

EFFECTS OF EARTHQUAKES.

chanced to be upon it, and leaving nothing but a yawn-
ing gulf, with sides nearly
vertical, from 300 to 400 feet
deep. In some instances an
immense quantity of water
was collected in the cavities,
and formed lakes more or less
considerable in magnitude,
some of which had no ap-
parent issue, while the water
from others iiowed. in enor-
mous torrents. In other cases
the contrary effects were mani-
fested, rivers and lakes sud-
denly disappearing as if they had sunk into the bowels of the earth.

112. While the principal of these earthquakes upon the Italian
peninsula was limited to the tract between Oppida and Soriano,
the phenomena were propagated under the straits of Messina to
that city, more than half of which, with twenty-nine surrounding
towns and villages, was swallowed up. The bottom of the sea
sunk, the shores were torn by clefts, and all the ground along the
harbour of Messina was inclined towards the sea, sinking sud-
denly to the depth of some feet. The entire promontory by which
the entrance to the harbour was formed, was swallowed up in an
instant.

113. The earthquakes which took place on the coast of Chili in
1822 — 35 — 37, produced effects not less remarkable. Different
paits of the coast from Yaldivia to Valparaiso, an extent of more
than 200 leagues, was manifestly raised above the water, as well
as several adjacent islands extending to that of Juan Fernandez.
The bottom of the sea, to a considerable distance, was similarly
affected. Upon the coasts, rocks formerly covered with water,
were raised eight or ten feet above the level of the sea, covered
Avith the shells attached to them. Rivers, which emptied them-
selves at different parts of the coast, and which were navigable
to vessels of small tonnage, became fordable. At sea, well known
anchorages were diminished in depth ; and various parts, where
vessels formerly passed safely, were now complete shoals, inacces-
sible to vessels except those of the lightest draught.

114. Similar effects were manifested in India in 1819. A hill,
60 miles long, and 18 wide, extending from S.E. to N.W., was
raised in the middle of a flat country, barring the course of the
Indus. Further south, and in a parallel direction, the ground was
sunk and with it the town and fort of Sindre, which remained, how-
ever, standing, though half submerged. The eastern embouchure

89

THE CRUST OF THE EARTH.

of the river became deeper at several places, and different parts of
its bed, formerly fordable, ceased to be so.

115. In the records of all ages and countries, effects of the
;?ame kind are recorded. Large crevices in the earth have
been formed, deep gulfs opened, into which cities, and even
whole countries, have sometimes been swallowed up. From these
openings, mephitic vapouts, water in enormous quantities, some-
limes cold, sometimes warm, have been ejected, and, occasionally,
even flame has issued. Plains have been suddenly transformed
into mountains, shoals produced in the deepest seas, mountains
cracked and overturned, and mountainous tracts, many hundred
miles in extent, suddenly levelled or replaced by deep lakes ;
rivers, turned from their beds, have discharged their waters into
cavities thus formed ; lakes, breaking through their banks, have
been emptied, and their bottoms left dry, or have been turned
through subterraneous openings suddenly formed beneath them.
•On the contrary, in some cases numerous springs, natural artesian
wells, have been formed, supplying waters which suddenly issued
i'rom crevices of the rock or from tunnels. Thermal springs have
been suddenly rendered cold, or altogether dried up, while others,
on the contrary, have been produced where none existed.

All these, and many other phenomena, indicate the existence of
internal convulsions, by which the matter subjacent to the crust of
the earth is driven upwards through its crevices.

116. Independently of the phenomena of this class which are
authentically recorded in history, many others are subjects of
tradition. Thus, Pliny relates a tradition that Sicily had been
separated from Italy, Cyprus from Syria, and Euboea from Boeotia,
by earthquakes. According to another classical tradition, a great
island called Atlantis existed in ancient times west of the Straits
of Gibraltar, having a numerous population. Its princes invaded
Africa and Europe, but were defeated by the Athenians and their
allies. Its inhabitants afterwards became wicked and impious,
and the island was visited with the vengeance of the Gods, and
swallowed up by the ocean in a single day and night. This
legend is given by Plato, and is said to have been related to
Solon by the Egyptian priests. According to all the analogies
supplied by the phenomena described above, there is nothing
impossible, or even improbable, in that part of this legend which
refers to an island being engulfed by the ocean.

117. The cases in which the dry land has been invaded by the
sea, or the bed of the sea left dry by the retirement of the waters,
has been popularly, and even by the scientific of former days,
ascribed to the change in the level of the waters of the ocean,
their elevation producing inundations, and their fall leaving

90

SLOW UNDULATIONS OF THE CRUST.

tracts, formerly covered, dry. Considering the solid and appa-
rently permanent character of the land on the one hand, and the
extreme mobility of water on the other, such a conclusion was
natural, and almost inevitable, until clear evidence of the con-
trary was obtained. It has, however, been proved that the very
reverse has been the case of such phenomena, the mobility apper-
taining to the land, and the permanence to the sea. It has been
shown, by observations made upon the level of the ocean, that it
has not suffered any general change within historic times ; but,
on the contrary, that the cases in which the land has been inun-
dated by the ocean must be ascribed to the sinking of the land,
and those in which the waters have deserted their bed to the
rising of the bottom of the sea.

118. These changes in the level of the crust of the earth have
been in some cases sudden, as when they are produced by earth-
quakes, but in others they have been so gradual that they could
only be ascertained by observation extending over long intervals
of time. It had been long observed in different parts of Sweden,
that the level of the surrounding ocean was subject to an apparent
but slow and gradual change, in some places rising, in others
falling. The Academy of Upsal, in 1731, commenced a series of
observations with a view of determining the fact whether this
apparent change of the ocean were real, or whether it were not
caused by an actual change in level of the land. Marks were
accordingly cut upon the faces of rocks at the level of the sea, and
at the end of some years these marks were several inches above
that level, from whence it was in the first instance inferred that
the Baltic had suffered a depression of its level, so as to leave
more or less of the bottom dry. These observations, however,
being continued and multiplied, it was soon rendered apparent
that, while the level of the Baltic remained unchanged, the
phenomena were produced by actual changes in the level of the
land. It was found that the apparent depression of the level of
the ocean was different in different places, and that in some
places, on the contrary, it appeared to have been even raised.
Thus, while at certain points the apparent depression amounted to
several inches, in others it did not exceed a fraction of an inch ;
at some places, such as the coast near Christianstad, the level of
the sea appeared to be elevated. The conclusion deduced from
all these observations was, that the apparent change of level of the
sea was produced by a slow and gradual upheaving of the land in
some places, and a sinking in others ; that in Finland, and in a
great part of Sweden, the surface was gradually raised without
any perceptible shock, while in the southern parts of the peninsula
a corresponding depression was produced.

91

THE CRUST OF THE EARTH.

119. Similar swellings and depressions of the land have been
manifested elsewhere. Thus, for four centuries, the western
coast of Greenland has been continually sinking through an
extent of 600 miles north and south. Ancient buildings, as well
upon the low islands of the coast as upon the mainland, have been
gradually submerged, and the removal to considerable distances
inland of various establfshments which formerly existed upon the
coast, has been rendered necessary. Similar sinkings of the surface
have been manifested in certain islands of the Southern Ocean,
especially in the Indian Archipelago.

120. It would even appear, from a comparison of the observa-
tions of Messrs. Boussingault and Humboldt, separated by an
interval of thirty years, that the whole continent of South
America is gradually sinking, and, if this process be continued,
at some distant epoch it may even be submerged. The observa-
tions referred to show that the altitudes of the Andes at the
epoch of the observations of Boussingault were less than those
given by the anterior observations of Humboldt; and these
results are confirmed by the fact that the snow-line in this
range of mountains has, in the interval referred to, apparently
risen.

121. An interesting modern example of the subsidence of a con-
siderable tract of country, clothed with forests, the trees remain-
ing erect, although submerged beneath a river which still flows
over them, is described by a late American writer, and will
serve to illustrate these remarks. The whole district, from
the Rocky Mountains on the east, and the Pacific Ocean on the
west, and from Queen Charlotte's Island on the north, to Cali-
fornia on the south, presents one vast tract of volcanic formation.
Basalt — both columnar, and in amorphous masses, veins, and
dykes — everywhere occurs, and craters of extinct volcanoes are
still visible. Elevations and dislocations of the strata have taken
place on an immense scale ; and successive beds of basalt, amyg-
daloidal trap, and breccia, prove the alternation of igneous action
and periods of repose. Within a few miles of the falls of the
river Columbia, and extending upwards of twenty miles, trees are
seen standing in their natural position, in a depth of water from
20 to 30 feet. The trees reach to high or fresh- water mark, which
is 15 feet above the lowest level of the tide ; but they do not
project beyond the freshet rise, above which their tops are decayed
and gone. In many places the trees are so numerous, that "we
had to pick our way with the canoe, as through a forest. The
water of the river was so clear, that the position of the trees could
be distinctly seen, down to their spreading roots, and they are
standing as in their natural state, before the country had become

92

TEMPLE OF JUPITER SERAPIS.

submerged. Their undisturbed position proves that the subsidence
must have taken place in a tranquil manner." *

122. Another most interesting and remarkable example of the
alternate elevation and depression of the surface of the earth,
manifested within historic times, is presented in the case of the
ground upon which the temple of Jupiter Serapis stands, at
Pozzuoli, near Naples. These ruins, standing on the northern
shore of the Bay of Baise, at a short distance from the Solfa-
tara, consist of the remains of a large building of quadran-
gular form, 70 feet in diameter, the roof of which was originally
supported by 46 columns, 24 of which were granite, and 22
marble, each column consisting of a single stone. Many of
these pillars are broken, and their fragments strewn about
the pavement, but three of them still remain standing (fig.
52). Their base is near the level of the sea. The surface of
the columns, the tallest of which is 42 feet in height, is smooth up
to an elevation of 12 feet from the pedestal, where a band of
perforations 9 feet wide commences, made by a species of mussel,
called the Modiola Uthophaga, which could only have lived in
sea- water. Above this band, at the height of 21 feet from the
pedestal, these cavities are discontinued, and the remainder of the
pillars are smooth, like the lower part. The cavities, many of
which still contain shells, sand, and microscopic shields, are of
elongated elliptical shapes, and so numerous and deep as to prove
that the pillars must have been submerged in sea-water to a
height more or less above the limit of these borings, for a long
period of time. The lower part of the columns, which are not
similarly affected, must have been protected, while they were
submerged, from the depredations of these boring mussels by
surrounding accumulations of rubbish and tufa, while the upper
parts projected above the water, and were consequently beyond
the reach of these animals.

The platform of the temple is now about one foot below high-
water mark, and the sea, which is only 40 yards distant, penetrates
the intervening soil. The upper part of the band of perforation is
.at least 23 feet above the level of the sea.

It appears from all this, that the ground on which the temple
.stands must have changed its level more than once, being alter-
nately heaved upwards and downwards. It is clear that when
Ihe temple was built, the ground of its foundation must not only
liave been high and dry, but remote from the shores of the bay,
and at some subsequent period must have sunk gradually, and so

* Journal of an Exploring Tour beyond the Rocky Mountains, by the
Tlev. Samuel Parker, A.M. New York, 1838.

93

THE CRUST OF THE EARTH.

tranquilly as not to overturn the columns, to a depth above the
band of perforation, and at a still more recent period must have
again been raised to its present level.

123. It results from the researches of Professor Forbes upon
this subject, that historical evidence is extant illustrating the
respective dates of these changes of level. From inscriptions
recording the embellislftnent of the temple by Septimius Severus
(A.D. 193—211), and Marcus Aurelius (A.D. 161—180), it appears
that the building was still entire, and occupied its present posi-
tion at the close of the second and commencement of the third
century; that in A.D. 1198, the eruption of the volcanic lake of
Solfatara took place, attended with earthquakes, and a general
subsidence of the coast ensued, which caused the temple to sink
to a depth which submerged its columns in water to a height
above the band of perforations. In this state it appears to have
continued until the commencement of the sixteenth ^century; for
the flat district called La Starza, in which the building stands, is
described by contemporary observers as being covered by the sea
in 1530. Eight years later, frequent and violent earthquakes
prevailed along all that part of the Neapolitan coast ; and on the
29th of September in that year, the volcanic eruptions burst forth
which threw up the Monte Nuovo already described. During
this catastrophe the coast on the north of the Bay of Baiai was
permanently raised 20 feet, forming a tract 600 feet in breadth,
and including the area in which —

" Those lonely columns stand sublime,

Flinging their shadows from ou high,
Like dials, which the wizard Time

Had raised to count his ages by !" — MOORE.

These were accordingly left above the water, several of the
columns still retaining their original position. They seem to
have been wholly neglected by antiquaries till 1750, when the
shrubs and weeds with which they were overgrown and concealed
were removed, and the earth accumulated in the court of the
temple cleared away. For the last thirty or forty years a slow
subsidence of the coast appears to have been going on, and the
floor of the temple is often submerged.*

124. We are not informed whether the Irish tradition, repro-
duced by Moore so beautifully in the following lines, has been.
verified by any scientific observers ; but, if so, it would seem as

* See a paper by Professor Forbes on the subject in Brewster's "Journal
of Science," vol. i. second series ; and also a letter quoted by Dr. Mantell,
and addressed to him by M. Hullmandel, "Wonders of Geology," voL i.
p. 458.
94

VOLCANIC PHENOMENA.

though Lough Keagh were the result of a post- Adamite sinking of
the ground.

" On Lough Neagh's banks as the fisherman strays,

When the clear cold eve's declining,

He sees the round towers of other days

In the wave beneath him shining !

Tli us shall memory, often, in dreams sublime,
Catch a glimpse of the days that are over ;

Thus, sighing, look through the waves of time
For the long faded glories they cover !" — MOOKE.

125. It appears, then, that the crust of the earth, instead of
being endowed with that character of stability and immobility
popularly ascribed to it, is subject to incessant as well as occa-
sional upheavings and depressions. It may indeed be regarded aa
in a certain degree elastic, yielding to the undulations, whether
slow and gradual, or sudden and more violent, of the agencies
within it.

Such phenomena, however, will cease to astonish when we
reflect what an enormous disproportion exists between the thick-
ness of the solid crust of the globe, and the mass of matter in a state
of igneous fusion which it encloses ; the crust being relatively"
thinner than a piece of card-board attached to the rind of an
orange, it cannot be matter of surprise that it should be subject to
more or less derangement of form, and even occasional disruption,
by the action of the fluid matter within it. That such changes
and such disruptions and their consequences should be much more
considerable at earlier than at more recent epochs, is also a
natural consequence of the growth of the crust of the globe by
the process of cooling. The earlier the epoch the thinner that
crust must have been, and the less its resistance to internal force.
Forces which would now fail to produce any sensible effect upon
its form, would at those earlier epochs have been sufficient to
disrupt it. That such effects have been actually produced at
various geological epochs is proved by the most incontrovertible
evidence presented by the crust of the globe itself, as will pre-
sently be more fully explained. Volcanic phenomena are closely-
connected with those of earthquakes, and, like them, supply
analogies by which various geological phenomena are explained.

126. "When the crust of the earth is disrupted in the manner
explained above, openings are made in it which supply communi-
cation between its internal fluid nucleus and its external surface,
and through these openings matter of various forms is often ejected
with vast force. The matter thus ejected consists sometimes of
the disrupted and broken parts of the crust itself, which are
projected upwards, vertically or obliquely as the case may be,

95

THE CRUST OF THE EARTH.

and often scattered over the surrounding country to vast distances.
Sometimes the matter thus thrown up is in a state of pasty fusion,
and is incandescent, scoriaceous, and pumiceous. In this semi--
fluid state it is projected sometimes to a distance, and sometimes
it flows in streams along the slopes of declivities, or collects in a
sheet or layer of more or^ less thickness round the crater from
which it is ejected.

127. These volcanic phenomena have been already, in part,
explained in our Tract on Earthquakes and Volcanoes, but their
connection with the condition and history of the crust of the
earth is so close, and the aids they afford for the explanation of
geological phenomena so important, that it will be necessary here
further to enlarge upon them.

In the month of May, 1808, the ground in the island of San
Jorge, one of the Azores, in the midst of an open plain and culti-
vated fields, was suddenly upheaved, after which it cracked at
several places with a terrific noise. A vast cavity or crater was
formed in the middle of it, having an area of nearly thirty acres,
and surrounded within the distance of three miles by from twelVe
to fifteen smaller craters. An enormous quantity of scoriaceous
and pumiceous matter was projected from it which covered the
surrounding ground to the depth of five feet for an extent of
twelve miles in length by three in breadth. Streams of molten
matter issued from it, which continued to flow for more than three
weeks from the principal crater to the sea.

128. The Monte Nuovo, which was formed upon the Neapolitan
coast in the Bay of Baise in 1538, presents an example of like
phenomena. A violent earthquake had prevailed for two years,
which, on the 27th and 28th of September, 1538, suddenly
increased so as to be attended with incessant movement of the"
ground day and night. The plain extending between the Lake of
Averno (the ancient Avernus), the Monte Barbaro and the sea
was then suddenly upheaved, and various crevices were formed*
in it ; the ground, rising still more, assumed the form of a moun-
tain. During the succeeding night, the summit of this mountain
opened with prodigious noise, and vomited great masses of flame,
accompanied by pumice stones and ashes. The eruption con-
tinued for seven days, the matter ejected filling up the Lucrine
Lake. Since this eruption the most perfect tranquillity has
continued at this place.

96

Fig. 71. — THE GEYSERS, ICELAND.

THE CRUST OF THE EARTH ;

OR, FIRST NOTIONS OF GEOLOGY.

CHAPTEE Y.

129. Volcanoes of the Andes — Pichiucha, Cotopaxi, and Tunguragua. —

130. Mexican volcanoes — Orizaba, Popocatapetl, Jorullo, and Colima.

131. Singular circumstances attending the elevation of Jorullo. — 132.
Ancient state of Vesuvius, according to Strabo — Eruption of 76 A.D. —
Destruction of Herculaneum and Pompeii. — 133. Volcano of Kirauea
in Owhyhee — Mr. Ellis's account of it. — 134. Visit of Mr. Stewart
and Lord Byroti to it in 1825 — Mr. Stewart's account of it. — 135.
Illustrations of the ejection of lava through the strata. — 136. Effect
of obstruction — formation of lateral craters. — 137. Submarine eruptions
— Formation of volcanic islands. — 138. Volcanic islands off Santorin,
in the Grecian Archipelago. — 139. Phenomena attending the formation
of such islands. — 140. Craters of elevation. — 141. Stratification around
these craters — Island of Palma. — 142. Formation of islands not
necessarily preceded by eruptions. — 143. Mount Etna. — 144. His-
torical accounts of its eruptions. — 145. Eruption of 1832. — 146. The

LARDNER'S MUSEUM OF SCIENCE.
No. 139.

97

THE CRUST OF THE EARTH.

Val del Bove. — 147. Section and plan of Etna.— 148. Volcanic lakes.
— 149. Crescent form of volcanic islands. — 150. Craters of elevation,,
temporary and permanent. — 151. Barren island in the Bay of Bengal.
— 152. Traditional volcanic origin of Santorin.

129. THE famous volcano of Jorullo in the Anahuac mountains
in Mexico had its origin in similar phenomena. The elevated
plateau which forms the province of Quito, in South America, has
been the theatre of extraordinary volcanic phenomena. Beneath
it is a focus of volcanic energy, the channels of which communi-
cate with the atmosphere by the craters of the great volcanoes of
Pichincha, Cotopaxi, and Tunguragua, part of the chain of the
Andes. These, by their groupings as well as by their lofty eleva-
tion and grand outlines, present the most sublime and picturesque
aspect which is anywhere concentrated within so small a space
in a volcanic landscape. The extremities of the chain are con-
nected by subterranean communications; and this fact, which
experience has made known to us in numerous instances, reminds
us of the old and just statement of Seneca, that the crater is only
the issue of more deeply-seated volcanic forces.

130. The Mexican volcanoes of Orizaba, Popocatapetl, Jorullo,
and Colima also appear to be connected with each other, being
placed in the direction of a line running transverse to the former,
and passing east and west from sea to sea.

131. As was first observed by Humboldt, these mountains are
all situated between north latitude 18° 59' and 19° 12'. In an

Fig. 53.— Volcano of Jorullo, Mexico.

exact line of direction with the other volcanoes, and over the
same transverse fissure, Jorullo was suddenly elevated on the
93

VOLCANO OF JOKULLO.

29th of September, 1759. The circumstances attending the
production of this volcano are so remarkable, that we shall here
notice them in some detail.*

An extensive plain, called the Malpays, was covered by rich
fields of cotton, sugar-cane, and indigo, irrigated by streams, and
bounded by basaltic mountains, the nearest active volcano being
at the distance of eighty miles. This district, situated at an
elevation of about 2600 feet above the level of the sea, was
celebrated for its beauty and extreme fertility. In June, 1759,
alarming subterranean sounds were heard, and these were accom-
panied by frequent earthquakes, which were succeeded by others
for several weeks, to the great consternation of the neighbouring
inhabitants. In September tranquillity appeared to be re-
established, when, in the night of the 28th, the subterranean
noise was again heard, and part of the plain of Malpays, from
three to four miles in diameter, rose up like a mass of viscid fluid,
in the shape of a bladder or dome, to a height of nearly 1700 feet ;
flames issued forth, fragments of red-hot stones were thrown to
prodigious heights, and, through a thick cloud of ashes, illumined
by volcanic fire, the softened surface of the earth was seen to
swell up like an agitated sea. A huge cone, above 500 feet high,
with five smaller conical mounds, suddenly appeared, and thousands
of lesser cones (called by the natives hornitos, or ovens,) issued
forth from the upraised plain. These consisted of clay inter-
mingled with decomposed basalt, each cone being a fumarolle, or
gaseous vent, from which issued thick vapour. The central cone
of Jorullo is still burning, and on one side has thrown up an
immense quantity of scoriaceous and basaltic lavas, containing
fragments of primitive rocks. Two streams, of the temperature of
186° of Fahrenheit, have since burst through the argillaceous vault
of the hornitos, and now flow into the neighbouring plains. For
many years after the first eruption, the plains of Jorullo were
uninhabitable from the intense heat that prevailed.!

132. It appears that the cone from which Vesuvius takes its
present character has been the result of similar effects.

In the description of the mountain given by Strabo, no mention
whatever is made of the cone which now forms its most remark-
able feature. The slopes of the mountain, says Strabo, were
regions of the greatest fertility; its summit was truncated,
entirely sterile, and had a burnt aspect, displaying cavities full
of crevices and calcined stones, from which it must be conjectured
that they had been formerly volcanic craters. It seems, there-
fore, that the cone to which the name of Vesuvius now more

* Cosmos, vol. i. p. 229. Trans,
t Mantell, p. 837.

H 2 99

THE CRUST OF THE EARTH.

properly belongs, and of which all the products differ altogether
from the rocks of the semicircle called the Somma which existed
in ancient times, was not formed until a much more recent period,
and probably dates from the famous eruption of the year 76 A.D.,
which was signalised by the loss of Pliny. It was then, probably,

Fig. 54.— Vesuvius as now formed. Fig. 55.— Vesuvius in the time of Strabo.

that a permanent communication was opened between the crater
and the internal parts of the earth. This catastrophe appears to
have produced but little lava, although it was attended with
violent effects on the surrounding country, throwing a great part
of the mountain into the sea, and burying Herculaneum and
Pompeii, not, as is vulgarly supposed, under molten matter
ejected from the crater, but under avalanches of pumiceous
substance, which existed previously upon the slopes of the
mountain.

133. Of all existing volcanoes, that of Kirauea, in Hawaii,
one of the Sandwich Islands, better known under the popular
name of Owhyhee, and noted as the theatre of the murder of
Captain Cook, exhibits volcanic phenomena under their most
sublime and imposing aspect. The island of Hawaii, which is
about seventy miles long, and covers an area of 4000 square miles,
is a complete mass of volcanic matter, perforated by innumerable
craters. It is, in fact, a hollow cone, rising to an altitude of
16000 feet, having numerous vents over a vast incandescent mass,
which doubtless extends beneath the bed of the ocean, the island
forming a pyramidal funnel from the fluid nucleus beneath to the
atmosphere. The following graphic account of a visit to the
crater, by Mr. Ellis, affords a striking picture of the splendid,
but awful, spectacle which this volcano presents.

" After travelling over extensive plains and climbing rugged
steeps, all bearing testimony of igneous origin, the crater of
Kirauea suddenly burst upon our view. We found ourselves
upon the edge of a steep precipice, with a vast plain before us
fifteen or sixteen miles in circumference, and sunk from two to
four hundred feet below its original level. The surface of this
plain was uneven, and strewed over with large stones and volcanic
rocks ; and in the centre of it was the great crater, at the distance
of a mile and a half from the precipice on which we were standing,
We proceeded to the northern end of the ridge, where, the sides
being less steep, a descent to the plain below seemed practicable ;
100

VOLCANO OF KIRAUEA.

but it required the greatest caution, as the stones and fragments
of rocks frequently gave way under our feet, and rolled down
from above. The steep which we had descended was formed of
volcanic matter, consisting apparently of light red and grey vesi-
cular lava, lying in horizontal beds, varying in thickness from one
to forty feet. In a few places the different masses were rent in
perpendicular and oblique directions, from top to bottom, either by
earthquakes, or by other violent convulsions of the ground. After
walking some distance over the plain, which in several places
sounded hollow beneath our feet, we came to the edge of the great
crater. Before us yawned an immense gulf in the form of a
crescent, about two miles in length from the north-east to south-
west, one mile in width, and 800 feet deep. The bottom was covered
with lava, and the south-west and northern parts were one vast
flood of burning matter. Fifty-one conical islands of varied form
and size, containing as many craters, rose either round the edge
or from the surface of the burning lake. Twenty- two constantly
emitted either columns of grey smoke or pyramids of brilliant
flame, and at the same time vomited from their ignited mouths
streams of lava, which rolled in blazing torrents down their black
indented sides into the boiling mass below. The existence of
these conical craters led us to conclude that the boiling cauldron
of lava did not form the focus of the volcano, but that this liquid
mass was comparatively shallow, and the basin which contained
it separated by a stratum of solid matter from the great volcanic
abyss, which constantly poured out its melted contents through
these numerous craters into this upper reservoir. We were
further inclined to this opinion from the vast columns of vapour
continually ascending from the chasms in the vicinity of the
sulphur banks and pools of water, for they must have been pro-
duced by other fire than that which caused the ebullition in the
lava at the bottom of the great crater ; and also by noticing a
number of small vents in vigorous action high up the sides of the
great gulf, and apparently quite detached from it. The streams
of lava which they emitted rolled down into the lake, and mingled
with the melted mass, which, though thrown up by different
apertures, had perhaps been originally fused in one vast furnace.
The sides of the gulf before us, although composed of different
beds of ancient lava, were perpendicular for about 400 feet, and
rose from a wide horizontal ledge of solid black lava, of irregular
width but extending completely round. Beneath this ledge the
sides sloped gradually towards the burning lake, which was, as
nearly as we could judge, three or four hundred feet lower. It
was evident that the large crater had been recently filled with
liquid lava up to this ledge, and had, by some subterranean

101

THE CRUST OF THE EARTH.

channel, emptied itself into the sea, or upon the low land on the
shore; and in all probability this evacuation had caused the
inundation of the Kapapala coast, which took place, as we after-
wards learned, about three weeks prior to our visit. The grey, and
in some places apparently calcined sides of the great crater before
us — the fissures which intersected the surface of the plain on
which we were standing — the long banks of sulphur on the
opposite sides of the abyss — the vigorous action of the numerous
small craters on its borders — the dense columns of vapour and
smoke that rose out of it, at the north and south ends of the
plain, together with the ridge of steep rocks by which it was
surrounded, rising 300 or 400 feet in perpendicular height —
presented an immense volcanic panorama, the effect of which was
greatly augmented by the constant roaring of the vast furnaces
below." *

134. This volcano was also visited in 1825 by Mr. Stewart,
accompanied by Lord Byron and a party from the " Blonde"
frigate, who descended to the bottom of the crater. Mr. Stewart
has left the following description of it : — " The general aspect of
the crater," observes he, " may be compared to that which the
Otsego Lake would present, if the ice with which it is covered in
winter were suddenly broken up by a heavy storm, and as suddenly
frozen again, while large slabs and blocks were still toppling, and
dashing and heaping against each other, with the motion of the
waves. At midnight the volcano suddenly began roaring, and
labouring with redoubled activity, and the confusion of noises
was prodigiously great. The sounds were not fixed or confined to
one place, but rolled from one end of the crater to the other ;
sometimes seeming to be immediately under us, when a sensible
tremor of the ground on which we lay took place ; and then again
rushing on to the farthest end with incalculable velocity. Almost
at the same instant a dense column of heavy black smoke was
seen rising from the crater directly in front, the subterranean
struggle ceased, and immediately after flames burst from a large
cone, near which we had been in the morning, and which then
appeared to have been long inactive. Red-hot stones, cinders,
and ashes, were also propelled to a great height with immense
violence; and shortly after, the molten lava came boiling up,
and flowed down the sides of the cone and over the surrounding
scoriae, in most beautiful curved streams, glittering with a
brilliancy quite indescribable. At the same time, a whole lake of
fire opened in a more distant part. This could not have been less
than two miles in circumference, and its aspect was more horribly

* Ellis's Polynesian Researches, vol. iv.
]02

SUBMAK1NE VOLCANOES.

sublime than anything I ever imagined to exist, even in the ideal
visions of unearthly things. Its surface had all the agitation of
the ocean ; billow after billow tossed its monstrous bosom into the
air, and occasionally those from different directions burst with
such violence, as in the concussion to dash the fiery spray forty or
fifty feet high. It was at once the most splendid and tearful of
spectacles."

135. The manner in which the liquefied matter is driven
upwards through the superjacent crust of the earth, and ejected
from the crater is illustrated in fig. 56, where F F represents

the fluid nucleus of the globe, and F c the passage along which
the fluid matter has burst its way through the successive strata
to the crater from which it issues.

136. It sometimes happens that the passage, through which
it is thus forced in the first instance becomes obstructed, so

103

THE CRUST OF THE EARTH.

that the escape of the lava through it is intercepted. In
such cases the pressure of the fluid matter acting against the
walls of the cleft, or channel, breaks through them at one or
more points of least resistance, and new issues are formed on
another side of the original crater. These lateral craters may
also be produced, even though the central crater is still in activity
(%• 57).

137. Volcanic eruptions are not confined to the land, but are
often produced in the bed of the ocean. In such cases islands
often rise suddenly out of the bottom of the deep. Thus, in
1831, the island of Julia arose in the Mediterranean, about 30
miles to the south-west of Sicily. Bogoslaw appeared in like
manner, in 1814, in the Aleutinian Archipelago ; Sabrina and
another among the Azores, in 1811 ; besides various others around
Iceland, in the Indian Archipelago, the Philippines, the Moluccas,
and off the coast of Kamtschatka. One of the most remarkable
examples of these was presented in the case of the island which
rose above the waters, in 1796, at 30 miles from the northern
point of Unalaska, one of the Aleutinian islands. A column of
smoke was first seen to rise out of the sea. This was followed by

the appearance of a black point at the
surface of the water, from the summit
of which blades of flame and incandes-
cent matter were launched with violence.
These phenomena continued for several
months, during which the island in-
creased greatly in magnitude and height.
Afterwards, smoke alone issued from
it, which, after continuing four years,
ceased. The island nevertheless, still
continued to increase in magnitude and height, without manifest-
ing volcanic phenomena. In 1806, it had so augmented that
it formed a cone, which could be perceived from Unalaska, upon
which were formed four other smaller cones in the north-west side.

138. Some remarkable examples of submarine eruptions have
been presented from the most remote times, in the Mediterranean
and the Levant. According to ancient historians, the bay included
between the islands of Santorin and Theresia (fig. 58), in the
Grecian Archipelago, has been the special theatre of these
phenomena. The island of Santorin is a half-moon-shaped tract
of land, evidently of volcanic origin, and the islands of Theresia
and Aspronisi, extending between the horns of the crescent,
enclose the bay. Within this enclosure, the island of Hiera rose
above the water in 186 B.C., and round it, and close to its coast
several islets appeared in 19 A.D., 726 A.D., and 1427 A.D. In

104

SANTORIN ISLAND.

1573 A.D., appeared the large island called Micra Kamini, and in
1707, the still larger one, Nea Kamini. The latter underwent a
gradual increase of magnitude during several succeeding years.

Fig. 58.

No volcanic crater was formed upon these islands, which appear
to cover the orifices in the subjacent crust through which the
liquid matter which forced them up acted.

139. These submarine volcanic phenomena are generally pre-
ceded by incandescent matter thrown above the waters, by
scoriaceous and pumiceous matter appearing on the surface, by
burning rocks which appear in the midst of vaporous waves, and
by the ebullition of the sea, the temperature of which then
becomes very elevated. All these effects were manifested in
modern times in the appearance of the islands of Julia, Sabrina,
and others, and the more ancient phenomena are similarly
described in historic narratives.

The circumstances attending them, however, are not always
identical. Sometimes no solid rock is raised above the water.
Thus, for example, at Kamtschatka, in 1737, jets of vapour only
were thrown up. Great ebullitions of the sea took place, and
pumiceous matter flowed on the surface. On the subsidence of
the eruptions, it was found that chains of submarine mountains
had been formed where previously there was a depth of 100
fathoms. In other cases there are not even jets of vapour, and
the phenomenon is manifested only by the increased temperature

105

THE CRUST OF THE EARTH.

of the waters, and by the sudden elevation of the matter which
existed at the bottom of the sea. This took place in 1820 at the
island of Banda in the Moluccas, where the bay, which previously
had a depth of 50 fathoms, was elevated by the tranquil upheaving
of compact basaltic matter which previously existed there, and
which formed a promontory composed of large blocks piled one
upon the other, without any accessory phenomena except the
increased temperature of the waters.

It has been ascertained also that violent submarine eruptions
are often followed by slow and gradual upheavings of the bottom
of the sea. This effect was manifested in the case of the island
near Unalaska, and also in those produced near Santorin. It may
be added also, that the islands thus produced are not always
permanent ; many of them disappear after intervals of more or
less duration, being either swept away by the water or sinking
into abysses formed under them.

140. Yolcanic craters, which result from the upheaving of the
crust of the earth in places where no volcano previously existed,
are distinguished by the name of CEATERS OF ELEVATION, from
those craters which break out at different points in existing vol-
canoes. In like manner, the conical mounds similarly formed are
called cones of elevation. Craters of elevation are distinguishable
even in places where no record or tradition of eruption exists, by
the arrangement of the strata elevated, which is altogether dis-
similar from what is found elsewhere. These strata are always
inclined in all directions round the axis of the cone, as shown in
fig. 59, presenting their edges abruptly towards the centre of the
cavity. The Monte Nuovo, already mentioned, presents an example
of this upon a small scale. The same formation is presented in
the case of the semicircular Somma of Vesuvius.

141. Another characteristic, not less important, and especially

useful for geological purposes in
Fig. 59. cases where the matter elevated is

not stratified, is supplied in all
great craters of elevation by the
crevasses which extend from the
borders of the crater to the external

base of the mountain, and which are presented in so remarkable
a manner in the Canaries, where they are called Barancos. These
radiating crevasses are admirably shown upon the plan of the
island of Palma, one of the Canaries, drawn by M. de Buch.

One of these Barancos, much deeper than the others, extends
from the foot of the mountain, at a place called Tazacorte, to the
base of the crater, as shown in the plan, and which is rendered
still more apparent in the perspective view shown in fig. 61.
106

ISLAND OF PALMA.

The volcanic islands which have arisen out of the ocean have
assumed forms, all of which are analogous to those here described.

Fig. 60. — Plan of the island of Palma, one of the Canaries.

Thus for example, the island of Sabrina, at the moment of its

Fig. 61. — Perspective view of Palma.

appearance, took the form of a crater which had an opening to

107

THE CRUST OF THE EARTH.

the south, and which was terminated by crevasses from which
boiling- water issued.

142. Although the crater-formed cavities which are so frequently
observed at various parts of the surface of the earth have been in
many cases preceded by volcanic eruptions, they are not invariably
to be ascribed to that cause, and indeed in many cases there is
abundant evidence that no such eruption could have taken place
at the moment of their formation. We have already explained
how the upheaving of the crust of the earth, produced by an earth-
quake shock, can produce not only radiating clefts, as in fig. 47,
but also an open cavity, as in fig. 51, the edges of which are sur-
rounded by divergent clefts.

143. Mount Etna, the most remarkable of European volcanoes,
situated on the island of Sicily, and composed entirely of erupted
mineral substances, rises to a height above the level of the Medi-
terranean of nearly eleven thousand feet. The circumference of
its base is more than a hundred and eighty miles, and on a clear
day it may be distinctly seen from any elevated point of the island
of Malta, a distance of a hundred and fifty miles. Compared with
this volcano, Vesuvius is insignificant. While the streams of
lava from the latter never exceed seven miles in length, those of
Etna very often are from fifteen to thirty miles, being five miles
in breadth, and from fifty to a hundred feet in thickness. The
surface of Etna presents three distinct regions. Around the base
for an extent of twelve miles, the country is richly cultivated, and
abounds in vineyards and pastures, and is the site of many towns,
monasteries, and villages. The middle or temperate zone above
is covered with forests of oak and chestnut, and a luxuriant vege-
tation reaches to within a mile of the summit. Above this all is
sterility and desolation, and the highest point of the mountain is
covered with eternal snow. The crater is about a quarter of a
mile in height, and three-quarters of a mile in circumference, and
is situated in the centre of a gently inclined plain, three miles in
diameter. From the crater a column of vapour constantly issues,
emanating from the mass of incandescent mineral matter which
fills up the interior, and may be seen, in a state of ebullition, in
the fumarolles in some of the lateral crevices, of which there are
generally several accessible.

144. Etna is recorded as having been in a state of activity
before the Trojan war; and ever since, at varying intervals,
violent eruptions have occurred. In an eruption of 1669, the
torrent of lava inundated a space of fourteen miles in length, and
four in breadth, burying beneath it five thousand villas and other
habitations, with part of the city of Catania, and at length fall-
ing into the sea. During several months before the lava burst

108

MOUNT ETNA.

out, the old mouth, or great crater, was observed to send forth
more smoke and name than usual, and the top fell in, so that the
cone became much lowered.

In 1809, twelve new craters opened, about half-way down the
mountain, and threw out rivers of burning lava, by which several
estates and farms were covered to the depth of thirty or forty feet;
and in 1811, other vents appeared on the eastern side, and dis-
charged torrents of liquid lava with amazing force.

145. In 1832, a violent paroxysm took place, and continued
with but little intermission for several weeks. On the 31st of
October, in the middle of the night, there arose, without any pre-
vious indication, a column of smoke and flame from the base of
the large cone on the northern side ; and, shortly after, an immense
quantity of fluid matter was discharged from the crater, on the
western side, divided into numerous streams. Next morning,
repeated earthquakes, the increased noise of the lava, which now
flowed rapidly, and the immense volumes of thick black smoke at
the foot of Monte Scavo, announced that the eruption had greatly
increased in violence, and several streams of lava were seen
descending. On the 2nd of November, contrary to all expecta-
tion, the eruption ceased, and the lava was found to be so far
cooled, that several adventurous observers were enabled to get
upon it, and walk a few paces. On the 3rd, the hope that the
fire was almost extinct was nearly certain ; but in the evening, a
violent earthquake, followed by several others less violent, with an
increased quantity of smoke, foreboded an eruption ; and two hours
before midnight, another severe shock occurred, and was succeeded
by black smoke mingled with flames, and incessant thunder.

" Having approached," says Signor di Luca, " as nearly as was
prudent, to the hollow from which the fire issued, we found four
apertures, which threw out burning matter. Raising our eyes
from these vents, we observed a cleft or rent, about a mile in
length, from which volumes of smoke arose from time to time ;
and, as at the bottom it reached the openings above-mentioned,
it enabled us to behold the burning furnace in the interior of the
mountain. Meanwhile the thunder was incessant, and the de-
tonations were terrible ; the lava continued to flow, and enormous
masses of red hot substances were thrown to a great height mingled
with vast volumes of flame and smoke. The shocks of earthquake
were likewise so violent, that horses and other animals fled in
terror from the places where they were feeding."

146. But by far the most interesting feature of Etna is an
immense depression or excavation on the eastern side of the moun-
tain, called the Val del Bove. This vast plain, or rather circular
hollow, is five miles in diameter, and from two to three thousand

109

THE CEUST OF THE EARTH.

feet in the height of its bounding precipices, which in most places-
are nearly perpendicular. This remarkable area appears to have
resulted from the giving way and subsidence of part of the crust
of the volcano, from some violent action in the interior, which
occasioned the sudden removal of an enormous mass of mineral
matter. This plain is ^ncircled by subordinate volcanic moun-
tains, some of which are covered by forests, while others are bare
and arid like many of the cones of Auvergne. The walls or cliffs
surrounding this depression are formed of successive layers of lava
of variable thickness, with interposed beds of tufa, ashes, and
igneous conglomerates of different colours and degrees of fineness.
They slope downwards towards the sea at an angle of from twenty
to thirty degrees, and have evidently been formed at various
intervals by successive eruptions from the top of the mountain,
and were continuous before the subsidence took place which gives
this region its present character.

The perpendicular sides of this natural amphitheatre are every-
where marked by vertical walls or dykes, which not only intersect
the concentric sheets of lava and tufa, but, standing out in bold
relief, like prodigious buttresses, impart a most extraordinary
character to the scene ; the greater induration of these intruded
dykes having enabled them to resist the denuding action which
has removed the less coherent pre-existing erupted materials.
These buttresses are from two to twenty feet in thickness, and
being of immense height, are extremely picturesque ; some of them
are composed of trachyte, and others of blue compact basalt with
olivine. The surface of the plain is wild and desolate in the
extreme, presenting the appearance of a tempestuous sea of liquid
lava, suddenly congealed. Innumerable currents of lava are seen
piled one upon the other, some of which terminate abruptly, while
others have extended across the Val, and descended in cascades
into the lower fertile regions, where they are spread out in sterile
tracts amid the vineyards and orange groves.*

147. A section of Mount Etna, extending north and south
between Catania on the south, and Taorminia on the north, is

shown in fig. 62 ; the east, upon the slope of which the cavity in
question is observable, being presented to the observer.

* See Captain Basil Hall's graphic description of a visit to the Val del
Bove, "Patchwork," vol. iii. p. 31.
110

VOLCANIC ISLANDS.

A ground-plan of the Yal del Bove and the surrounding
parts of the mountain is shown in fig. 63.

Lava of 1832

Terminal cone
Val del Bove

Lava of 1600
Catania

Cycloptean isles

Taormiuiu

Fig. 63.— Plan of the Val del Bove, Mount Etna.

148. The sudden sinking of the ground by which these crater-
formed cavities are produced, is often attended with the sudden
production of lakes, filling such cavities with water from sub-
terranean sources. Such lakes are sometimes supplied with water
at a high temperature, as was the case in one produced in 1835
in Cappadocia, near the ancient Ceesarea, and in 1820 in the
island of St. Michael, one of the Azores.

149. Volcanic islands generally are found to affect the semi-
lunar form, of which Santorin, fig. 58, is an example. Thus the
island of Sabrina, already mentioned, which appeared in 1811
among the Azores, at the moment of its rise above the waters
presented a crater which opened towards the south and was ter-
minated by crevasses, or openings, from which issued a current of
boiling water, figs. 64 and 65.

The island of Julia, which appeared to the south-west of Sicily
in 1831, assumed a similar form, and on the 6th of September,
1835, Captain Thayer, the French navigator, found to the north
of New Zealand a similar rock recently formed near the surface
of the water, which included a lagoon, having a single issue, and
within which the water was boiling.

150. These craters of elevation have sometimes continued per-
manently in the form which they first assumed, but they have
also frequently been subject to subsequent changes of form from
age to age. The case of Vesuvius, which underwent a remarkable
change in 79 A.D., has been already mentioned. The Peak of
Teneriffe rises within a circular enclosure, the sides of which are
vertical, and rise to a height of from twelve hundred to two

111

THE CRUST OF THE EARTH.

thousand feet. The volcano of Taal, in the island of Lucon, one
of the Philippines, is placed in the centre of a basin filled with

Fig. 64. Forms of volcanic islands.

Fig. 65.

water and surrounded by steep and elevated rocks, which leave a
single issue from it as in the cases already described.

151. A similar example of this form is presented in the case of
Barren Island in the Bay of Bengal (fig. 66, p. 65), which consists
of a circle of high mountains, into which the sea flows through a
single opening, and of which the centre is occupied by a volcano
two thousand feet high, which was in full activity at the time
of the discovery of the island.

152. The islands of Santorin and Theresia, already described,
form probably the borders of vast craters of elevation. Ancient
historians cite them as having appeared long before the Christian
era, after a succession of violent earthquake shocks, and in
accordance with this tradition, they present strata inclined out-
wards, as shown in fig. 67 ; the islands more recently elevated

Fig. 07.

having issued from the middle of the crater. The strata observed
in Santorin and Theresia, are inclined from the centre in accord-
ance with the principle explained in (140).

112

Fig. 72. — THE SALSES, OR MUD VOLCANOES, OF CARTHAGENA.

THE CRUST OF THE EARTH ;

OR, FIRST NOTIONS OF GEOLOGY.
CHAPTER VI.

1 53. Form of craters. — 154. Stromboli — Formation of internal adventitious
cones and craters. — 155. Strata of lava — how formed. — 156. Formation
of lava ndykes. — 157. Salses, or mud volcanoes. — 158. Fumarolles. —
159. Geysers. — 160. Valleys of elevation. — 161. Formation of parallel
ridges. — 162. Slow operation of air, water, and heat. — 163. Atmos-
pheric effects upon rocks. — 164. Effects of water on rocks. — 165.
Effects of the sea on cliffs.— 166. Effects of waterfalls.— 167.
Effects of the sea and of icebergs on the forms of rocks. — 168. Geo-
logical phenomena explicable by natural causes still in operation. — 169.
Portland dirt-bed — Its organic deposits. — 170. Climate of England
tropical at the epoch of these deposits. — 171. Example of coal deposits.
— 172. Northumberland coal mines. — 173. Colliery near Wolver-
hampton. — 174. Deposits in Treuille mine at St. Etienne. — 175.
The character of the waters shown by the organic deposits — Fossil
shells.

153. IT has been impossible to obtain direct observations of the
interior of craters when in a state of active eruption, but when
they have been approached immediately after the cessation of an

LA RISER'S MUSEUM OF SCIENCE. i 113

No. 141.

THE CRUST OF THE EARTH.

eruption, these cavities appear to have generally a conical form,
the base of the cone being presented upwards, and the lower part
filled with consolidated lava, by which the principal chimney of
the crater is covered. Sulphurous vapour is observed to issue
from its fissures and interstices, sometimes several open gulfs are
seen, from some of which vapours are emitted, and at the bottom
of others incandescent lava is seen. Others again are silent and
dark, and inspire an indescribable sense of terror.

154. The crater Stromboli, which has been in activity since the
most ancient times, presents at present the same appearances as
those which were described by Spallanzani, in 1788. It is
constantly filled with lava in a state of fusion, which alternately
rises and falls in the cavity. Having ascended to ten or twelve
yards below the summit of the walls, this boiling fluid is covered

'•^m.

}-:^M

Fig. 67.— Vesuvius in 1829.

with large bubbles, which burst with noise, letting enormous
quantities of gas escape from them, and projecting on all sides
scoriaceous matter. After these explosions, it again subsides,
but only to rise again and produce like effects — these alternations
being repeated regularly at intervals of some minutes. In craters
114

DYKES — SALSES.

-where the lava is less fluid than in that of Stromboli, new cones
•are sometimes formed in the midst of the crater, which first rise
in the form of a dome, and then burst out so as to form a small
-active volcano in the middle of the crater of the great one. This
phenomenon is often presented within the crater of Vesuvius
'(fig. 67) and was more particularly witnessed in 1829.

155. Sometimes the lava, which is pressed upwards instead of
being violently ejected, spreads itself in a sheet of greater or less
thickness over the surface (fig. 68) where it hardens, and is subse-
quently covered by other deposits. Cases have been found also
where a succession of these strata, formed at different intervals,
with interposed strata of other matter, have been observed (fig. 69).
In such cases the matter forming the superior stratum is seen to

Fig. 68.

Fig.

Fi*- 70-

;have passed in a liquid state through the inferior strata pre-
viously described.

156. It .of ten happens that the lava is solidified in clefts, which are
nearly vertical, and thus forms

walls, called dykes, which fre-
quently rise to the surface.
In such cases the solidified
lava, being much harder and
less susceptible of degradation
from atmospheric and aqueous
influences, remains standing
when the matter surrounding
it is swept away, and thus
forms a wall rising above the general surface (fig. 70).

157. The phenomena of Salses, or mud volcanoes, has been
already briefly noticed in our Tract upon " Earthquakes and
Volcanoes," Vol. IV. p. 169. The mud volcanoes are characterised
also by the conical form, but their cones are much less elevated,
their slopes being flatter (fig. 72). They have at their summit a
crater-formed cavity frequently filled with liquid mud, on which
large bubbles are continually formed which, bursting, scatter
around them earthy matter. There are sometimes, over a surface
of little extent, a great number of these cones in full activity,
some of which have a height of ten or twelve yards. Sometimes
such an assemblage of cones is found at the summit of a mound
from fifty to two hundred yards in height, formed of argillaceous

i 2 115

THE CRUST OF THE EARTH.

matter, which appears to have been the result of former ejections.
The middle is often formed of a lake of mud, the surface of
which is more or less consolidated. In certain countries these
mounds are found permanently dried, all disengagement of gas.,
water, and earth having altogether ceased, but it sometimes
happens that the samg phenomena after long cessation are
renewed with violence.

158. Fumarolles and Geysers are the names given to eruptions
of steam or boiling water issuing from crevices in volcanic dis-
tricts, remarkable examples of which are presented in the country
surrounding the celebrated volcano of Hecla, in Iceland. Erup-
tions of hot steam are projected from the crevices of the soil
in the form of white columns, rising to heights of from 30 to
60 feet, and often with noise similar to that with which high
pressure steam issues from the safety-valve of a boiler. Such
phenomena are manifested on a considerable scale in Tuscany, in
the neighbourhood of Monte Cerboli, Castel Nuovo, and Monte
Rotondo, and are generally disposed in a single line of from 20 to
25 miles in length.

These jets of vapour in all cases include chemical agents, which
attack the rocks with which they come in contact; thus the
vapour ejected from Vesuvius includes hydrochloric acid, that of
the Solfatara, of Pozzuolt, includes sulphurous acid, and that of
Tuscany, boric acid.

159. The Geysers are volcanic eruptions of boiling water, some
continued, others intermitting, which prevail in immense numbers
in Iceland. One of these hot springs is mentioned which, from
half-hour to half-hour, projects a column of boiling water 18 feet
in diameter to a height of 150 feet (fig. 71).

The water thus ejected contains a certain proportion of silica,
which is deposited in a state of hydrate upon all the surround-
ing bodies, and forms sometimes mounds of considerable extent,
at the summit of which is an opening, from which the liquid
issues.

Besides silica, the water of the geysers also contains, in a small
proportion, the carbonates or sulphates of soda, ammonia, potash,
and magnesia, besides a minute proportion of carbonic acid.

160. Calcareous as well as volcanic countries present vast
depressions of the ground analogous to craters, but instead of
being nearly circular like those of volcanoes, they are most fre-
quently oblong and very irregular. Such cavities are frequent in
the mountains of the Jura. These are generally oblong hollows,
like clefts, which sometimes extend to a great distance, forming
oblong mounds parallel to each other, with salient summits.
These depressions or cavities (fig. 73) have received the name of

116

VALLEYS OF ELEVATION.

valleys of elevation, though they differ in [nothing except their form
from craters of elevation.

161. Elevations of the superficial crust often take place in a
series of parallel lines forming so many parallel ridges with inter-
mediate hollows, as if an upheaving force had been exerted by
the subjacent strata in a series of parallel directions vertically
under the ridges thus produced. The Jura mountains present
great numbers of examples of this (fig. 74).

Fig. 74. — Section ot ridges in the Jura Mountains.

162. The changes in the condition of the earth's surface
attending the undulations and disruptions of its crust, which
have been noticed above, excite attention because of the sudden
catastrophes which often attend them, and the wide devastation
which they sometimes spread. There are, however, other agencies
exterior to the surface, of which the accumulated effects, produced
in long intervals of time, are not less important. The principal of
these are air, water, and heat, acting separately or together, upon
the solid matter of the external surface of the terrestrial crust.
These operate mechanically by fracture and abrasion, physically by
dissolution and disintegration, and chemically by decomposition.

117

THE CRUST OF THE EARTH.

The waters of the ocean evaporated by heat rise into the-
superior parts of the atmosphere, and are carried by atmospheric
currents in their course towards the most elevated parts of the-
land, where they are condensed, and upon which they are pre-
cipitated in the liquid state ; thence they descend along the
declivities, forming rivers and lakes, and sometimes penetrating
the crust so as to form springs, until at length, sweeping over the
land, they return to the deep, carrying with them, however, a
large quantity of detritus of the solid crust, over which they have
passed, and which they deposit in the bottom of the sea to form
new systems of strata.

Independently of water, air itself by its mechanical action upon
solid matter detaches, fractures and abrades more or less of it.
The water suspended in the form of vapour in the atmosphere
penetrates the pores and interstices of rocks to a greater or lesser
extent, according to their density and structure. In times of
drought it is again expelled by evaporation, and being thus alter-
nately and incessantly absorbed and dismissed, it at length dis-
integrates the superficial strata of the rocks, to whatever depth it
may penetrate.

Such effects are observable in all cases where extensive sections
of the solid crust are made, whether by natural or artificial!
causes. They are thus seen upon the face of the cliffs which
overhang the sea, in the escarpments of ravines which pass
through mountain-chains, and in the sides of the vast cuttings
artificially produced in quarrying, and still more in the con-
struction of roads, railways, and canals. These effects are, of
course, the more prompt and sensible, as the matter composing the
rocks is more susceptible of imbibing humidity and of being
deprived of it by evaporation. All mountains exhibit traces of
such effects in some forms, determined by the various degrees in
which their strata are susceptible of them. Thus, while some, like
volcanic cones, assume uniform slopes in a conical iprm, fig. 75 A;
others, those composed of gneiss, for example, assume the forms of
pointed and dentated peaks, fig. 75 r,. Numerous examples of

these are seen in the chains of the Alps, where they take the
names of needles, teeth, and horns, (aiguilles, dents, and cornes,}
118

ATMOSPHERIC EFFECTS.

according to their varying forms. Calcareous cliffs assume
cylindrical forms, fig. 75 c, which seem at a distance to resemble
fortifications. The faces of these cliffs are often worn into a
succession of terraces or steps, as in fig. 75 D.

163. The effects of long- continued atmospheric action upon the
forms of solid rocks, are seen in many places on the surface of
continents, which the sea has not approached within historic times.
Certain granites are thus disintegrated so profoundly, as to reduce
the under-surface of the strata to a mass of gravel, forming holes
into which the pluvial water from ravines flows in all directions.
These rocks are also sometimes met with worn into rounded forms,
and piled one upon another, so as to he supported only at a
single point, forming what are popularly called rocking-stones,
fig. 76 B. Cases of this kind are especially presented in the case
of certain porphyritic granites. In mountains where granite is
decomposed with facility, it has been remarked that masses of
these rocks, more or less divided, present a sort of horizontal
layers, separated by vertical fissures, so as to reduce the whole to
a pile of irregular parallelepipeds, fig. 76, c. The angles and
edges being often worn away, the mass is reduced to a form
resembling a pile of cheeses, fig. 76 A.

Fig. 76.

164. Solid rocks are often traversed by vertical crevices filled
with matter more easily penetrable by water. In such cases, the
pluvial waters entering these crevices dissolve and ultimately
sweep away the matter which fills them, leaving the parts thus
separated without support. These ultimately fall to the foot of
the cliff (fig. 77).

165. When waters bathe the foot of steep cliffs, they have a
tendency to dissolve and decompose their lower strata, leaving

119

THE CRUST OF THE EARTH.

Fig. 77.

the superior ones undisturbed and, consequently, overhanging.

When this action is continued to a certain point, the cliffs thus

overhanging fall by their weight (fig. 78).

It sometimes happens that the accumulation of the debris of
such cliffs which takes place below
^hem, operates as a barrier to the
waves, and so, for a time, protects
them from further degradation (fig. 79).
In some cases the natural form of the
rocks exposed to the action of the
waves enables them to resist these
effects, and such forms are accord-
ingly often imitated in the construction
of harbours and breakwaters (fig. 80).
166. Cascades have often the same effect in the degradation

of the cliffs over which they fall, as have the action of waves

directed against their base. When such cliffs are formed of

Fig. 78.

alternate calcareous and argillaceous matter, the former, being
more susceptible of disintegration than the latter, absorbs and is

Fig. 79.

Fig. 80.

worn away by the water either in its fall, or by its recoil from the
foot of the cascade ; the cliff, therefore, over which the torrent falls
soon overhangs (fig. 81) and at length is broken off by its weight.
120

EROSION BY WATER,

167. It is to the operation of such causes, as well as to
the action of icebergs float-
ing from the Pole, that various Flg- SL
forms of rocks found in the
ocean, but more especially in
the neighbourhood of conti-
nents, must be ascribed. The
action of the sea disintegrates
the softer parts, leaving the
harder standing, and thus
forms the most capricious are
produced. Wide clefts and
openings are made between
solid rocks through which the sea passes, and in some cases the
rocks are broken into rude and irregular columns and needles
(figs. 82 and 83.)

Fte. R2.

Fisr. S3.

Other similar examples are presented in the case of the chalk-cliffs
near the French village of Etretat on the Channel coast (fig. 84)
and also in the porphyritic and granitic columnar rocks of the
Shetland Isles (fig. 85).

168. Thus, in fine, it appears that there are, and constantly
have been, within historic times, natural agencies in operation
upon the surface of the globe, sufficient to explain all those
phenomena observed by modern geologists, which, when first
brought under notice, excited sentiments of such unmixed wonder.
Alternate elevations and depressions of the earth's crust, either
sudden or gradual, are recorded in all times ; and it is easy to
imagine that, in proportion as the shell of solid matter which
incloses the igneous central fluid was less and less thick, and

121

THE CRUST OF THE EARTH.

consequently less and less resisting, so at more and more remote
periods, these undulations, and their consequent disruptions and
explosions, must have been much more frequent, and attended by
catastrophes infinitely more violent. The volcanic eruptions
which have taken place within historic times, may be regarded as
miniature reproductions oj: the phenomena of which the globe was
the theatre at much more remote geological dates. The wear,
abrasion, decomposition, and transport of the solid materials of
the earth's crust by the action of atmosphere and the waters of
the ocean, when continued through periods compared with which
that limited by the existence of the human race is but a unitr
can easily be imagined to have produced all the effects which
are visible on the earth's surface, and to greater or less depths-
within its crust. The deposits formed by the detritus of the land

Fig. 84.

carried by the currents of rivers to their embouchures, exhibit on
a small scale the stratification produced by pre- Adamite seas. In
a word, all the geological phenomena discoverable by the sections,
natural or artificial, of the earth's crust, admit of clear and satis-
factory explanation, by merely imputing to the physical agents-
now in operation an energy proportional to the diminished thick-
ness of the earth's crust, and effects due to a continuance of action
for periods of time, compared with which the common chrono-
logical units must be regarded as insignificantly minute.

Having thus briefly indicated the natural causes to which geo-
logical phenomena must be ascribed, we shall resume the subject
which we had dropped, and continue our notice of some of these
resultst which illustrate the past condition of the earth.

169. There exists in England, in the Isle of Portland, as well
as elsewhere, and on various parts of the continent, a stratum
called by miners and quarrymen the " dirt-bed." This consists of
122

PORTLAND DIRT-BED.

a layer about one foot in thickness, composed of dark brown friable-
loam, containing a large proportion of earthy lignite, and, like the
recent soil of the island, many water-worn stones and pebbles.
It seems to have been a bed of vegetable mould, which at a remote
geological epoch supported an abundant and luxuriant vegetation,,
for we find in it and upon it innumerable trunks and branches of
cone-bearing trees and cycadeous * plants. Above this bed are
found layers of finely-laminated cream-coloured limestones, the
total thickness of which is about ten feet, and upon which is
deposited the modern vegetable soil ; but this latter at present,,
instead of supporting cycadeous plants and pine forests, barely
maintains a scanty vegetation.

The most remarkable circumstance attending this dirt-bed, as it
is called, is the position of the trees and plants found on it. They
are still erect, as though they had been suddenly petrified while
growing in their native forests, with their roots in the vegetable
soil and their trunks extending into the limestone above it.

Immediately below it is a thick stratum of fresh-water lime-
stone, of little value for building ; and below this again is the
stratum of the celebrated Portland stone so extensively used for
that purpose. The consequence is that the dirt-bed and its-
interesting materials, little regarded by quarrymen, are cast
away and scattered about as mere rubbish, in order to get at
the layer of building-stone which lies below them. " On one of my
visits to the island (in the summer of 1832)," says Dr. Mantell,
" the surface of a large area of the dirt- bed was cleared, prepa-
ratory to its removal, and the appearance presented was most
striking. The floor of the quarry was literally strewn with fossil
wood, and before me was a petrified forest, the trees and plants,,
like the inhabitants of the city in Arabian story, being converted,
into stone, yet remaining in the places which they occupied when
alive ! Some of the trunks were surrounded by a conical mound
of calcareous earth, which had evidently, when in a state of mud,
accumulated round the stems and roots. The upright trunks, wore
generally a few feet apart, and but three or four feet high ; their
summits were broken and splintered, as if they had been snapped
or wrenched off by a hurricane, at a short distance from the ground.
Some were two feet in diameter, and the united fragments of one
of the prostrate trunks indicated a total length of from thirty ta
forty feet ; in many specimens portions of the branches remained
attached to the stems. In the dirt-bed, there were numerous
trunks lying prostrate, and fragments of branches.

" The external surface of all the trees I examined was weather-

* Such as palms and ferns.

123

THE CRUST OF THE EARTH.

worn, and resembled that of posts and timbers of groins or piers
within reach of the tides, and subjected to the alternate influence
of the water and atmosphere ; there are but seldom any vestiges
of the bark.

" The fossil plants related to the recent Cycas andZamia,* occur
in the intervals between th$ pine-trees; and the dirt-bed is so little
consolidated, that I dug up with a spade, as from a parterre,
several specimens that were standing on the very spot where they
originally grew, having, like the columns of the temple of Poz-
zuoli, preserved their original erect position amidst all the revo-
lutions which have subsequently swept over the surface of the
earth, and buried them beneath the accumulated detritus of
innumerable ages. These fossil plants, though related to the
recent Cycadese, belong to a distinct genus, f There are two
species — one is short, and of a spheroidal form (M. nidiformis) ;
the other is longer, and subcylindrical (M. cylindrica). t

" The trees and plants are completely silicified, and their internal
structure is beautifully preserved in many examples ; the wood,
microscopically examined, displays the organisation of the Arau-
caria. A cone has been found in the dirt-bed, which Dr. Brown
considers to be nearly related to the fruit of the Norfolk Island
pine (Araucaria excelsa). The Portland and Isle of Wight fossil
trees appear to belong to the same species of Conifer a)." §

Fig. 86.— Section of the Portland dirt-bed.

170. The presence of plants analogous to the modern Cycas
and Zamia shows that the climate of England, at the time when
the vegetation of this stratum flourished, must have been

* These plants are so common in conservatories that their general
appearance must be familiar to the reader. In the Botanic Gardens at
Kew there are magnificent specimens of Cycas and Zamia, and of other
plants of hot climates, of which related forms occur in the Wealden.

f Named by M. Adolphe Brongniart, Mantellia.

J Specimens of the former species are called " crows' -nests" by the
quarrymen, who believe them to be birds' nests originally built by crows
in the pine-trees, and which have since become petrified.

§ Mantell, p. 387.
124

SECTION OF TREUILLE MINE.

analogous to that of the tropics, a fact which is in conformity
with what has already been explained.

171. The coal deposits are everywhere attended with similar
results. Entire trees are found, some of which are standing
upright with their roots penetrating the stratum below them,
exactly as they penetrated the soil on which they grew. Several
examples of these have been presented in England, one of the
most remarkable of which occurred in the construction of the
railway between Manchester and Bolton. Near Dixonfold five
large stems of Sigillarise were found erect with their roots striking
into layers of clay below. They stood upon the same level one
beside the other, the trunks being surrounded and filled by soft
blue shale, and the carbonised bark being all that remained of the
original structure. All these trunks seemed to have been broken
violently off at a point four or five feet above the roots, no
traces of the upper parts of the trees being discovered.

172. On the coast of Northumberland, within a space of half a

Fig. 87.— Section of the Treuille Mine at St. Etienne.

mile in length, twenty upright trees were discovered by Mr.
Trevelyan, and similar ones were found in the same coal-field at
some distance, as if they had been the continuation of a sub-
merged forest like that of the Isle of Portland.

In the Newcastle coal-field a stratum of sandstone occurs

125

THE CRUST OF THE EARTH.

nearly five hundred feet below the surface, on which numerous
trees have been found standing erect from two to eight feet
in circumference, with their roots struck into thin layers of
coal.

173. " In a colliery near "Wolverhampton," says Hugh Miller,
" the bottom coal rises to view, and where the surface has been
•cleared of the alluvial covering, it presents the appearance of a
moor on which a full-grown fir- wood had been cut down a few
months before, and only left the stumps behind. Stump rises
beside stump, to the number of seventy-three in all : the thickly
•clinging roots strike out on every side into what seems once to
have been vegetable mould, but now exists as an indurated
brownish-coloured shale. Many trunks, sorely flattened, lie
recumbent on the coal; several are full thirty feet in length^
while some of the larger stumps measure rather more than two
feet in diameter. There lie> thick around, Stigmariae, Lepidof-
dendra, Calamites, and fragments of Ulodendra; and yet with
all the assistance which these lent, the seam of coal formed by
this ancient forest does not exceed five inches in thickness. Not
a few of the stumps in this area are evidently water-worn. The
prostrate forest had been submerged, and molluscs lived, and
fishes swam over it. This upper forest is underlaid by a second,
and even a third : we find three full-grown forests closely packed
oip in a depth of not more than twelve feet." *

174. M. Alexandre Brongniartf describes a coal-pit at Tfeuille
mear St. Etienne, in the neighbourhood of Lyons, which contains
•enormous stems of Calamites and other trees in erect positions
(fig. 87). These and similar effects are considered as proofs that
the coal was produced by the submergence of a forest which grew
upon the spot. This particular mine is very favourable for observa-
tions being in the open air, and presenting a natural succession of
the strata of clay, slate, and coal, with four layers of compact
iron-ore in flattened nodules, accompanied and even penetrated
by vegetable remains.

The upper ten feet of the quarry consist of micaceous sand-
stone, which is in some instances stratified, and in others has a
slaty structure. In this bed are enormous vertical stems
traversing all the strata, and appearing like a forest of plants
resembling the bamboo or large Equiseta petrified on the spot on
which they grew. The stems are of two kinds, one long and thin,
from one to four inches in diameter, and nine or ten feet high,

* First Impressions of England and its People, by Hugh Miller, p. 223.
1* Notice sur les Vegetaux fossils traversants les Couches du Terrain
houilleux, par M. A. Brongniart, Paris, 1821.
126

FOSSIL SHELLS.

consisting of jointed and striated cylinders with a thin coaly
hark. The other and less common species consist of hollow
cylindrical stems spreading out from the hase like a root.

Fig. 88. Fig. 89. Fig. 90. Fig. 91. Fig. 92.

Limnea longiscata.

Planorbis evouphalus.

Melania.

175. The character of the waters, according as they may have
heen fluviatile and lacustrine or marine, from which the several
strata forming the crust of the earth were deposited, is betrayed

Fig. 93.

Fig. 94.

Fig. 95.

Fig. 96.

by the nature of the organic remains which these strata severally
contain. Thus, if we find shells (figs. 88 to 92) analogous in their
character to existing fresh-water shells, it may be inferred that
the deposits were fluviatile and lacustrine, or at all events that
they were fresh-water deposits.

If, on the other hand, none but marine shells (figs. 93 to 96) be
found in any stratum, it may be inferred that such stratum was
submerged by the ocean from which the deposits were made.

127

THE CRUST OF THE EARTH.

In cases where the organic remains are of a mixed character,
containing shells and other fossils, some analogous to existing
marine and other species, it may be inferred that such deposits
were made at the embouchures of rivers.

By such inductions it has been ascertained that extensive tracts
of the surface of the globe, which are now dry land and raised to
elevations considerably ab'ove the level of the sea, must, at various
former epochs, have been submerged in the waters of the ocean.
A great part of France, including the country around Paris,
Normandy, Artois, Picardy, Franche-Comte, Burgundy, the
Cevennes, Dauphiny, and Provence, present examples of this.

128

Fig. 108. — Fossil footprints in the strata 01 the new red sandstone.

THE CRUST OF THE EARTH;

OR, FIKST NOTIONS OF GEOLOGY.

CHAPTEE VII.

176. Fossil foraminifera— 177. Fossil infusoria— Researches of Ehrenberg.
— 178. Marine deposits at great heights, produced by former undula-
tions of the land. — 179. Organic deposits show that the same parts
of the land have undergone a series of alternate elevations and depres-
sions.— 180. Example of the Portland beds. — 181. Fossil footsteps. —
182. Such traces common in the new red sandstone, at Hesseburg, and
near Liverpool. — 183. Footprints of birds. — 18)4.. FossU rain-dropa
and ripple-marks. — 185. Conclusion.

LARDNER'S MUSEUM OF SCIENCE.
No. 143.

129

THE CRUST OF THE EARTH.

176. BESIDES the larger class of fossil shells, there are numerous
infinitely more minute ones, which have largely contributed to
the formation of the actual crust of the globe, the principal of
which are the Foraminifera and Infusoria.

The Foraminifera are marine animals of low organisation,
and generally of such extreme minuteness, that an ounce of
sea-sand will contain thr^e or four millions of them. The body
consists of uniform granules enclosed in a skin or membrane

Fig. 97. Fig OS. Fig. 90.

Serpula.

Ammonites catena.

Nautilus trun-
catus.

having one or more cavities or digestive sacs. These animals
are regarded as polypes, and are protected by shells. Some of
these shells, such as the Orbiculina, contain a single cell.
Others, such as the Nodosaria, consist of several cells disposed
in conical or cylindrical directions. Other families have shells,
like that of the Nautilus, consisting of a succession of cells in
spiral forms.

The Foraminifera derive their name from the structure of the
shell, which consists of one or more series of chambers separated
from one another by septa or partitions, in each of which there is
a small perforation called a foramen.

Some specimens of these shells, on a highly magnified scale, are
given in figs. 100 to 106.

These microscopic shells, of which from seven to eight hundred
fossil species have been discovered, are accumulated in enormous
numbers in the strata of the earth, and, in many cases, exclu-
sively form very considerable calcareous deposits, of which the
chalk and the cretaceous and tertiary strata present numerous
examples in all parts of the world.

177. The Infusoria, the existing species of which are found in
130

FOSSIL INFUSORIA.

fresh and salt water, are smaller still than the Foraminifera,
being only visible by the aid of microscopes of great magnifying
power. There are innumerable species of them, which are fur-
nished with siliceous shells, and which, consequently, are accu-
mulated at the bottom of the waters with the contemporaneous
microscopic plants. Now, although these beings are so minute
that forty thousand millions of them would not fill a space of

Fig. 100.

Fig. 101.

Fig. 102.

Fig. 103.

Fig. 100. — a. Nodosaria limbata. Fig. 102. — a. Flabellaria rugosa.

6. Internal arrangement of b. Side view, to show the flat

the cells.
„ 101.— a. Marginulina trilobata.

b. The last cell seen from

above.

c. The internal arrangement

of the cells.

103. — a. Textularia turris.

b. Internal arrangement of
the alternate cells.

more than a cubic inch, M. Ehrenberg has demonstrated that
their accumulation in certain parts of the earth's crust has pro-

Fig. 104.

Fig. 105.

Fig. 106.

Fig. 104.— Rotulina Voltzii.
,, 105.— a. Cristellaria rotula.

Fig. 105. — b. Edge view, to show the flatness.
,, 106. — Orbiculina numismalis.

duced strata several yards in thickness, and of vast extent ; and
that in many other cases strata not less extensive are formed by
their combination with other conchiferous animalcules. They
constitute almost exclusively the polishing slate of Bilin, in
Bohemia, which occupies a surface of great extent, probably the
site of an ancient lake, and forms a stratum fourteen feet
in thickness, composed of the mineralised shields of these
animalcules. " The diameter of a single one of these creatures,"
says M. Ehrenberg, "amounts upon an average, and in the
greatest part, to the 3500th of an inch, which equals | of the
thickness of a human hair, reckoning its average size at the
570th of an inch. The globule of the human blood, considered at
the 3600th of an inch, is not much smaller. The blood globules

K 2 131

THE CRUST OF THE EARTH.

of a frog are twice as large as one of these animalcules. As the
Polirschiefer of Bilin is slaty, but without cavities, these animal-
cules lie closely compressed. In round numbers, about twenty-
four millions would make up a cubic line, and would, in fact,
be contained in it. There are 1728 cubic lines in a cubic inch ;
and therefore a cubic inch would contain, on an average, about
forty-one thousand milHbns of these animals. On weighing a
cubic inch of this mass, I found it to be about 220 grains. Of
the forty-one thousand millions of animals, a hundred and
eighty-seven millions go to a grain ; or the siliceous shield of
each animalcule weighs about ^ millionth part of a grain."

The remains of these Infusoria are often found in abundance in
flints, opals, and more especially in the earthy matter which
envelopes the translucent parts. They exist in large quantities,
also in most marls, especially in those of lacustrine depositions
in calcareous slates of the same formation, and in all chalk strata.
They form the chief part of the deposits which fill the gulfs and
arms of the ocean, and are found in all the earthy deposits raised
from the bottom of the waters in ancient and modern times.
They exist in strata sixty feet thick in the low plains of "Western
Germany, at a depth greater or less under the sands of those
countries. It is a remarkable fact, that one of the strata on
which the city of Berlin is placed, is formed of the shells of
Infusoria which still live and are propagated and sustained,
doubtless, by the waters of the Spree, on which that city is built.

M. Ehrenberg has described numerous fossil genera and species
of Infusoria found in all parts of the world, and in different
strata, a few of which are represented on a high magnified scale
in fig. 107.

178. At whatever heights upon the land fresh- water shells and
the remains of land animals may be found in the sedimentary
strata, no surprise can be excited, since it is perfectly conceivable
that at various epochs any portions of the land, whatever be its
level, may have been submerged by lakes or overflown by rivers.
But we find, also, at all levels, no matter how high, even at
the summits of lofty ranges of mountains, marine deposits in
strata of immense extent and vast thickness.

179. In many places an analysis- of the strata shows the most
curious alternations between fresh-water and marine deposits,
which can only be explained by the supposition that the crust of
the globe at these places has undergone a succession of elevations
and depressions, and that after being submerged for an indefinite
period, and receiving marine deposits, it was then upheaved so as
to become dry land ; and that during another indefinite period it
was peopled by land and fresh- water animals, and covered with a

132

FOSSIL INFUSORIA.

certain vegetation : that then it was again submerged, either by-
fresh water or by the sea, receiving during an indefinite period
another stratum, either with fluvial and lacustrine remains or
with marine deposits, as the case might be ; that subsequently it
was again upheaved, and again became the theatre of animal and
vegetable life upon dry land, and so on.

180. The Portland dirt-beds, already described, prove the
existence there, at remote epochs of the globe, of vegetable soil on
a land nearly if not altogether dry, beneath which lay a stratum
of marine deposits. It follows, therefore, that before this epoch,
and before the deposition of the dirt-bed itself with the remains
contained on it, that part of the land must have been submerged
by the ocean, and that being upheaved, it afterwards became the
theatre of animal and vegetable life, such as we find deposited in

Fig. 107.— Fossil infusoria.

a. Desmidium apiculosum.
6. Euastrum verrucosum.

c. Xanthidium ramosum.

d. Peridinium pyrophorum.

e. Gomphouema lanceolata.
/. Hemanthidium arcus.

g. Piunularia dactylus.

h. Navicula viridis.

i. Actinocyclus senarius.

j. Pixidula prisca.

k. Galliouella distans.

I. Syuedra ulna.

in. Bacillaria vulgaris.

n. Spicula spongiaria.

the dirt-beds. Over the dirt-beds are thick lacustrine deposits of
limestone, which lie under the green sandstone, which latter is
itself overlaid by the chalk, this latter being evidently a marine
formation. Let us see, then, what a curious succession of phe-
nomena is here indicated. The lower marine strata of limestone
was first upheaved, and, becoming dry land, was the theatre
of rich terrestrial vegetation. It was then submerged by fresh
water, and was the place of a lake or deep estuary, in which
were formed the strata of limestone, sand, and clay, filled with
fluviatile shells, forming a stratum, which from actual observation
appears to have a thickness of from seven hundred to a thousand
feet. Later still, this was covered by the sea, and marine deposits
of green sandstone and chalk were formed upon it, which, at

133

THE CRUST OF THE EAKTH.

certain places, had a still greater thickness. In fine, at a later
period still, another great elevation of the crust took place, which
raised the surface to its present elevation above the waters.

181. Independently of the evidence supplied by organic remains
proving that certain strata were at former epochs above the waters
and inhabited by land animals, and were subsequently submerged,
indications of another order have been supplied by the traces left
by animals in the soft surface of the strata, which were afterwards
hardened without these traces being effaced. Such traces, there-
fore, may be regarded as moulds thus accidentally preserved, from
which castings might be obtained of those parts of the animals,
then living upon the globe, which produced them. Many moulds
of shells have been thus found, but the most remarkable indica-
tions of this kind consist of the footsteps of certain animals which
appear to have been impressed upon the soft surface of the ground,
just as the footsteps of any animals might be at present impressed
upon the soft sand upon the sea-shore after the retirement of the
tide.

182. In 1834 an account was published of remarkable fossil
footprints in the new red sandstone at Hesseburg, near Hildburg-
hausen, in Saxony. The largest of these tracks appears to have
been made by an animal whose hind-foot was eight inches long,
and which, from its resemblance to the human hand, received
from Professor Kaup the name of Chirotherium. Some of the
tracks, however, appeared to be those of tortoises ; and Link sug-
gests that others are those of some colossal species of frog or sala-
mander, fig. 109. *

Fig. 109. — Labyrinthodon pachignatus (Owen).

The traces represented in fig. 108 are those found in a sand-
stone slab at Hesseburg. Various similar tracks have been found
in the sandstone quarries at Storeton Hill, near Liverpool. The
largest footprint was nine inches long and six inches broad, the
length of the step being nearly two feet. Similar tracks were
found by Professor Hitchcock in the quarries of Connecticut ; and
Mr. Scrope found similar marks on a surface bearing ripple-marks
in the vast marble- quarries near Bath.
134

FOSSIL FOOTPRINTS.

These last are supposed to have been made by some crustaceous
animal crawling along the bottom of an estuary ; for between the
rows of footprints are in some cases observed the impressions of
the belly, and in others the trail of the tail.

183. Footprints of birds have been discovered in several quar-
ries in the valley of the Connecticut river, and also in different
parts of the state of Massachusetts. Several specimens of these
are now in the British Museum. The most remarkable of these
is a slab, eight feet by six, exhibiting various tracks of birds,
(fig. 110).

Fig. 110. — Fossil footsteps of birds.

|

All these marks belong to the birds called waders, and appear
to have been made upon the sea-shore. Some are small, others of
a size so enormous that they could only have belonged to birds
twice the size of the largest ostrich. In one case the footprint
measures fifteen inches in length by ten in width, without count-
ing the hind-claw, which itself measures two inches. The distance

135

THE CRUST OF THE EARTH.

between the successive footprints varies from four to six feet. The
shorter footsteps may be taken as the length of the step when the
bird walked at its ordinary pace, and the larger one when it
moved more swiftly.

184. A very remarkable concomitant of some of these fossil
footprints is the distinct Impressions of rain-drops upon the strata.
Dr. Dean discovered a stratum containing more than a hundred
marks of the feet of birds of various species, the whole surface of
which was pitted by the marks produced by a heavy shower of
rain. Like marks were observed in Storeton quarry, near Liver-
pool. The impressions produced by the rain-drops were sometimes
perfect hemispheres, an indication of a heavy fall of rain in a vertical
direction, and consequently in a calm atmosphere. In other cases-
the impressions were irregular and oblong, as if the drops had
struck the surface obliquely, as when a shower is accompanied by
a strong wind.

Professor Hitchcock also mentions specimens of sandstone ob-
tained from various parts of the United States, showing at once
footprints, ripple-marks, and rain-drops, the latter being elongated
by the direction of the wind when the shower was falling.

These phenomena can only be explained by the fact that the
marks were made upon the moist sand formed on the shores of an
estuary or tidal river, between high and low water mark, which
then was allowed to dry and harden by the action of the sun and
air between two successive tides. The waters on the return of the
tide would wash up silt to cover up the impressions without im-
pairing their accuracy ; the two layers uniting so as to exhibit
when separated the one a shield, and the other a cast from it of
the form thus impressed.

185. Our necessary limits, rather than the exhaustion of the
subject, compel us here to close these first glimpses of geology.
We propose, however, in a succeeding paper, to resume the sub-
ject, and to give a brief sketch of the History of the Earth from
the first formation of its solid crust to the last great act of creation
which called the human race and its concomitant tribes into-
existence.

135

CUVJER .

Pig. 1.— CUVIER.

THE STEREOSCOPE.

1. Surprising effect of the instrument explained. — 2. Causes of visual
perspective and relief. — 3. Effects of binocular parallax. — 4. Example
of the bust of Cuvier. — 5. Principle of the stereoscope. — 6. Origin of
the name. — 7. Wheatstone's reflecting stereoscope. — 8. Sir David
Brewster's lenticular stereoscope. — 9. Method of obtaining stereoscopic
pictures. — 10. How the effects of relief are produced. — 11. Natural
relief greatly exaggerated.

1. THE surprise excited by the impressions of perspective and
relief produced by the stereoscope have never, as we think, been
fully or adequately explained. This emotion of astonishment does
not merely arise, as is commonly supposed, from the fact that
such impressions are stronger than those produced by the best
executed drawings or paintings, but that, paradoxical as it may
seem, they are actually in many cases stronger and more vivid,
than any which could be produced by the objects themselves. In
a word, the stereoscope has the property of exaggerating the
natural effects of perspective and relief. To comprehend this it
will only be necessary to revert for a moment to the principles
upon which the effects of vision are based.

The mind judges of the relative position, form, and magnitude
of visible objects, by comparing their apparent outlines and
varieties of light and shade, with previously acquired impressions
of the sense of touch. The knowledge that such and such visual
appearances and optical effects are produced by certain varieties
of form, position, and distance having been already acquired, it
substitutes with the quickness of thought the cause for the effect.
The continual repetition of such acts, which are necessarily re-
peated as often as the sense of vision is exercised, and the extreme
rapidity with which all such mental operations are performed,

137

THE STEREOSCOPE.

render us unconscious of them, and we imagine that shape, dis-
tance, and position are the immediate subjects of visual percep-
tion, instead of being consequences deduced from a set of percep-
tions of a wholly different kind.

2. In drawing and painting, the effects of perspective and relief
are therefore reproduced* by transferring to the canvas the same
outlines and the same varieties of light and shade, which the
objects delineated really present to the eye, and when this has
been accomplished with the necessary degree of fidelity and pre-
cision, the same impression of distance, perspective, and relief is
produced, as that which would be received from the immediate
view of the objects themselves which are delineated.

3. In certain exceptional cases, however, a class of visual phe-
nomena is manifested which are quite independent of mere
outline and varieties of light and shadow, and which no effort of
art can transfer to canvas. Inasmuch, also, as these phenomena,
like those already mentioned, are optical effects of distance, form,
and position, they become, like the others, indications by which
the mind judges of the relative forms and positions of the objects
which produce them. Phenomena of this class are manifested,
when the objects viewed are placed so near the observer, as to
have sensible binocular parallax. The aspects under which they
are seen in this case by the two eyes, right and left, are different.
Certain parts are visible to each eye which are invisible to the
other, and the relative positions in which some parts are seen by
one eye, differ from those in which the same parts are seen by the
other eye. This difference of aspect and apparent position, arises
altogether from the different position of the two eyes in relation
to the objects. It is a phenomenon, therefore, which can never
be developed, in the case of objects whose distance bears a large
proportion to the distance between the eyes, because there is no
sensible difference between the aspects under which such objects
are viewed by the one eye and the other. The phenomenon,
therefore, can only be manifested in relation to objects, whose
distance from the observer is a small multiple of the distance
between the eyes.

4. To render this more clear, let us imagine a bust presented to
an observer at a distance of a few feet, the face being turned
obliquely so that one side is presented more to view than the
other. Supposing the side which is turned towards the observer
to be on his right, it is evident that the nose will intercept, more
or less, the view of the side of the face which is on his left, but
the part which it thus intercepts will not be the same for both
eyes. It will evidently intercept more from the right than the
left eye. On the other hand, the right eye will see a part of the

EFFECTS EXPLAINED.

right side of the bust, which will be concealed from the left eye
by the projecting parts of the face.

It therefore appears that the two eyes, right and left, will have
different views of the bust ; so that if the observer were to make
an exact drawing of the bust with his left eye closed, and another
exact drawing of it with his right eye closed, these drawings
would not be identical. One of them would show a part of the
bust on the extreme right, which would not be exhibited in the
other, and the latter would show a part on the extreme left, which
would not be included in the former. Moreover, a part of the
cheek and the eye would be shown in the drawing made with the
right eye closed, which would not appear in the drawing made
with the left eye closed.

Two such views of the same object are shown in figs. 1 and 2,
the former being the view presented to the left and the latter to
the right eye.

Now it is evident that when such an object is looked at with
both eyes open, the two different visual impressions here described
are simultaneously perceived, and they become to the mind signs
and indications of the actual forms which produce them.

When objects, therefore, can be viewed at distances small
enough to be attended with a sensible degree of parallax, their
perspective and relief are perceived, not only by the outlines and
varieties of light and shade, which are the common indications of
perspective and relief at all distances, but also by the class of
binocular phenomena which we have just described.

Hence it follows that the perception of relief, and generally of
form and relative position in objects whose proximity is sufficient
to produce binocular parallax, is much stronger and more vivid
than those whose distances, rendering the binocular parallax
evanescent, leaves nothing but the outlines and the varieties of
light and shadow, by which the mind can form a judgment of
form, relative distance, and position.

But since binocular parallax is reduced to the very small
amount of half a degree at the distance of 24 feet, it is clear that
it can only enter into the conditions by which we perceive
perspective and relief, in the case of a very limited class of
objects, and is not at all applicable to objects in general whose
forms and perspective we habitually contemplate.

5. After what has been explained of the two different views
which a near object presents, when looked at successively with
the one eye and the other closed, the principle of the stereoscope
will be easily understood.

A bust being placed before a competent draughtsman, as above
described, at a distance sufficiently small to produce considerable

139

THE STEREOSCOPE.

binocular parallax, let him. make two exact drawings of it, one
with the right eye closed, and the other with the left eye closed.
These two drawings will then represent the object as it is actually
seen, when the optic axis of each eye is directed to it. Let us
suppose that, by some optical expedient, the two drawings thus
made can be so prese'hted'to the two eyes, that the optic axis, when
directed to them, shall converge at the same angle as when they
are directed to the object itself. In that case each eye will
obtain the same view which it would obtain if the object itself
were placed before it, and the visual perception must necessarily
be the same as would be produced by the object looked at with
both eyes open.

6. Now the optical expedient by which this is accomplished is
the stereoscope, a name derived from two Greek words, a-rsptdv
(stereon), a solid object, and o-/co7reo>, (skopeo) I look at ; inasmuch
as the effect is such as to make the observer imagine that a really
solid object (in the geometrical sense of the term), instead of a
flat surface, is placed before him.

Various optical combinations have been proposed and contrived,
for the purpose of producing this effect upon two such drawings
as we have here described. In some the visual rays proceeding
from the pictures are thrown into the requisite direction by
reflection, and in others by refraction.

7. In the first form given to the instrument by Professor
Wheatstone, its inventor, the visual rays proceeding from the
two pictures were deflected by two plane reflectors placed %t a
right angle, so that in entering the eyes they proceeded as if they
had diverged from a common point, at which the object repre-
sented by the pictures would therefore appear to be placed.

Fig. 3.

Let A B c D (fig. 3) be the ground-plan of a rectangular box, open upon

the side AD so as to admit the light.
Let R and L be two eye-holes made
in the side B c, at a distance apart
equal to the distance between the eyes
of the observer. Let E F and p G be
two plane mirrors placed at right
angles to each other. Let a drawing
of an object seen with the right eye,
the left being closed, be attached to
the inside of D c at r, and another
made from the object seen with the
left eye, the right being closed, be in
like manner attached at I to the
inside of A B. Supposing the eyes of
the observer to be placed at the holes R and L, the right eye will see by
reflection the drawing r in the direction R TO, and the left eye will see the
drawing I by reflection in the direction L m. If the lines L m and R n be
140

<•! V

WHEATSTONE AND BREWSTER.

imagined to be continued backwards, they will meet at a certain point 0
behind the reflectors ; and if the drawings r and I be made to correspond
with the views which the right and left eyes would have respectively of the
object itself, which they represent, placed at o, the impression produced by
the two drawings thus seen will be precisely the same as those which
would be produced on the right and left eye respectively by the object
itself seen at o.

8. In the lenticular stereoscope invented by Sir David
Brewster, the form of the instrument to which the public in general
in all countries have given the preference, the visual rays pro-
ceeding from the two pictures are deflected and made to diverge
from the desired distance, by means of two eccentric double
convex lenses.

These are formed by cutting a double^ convex lens A B c D (fig. 4), into
two semi-lenses BAD and BCD, in the
direction of a plane B D, passing through
the centre of the lens. The two eccentric
lenses are then cut out of these, so that
the diameters A E and c E shall be the
semi-diameters of the original lens. It
will be evident that a section of the origi-
nal lens, made by a plane passing through
A E c at right angles to its surface, will
have the form represented at ABC (fig. 5),
and consequently that the two eccentric
lenses A E and c E will have their thickest
part at E, and their thinnest at A and c.
While the geometrical centres of these
lenses are at o and o', their optical centres
are at the thickest point E of the radius.

Now, suppose these two lenses to be set with their edges A and c towards
each other in two eye-holes whose distance apart is equal to that of the
eyes, and let two objects, p and p' (fig. 6), be placed before them at a
distance equal to their common focal length. According to the properties
of lenses already explained, pencils of
rays diverging from p and p', and passing -pig. 5.

through the lenses, will be, after refrac-
tion, parallel respectively to lines drawn
from P and p', through the optical centres
E and E' of the lenses. Thus the visual
ray P p will, after refraction, issue in the
direction p L, and the ray p' p' will issue
in the direction p' R, so that the points p
and P' will be seen in the directions of
L p and R p' converging to the point o.

Now, if P be a picture of an object as it appears to the left eye, and P' a
picture of it as it appears to the right eye, these two pictures will be
brought together at o by the refraction of the lenses, and the eyes will see
the combined pictures at o, exactly as they would see the object itself if it
were placed there.

An advantage incidental to this arrangement is, that the

141

THE STEREOSCOPE.

Fig. C.

convexity of the lenticular eye-pieces A E and c E', may be such
as to produce
any desired mag-
nifying effect,
within practical
limits, upon the
two pictures.

The tubes con-
taining the eye-
glasses A E and
c E', are made to
draw in and
out so as to be
adapted to dif-
ferent eyes ; and
they are fixed
by pins, which
pass into slits
made in them
in that position
in which the de-
flected rays have
the proper degree
of divergence.

The form in
which this lenti-
cular stereoscope is usually constructed, is shown in fig. 7. The

pictures are either opaque or
transparent. If they are opaque,
they are illuminated through an
opening A B c D, covered by 9.
hinged lid, the inside surface of
which is coated with tinfoil so as
to reflect light upon the pictures.
If they are transparent, the base
of the instrument A' B' c' D' has
a plate of ground glass set in it,
which allows a diffused light to
pass through the pictures.

9. In what has been stated
above, it has been assumed that

two drawings of the same object can be produced, differing one from
another precisely as the two views of the same object would differ,
when viewed by the right and the left eye successively, subject
to a given degree of binocular parallax. Now, the difficulty, if
142

STEREOSCOPIC PICTURES.

not the total impracticability, of accomplishing this, with the
extreme precision which is indispensable, by any process of hand-
drawing, will be apparent ; and if the stereoscope were dependent
on such a process, the most remarkable effects manifested by it
would never have been witnessed. Fortunately, however, contem-
poraneously with this beautiful optical invention, another, still
more remarkable, was in progress of improvement. Photography
lent its powerful aid to the stereoscope, and supplied an easy and
perfectly accurate and efficient means of producing the right and
left monocular pictures. If two lines be imagined to be drawn
from the object inclined to each other at the angle which measures
theproposedbinocularparallax, two photographic instruments placed
one on each of these lines, at the proper distance from the object,
will produce the two desired pictures ; or the same instrument
would do so, placed successively in the directions of the two lines.

The stereoscopic pictures are accordingly produced by this
method either upon daguerreotype plates, photographic paper, or
glass. On daguerreotype plates they are necessarily opaque ; on
glass they are transparent ; and on paper may be either opaque or
transparent, according to the thickness and quality of the paper.

10. Since the greater number of stereoscopic pictures represent
views of objects which must be so distant from the observer as to
have no sensible binocular parallax, it may be asked how it is
that stereoscopic effects, so remarkable as those which are mani-
fested by such pictures, can be produced. If the stereoscopic
effects be the consequences of binocular parallax, and of that
alone, how can such effects be produced by pictures of objects,
which have no such parallax ?

This brings us back to a statement made in the commence-
ment of this notice, that the appearance of perspective and relief
produced by the stereoscope is, in most cases, exaggerated, as
compared with that produced by an immediate view of the objects
themselves, and that it is consequently such as can never be
perceived when the objects themselves are looked at ; and that
hence arises the sensation of surprise that such stereoscopic effects
never fail to excite.

If we desire to obtain a pair of stereoscopic pictures of any
object of considerable magnitude, a palace or a cathedral, for
example, we take a position at such a distance from it as will
enable us to obtain, in the camera obscura of the photographic
apparatus, a picture of it on a sufficiently small scale. Supposing,
then, two lines to be drawn from the centre of the object to the
place selected for the camera, making with each other an angle
equal to the amount of binocular parallax, which is necessary to
produce the stereoscopic effect of perspective and relief; let two

143

THE STEREOSCOPE.

photographic instruments be then placed one on each of these
lines, with their optic axes in the directions of the lines respec-
tively, and therefore converging towards the same point of the
object, and let the distances of their object glasses from that point
be equal. The optical pictures which they will produce will in
that case be those which would be seen by two eyes, right and
left, having a distance apart equal to the distance between the
object glasses of the two photographic instruments.

When the pictures are thus produced on a small scale they are
placed in the stereoscope, the eye-glasses of which will have the
effect of causing them to be viewed in lines converging at the same
angle, as that formed by the optic axes of the two photographic
instruments by which the pictures were produced.

11. It will be manifest, then, that the impression produced by
the view of such pictures in the stereoscope will be such, as could
never be produced by the immediate view of the objects them-
selves, inasmuch as they could never be seen with any such
degree of binocular parallax, as that which has been given to
them by the relative position of the two photographic instruments.
This parallax will be greater than the natural binocular parallax
of the object, in the same proportion as the distance between the
centres of the object glasses of the two photographic instruments,
is greater than the distance between the eyes. Thus if, in taking
such a pair of stereoscopic views of a building, the distance
between the photographic instruments is 50 inches, the parallax
thus produced will be greater than the natural binocular parallax
in the proportion of 50 to 2£, or 20 to 1, and so far as the percep-
tion of perspective and relief depends on binocular parallax, that
which is produced from viewing the pictures of the building in
the stereoscope, will be 20 times more strong and vivid than that
which is produced by the view of the building itself, seen from the
station at which the pictures are taken.

It is then rigorously true, that the surprise and admiration
excited by the stereoscope, does not arise from the truth of the
picture which it presents, but from the strong exaggeration of
perspective and relief which it exhibits. It is very true that no
art of the draughtsman or painter could produce any such effects :
but it is equally true that no such effects could be produced by
the objects themselves.

Among the most interesting and instructive as well as sur-
prising effects of the stereoscope, are those which it exhibits when
stereoscopic views of geometrical solid figures are exhibited in it.
The variety of these is endless. But since no mere verbal de-
scription could convey any adequate idea of them, we can only
invite the reader's attention to this class of objects.
144

CHAPTER I.

I. COMETARY ORBITS : 1. Prescience of the astronomer. — 2. Strikingly
illustrated by cometary discovery. — 3. Motion of comets explained by
gravitation. — 4. Conditions imposed on the orbits of bodies which
are subject to the attraction of gravitation. — 5. Elliptic orbit. — 6.
Parabolic orbits. — 7. Hyperbolic orbits. — 8. Planets observe in their
motions order not exacted by the law of gravitation. — 9. Comets
observe no such order in their motions. — 10. They move in conic
sections, with the sun for the focus. — 11. Difficulty of ascertaining in
what species of conic section a comet moves. — 12. Hyperbolic and
parabolic comets not periodic. — 13. Elliptic comets periodic like the
planets. — 14. Difficulties attending the analysis of cometary motions.
— 15. Periodicity alone proves the elliptic character. — 16. Periodicity

LAKDNER'S MUSEUM OF SCIENCE. L 145

COMETS.

combined with the identity of the paths while visible establishes
identity. — 17. Many comets recorded— few observed. — 18. Classifica-
tion of the cometary orbits. — II. ELLIPTIC COMETS REVOLVING WITHIN
THE ORBIT OF SATURN : 19. Encke's comet. — 20. Table of the elements
of the orbit. — 21. Indications of the effects of a resisting medium. —
22. The luminiferous etter would produce such an effect. — 23. Comets
would ultimately fall into the sun.

I. — COMETAHY OUBTTS.

1. FOR the civil and political historian the past alone has
existence — the present he rarely apprehends ; the future never.
To the historian of science it is permitted, however, to penetrate
the depths of past and future with equal clearness and certainty :
facts to come are to him as present, and not unfrequently more
assured than facts which are passed. Although this clear per-
ception of causes and consequences characterises the whole domain
of physical science, and clothes the natural philosopher with
powers denied to the political and moral inquirer, yet foreknow-
ledge is eminently the privilege of the astronomer. Nature has
raised the curtain of futurity, and displayed before him the suc-
cession of her decrees, so far as they affect the physical universe,
for countless ages to come ; and the revelations of which she has
made him the instrument, are supported and verified by a never-
ceasing train of predictions fulfilled. He " shows us the things
which will be hereafter," not obscurely shadowed out in figures
and in parables, as must necessarily be the case with other reve-
lations, but attended with the most minute precision of time,
place, and circumstance. He converts the hours as they roll into
an ever-present miracle, in attestation of those laws which his
Creator through him has unfolded ; the sun cannot rise — the moon
cannot wane — a star cannot twinkle in the firmament, without
bearing witness to the truth of his prophetic records. It has
pleased the " Lord and Governor" of the world, in his inscrutable
wisdom, to baffle our inquiries into the nature and proximate
cause of that wonderful faculty of intellect — that image of his
own essence which he has conferred upon us ; nay, the springs
and wheelwork of animal and vegetable vitality are concealed
from our view by an impenetrable veil, and the pride 'of philo-
sophy is humbled by the spectacle of the physiologist bending in
fruitless ardour over the dissection of the human brain, and
peering in equally unproductive inquiry over the gambols of an
animalcule. But how nobly is the darkness which envelopes
metaphysical inquiries compensated by the flood of light which is
shed upon the physical creation ! There all is harmony, and
order, and majesty, and beauty. From the chaos of social and
political phenomena exhibited in human records- — phenomena
146

COMETARY DISCOVERY.

-unconnected to our imperfect vision by any discoverable law, a
war of passions and prejudices, governed by no apparent purpose,
tending to no apparent end, and setting all intelligible order at
defiance — how soothing and yet how elevating it is to turn to the
splendid spectacle which offers itself to the habitual contemplation
of the astronomer ! How favourable to the development of all
the best and highest feelings of the soul are such objects ! the
•only passion they inspire being the love of truth, and the chiefest
pleasure of their votaries arising from excursions through the
imposing scenery of the universe — scenery on a scale of grandeur
;and magnificence, compared with which whatever we are accus-
tomed to call sublimity on our planet dwindles into ridiculous
insignificancy. Most justly has it been said, that nature has
implanted in our bosoms a craving after the discovery of truth,
and assuredly that glorious instinct is never more irresistibly
awakened than when our notice is directed to what is going on in
-the heavens. " Quoniam eadem Natura cupiditatem ingenuit
hominibus veri inveniendi, quod facillime apparet, cum vacui curis,
etiam quid in ccelo fiat, scire avemus ; his initiis inducti omnia
vera diligimus ; id est, fidelia, simplicia, constantia ; turn vana,
rfalsa, fallentia odimus." *

2. Such reflections are awakened by every branch of astronomy,
'but by none so strongly as by the history of cometary discovery.
No where can be found so marvellous a series of phenomena
:foretold. The interval between the prediction and its fulfilment
has sometimes exceeded the limits of human life, and one gene-
ration has bequeathed its predictions to another, which has been
filledjWith astonishment and admiration at witnessing their literal
accomplishment.

3. In the vast framework of the theory of gravitation constructed
:by Newton, places were provided for the arrangement and ex-
position not only of all the astronomical phenomena which the
observation of all preceding generations had supplied, but also for
a far greater mass which the more fertile and active research of
the generations which succeeded him have furnished. By this
theory all the known planetary motions were explained, and
planets previously unseen were felt by their effects, their places
ascertained, and the telescope of the observer guided to them.

But transcendently the greatest triumph of this celebrated
theory was the exposition it supplied of the physical laws which
govern the motions of comets as distinguished from those which
prevail among the planets.

.4. It is proved in the propositions demonstrated in the first

•* Gic. de Fin. Bon. et Mai.., ii. 14,

L 2 147

COMETS.

book of Newton's Principia, which propositions form in substance
the ground -work of the entire theory of gravitation, that a body
which is under the influence of a central force, the intensity of
which decreases as the square of the distance increases, must move
in one or other of the curves known to geometers as the " CONIC
SECTIONS," being those w^ch are formed by the intersection of
the surface of a cone by a plane, and that the centre of attraction
must be in the FOCUS of the curve ; and in order to prove that
such curves are compatible with no other law of attraction, it is
further demonstrated that whenever a body is observed to move
round a centre of attraction in any one of these curves, that centre
being its focus, the law of the attraction will be that of gravitation ;
that is to say, its intensity will vary in the inverse proportion of
the square of the distance of the moving body from the centre of
force.

Subject to these limitations, however, a body may move round
the sun in any orbit, at any distance, in any plane, and in any
direction whatever. It may describe an ellipse of any eccentricity,
from a perfect circle to the most elongated oval. This ellipse
may be in any plane, from that of the ecliptic to one at right
angles to it, and the body may move in such ellipses either in the
same direction as the earth or in the contrary direction. Or the
body thus subject to solar attraction may move in a parabola with
its point of perihelion at any distance whatever from the sun,
either grazing its very surface or sweeping beyond the orbit
of Neptune, or, in fine, it may sweep round the sun in an
hyperbola, entering and leaving the system in two divergent
directions.

To render these explanations, which are of the greatest interest
and importance in relation to the subject of comets, more clearly
understood, we have represented, in fig. 1, the forms of a very
eccentric ellipse, aba' b', a parabola a p p\ and an hyperbola
a h h'y having s as their common focus, and it will be convenient
to explain in the first instance the relative magnitude of some
important lines and distances connected with these orbits.

5. Ellipses or ovals vary without limit in their eccentricity.
A circle is regarded as an ellipse whose eccentricity is nothing.
The orbits of the planets generally are ellipses, but having
eccentricities so small that, if described on a large scale in
their proper proportions on paper, they would be distinguish-
able from circles only by measuring accurately the dimensions
taken in different directions, and thus ascertaining that they are
longer in a certain direction than in another at right angles to it.
A very eccentric and oblong ellipse is delineated in fig. 1, of which
a a' is the major axis. The focus being s, the perihelion distance
148

ELLIPTIC AND PARABOLIC ORBITS.

Fig. i.

d is 5 a, and the aphelion distance d' is s a', the mean distance a
being s c, or half the major axis.

The curvature of the ellipse continually increases from the mean
distance to perihelion, and constantly decreases from perihelion
to the mean distance, being
equal at equal angular dis-
tances from perihelion as seen
from the sun.

It is evident that if a body
move in a very eccentric
ellipse, such as that repre-
sented in fig. 1, whose plane
coincides exactly or nearly
with the common plane of the
planetary orbits, it may in-
tersect the orbits of several
or all of the planets, as it is
represented to do in the figure,
although its mean distance
from the sun maybe less than
the mean distance of several
of those which it thus inter-
sects. The aphelion distance
of such a body may, there-
fore, greatly exceed that of
any planet ; while its mean
distance may be less than
that of the more distant
planets.

6. The form of a parabolic
orbit having the same peri-
helion distance as the elliptic
orbit is represented at a p p,
in fig. 1. This orbit consists
of two indefinite branches,
similar in form, which unite
at perihelion a. Departing
from this point on opposite
sides of the axis a a', their curvature regularly and rapidly
decreases, being equal at equal distances from perihelion. The
two branches have a constant tendency to assume the direction
and form of two straight lines parallel to the axis a a'. To
actual parallelism, and still less to convergence, these branches,
however, never attain, and consequently they can never reunite.
They extend, in fine, like parallel straight lines, to an unlimited

149

COMETS.

distance, without ever reuniting, but assuming directions when*
the distance from the focus bears a high ratio to the peri-
helion distance, which are practically undistinguishable from
parallelism.

One parabolic orbit differs from another in its perihelion dis-
tance. The less this distance is, the less will be the separation at
a given distance from s between the parallel directions to which
the indefinite branches p p' tend. This distance may have any
magnitude. The body in its perihelion may graze the surface of
the sun, or may pass at a distance from it greater than that of the
most remote of the planets, so that, although it be subject to solar
attraction, it would in that case never enter within the limits of
the solar system at all.

A body moving in such an orbit, therefore, would not make,
like one which moves in an ellipse, a succession of revolutions-
round the sun ; nor can the term periodic time be applied at all
to its motion. It enters the system in some definite direction,
such asp' p, as indicated by the arrow from an indefinite distance.
Arriving within the sensible influence of solar gravitation, the
effects of this attraction are manifested in the curvation of its path,
which gradually increases as its distance from the sun decreases,
until it arrives at perihelion, where the attractive force, and con-
sequently the curvature, attain their maxima. The extreme
velocity which the body attains at this point produces, in virtue-
of the inertia of the moving mass, a centrifugal force, which
counteracts the gravitation, and the body, after passing perihelion,
begins to retreat ; the solar gravitation and the curvature of its-
path decreasing together, until it issues from the system in a
direction p p't as indicated by the arrows, which is nearly a straight
line, and parallel to that in which it entered. In such an orbit a
body therefore visits the system but once. It enters in a certain
Direction from an indefinite distance, and, passing through its
perihelion, issues in a parallel direction, passing to an unlimited
distance, never to return.

7. Hyperbolic orbits, like the parabolas, consist of two inde-
finite branches, which unite at perihelion, which at equal dis-
tances from perihelion have equal curvatures, and which, as the
distance from perihelion increases, approach indefinitely in direc-
tion and form to straight lines, but, unlike the parabolic orbits,
the straight lines to whose direction the two branches approximate
are divergent and not parallel.

Such an orbit having the same perihelion distance as the ellipse
and parabola, is represented by a h h', fig. 1.

The parabola is included between the ellipse and hyperbola.

8. When the theory of gravitation was first propounded by its.
150

HYPERBOLIC ORBITS.

illustrious author, no other bodies, save the planets and satellites
then discovered, were known to move under the influence of such
a central attraction. These bodies, however, supplied no example
of the play of that celebrated theory in its full latitude. They
obeyed, it is true, its laws, but they did much more. They dis^
played a degree of harmony and order far exceeding what the law
of gravitation exacted. Permitted by that law to move in any of
the three classes of conic sections, their paths were exclusively
elliptical ; permitted to move in ellipses infinitely various in their
eccentricities, they moved exclusively in such as differed almost
insensibly from circles ; permitted to move at distances subordi-
nated to no regular law, they moved in a series of orbits at dis-
tances increasing in a regular progression ; permitted to move at
all conceivable angles with the plane of the ecliptic, their paths
are inclined to it at angles limited in general to a few degrees ;
permitted, in fine, to move in either direction, they all agreed in
moving in the direction in which the earth moves in its annual
course.

Accordance so wondrous, and order so admirable, could not be
fortuitous, and, not being enjoined by the conditions of the law
of gravitation, must either be ascribed to the immediate dictates
of the Omnipotent Architect of the universe above all general
laws, or to some general laws superinduced upon gravitation,
which had escaped the sagacity of the discoverer of that principle.
If the former supposition were adopted, some bodies, different in
their physical characters from the planets, primary and secondary,
playing different parts and fulfilling different functions in the
economy of the universe, might still be found, which would illus-
trate the play of gravitation in its full latitude, sweeping round
the sun in all forms of orbit, eccentric, parabolic, and hyperbolic,
in all planes, at all distances, and indifferently in both directions.
If the latter supposition were accepted, then no other orbit, save
ellipses of small eccentricity, with planes coinciding nearly with
that of the ecliptic, would be physically possible.

9. The theory of gravitation had not long been promulgated,
nor as yet been generally accepted, when the means of its further
verification were sought in the motion of comets. Hitherto these
bodies had been regarded as exceptional and abnormal, and as
being exempt altogether from the operation of the law and order
which prevailed in a manner so striking among the members of
the solar system. So little attention had been given to comets
that it had not been certainly ascertained whether they were to be
classed as meteoric or cosmical phenomena ; whether their theatre
was the regions of the atmosphere, or the vast spaces in which the
great bodies of the universe move. Their apparent positions in

151

COMETS.

the heavens on various occasions of the appearances of the most
conspicuous of them had nevertheless been from time to time for
some centuries observed and recorded with such a degree of pre-
cision as the existing state of astronomical science permitted ; and
even when their places were not astronomically ascertained, the
date of their appearanc^ was generally preserved in the historic
records, and in many cases the constellations through which they
passed were indicated, so that the means of obtaining at least a
rude approximation to their position in the firmament were thus
supplied.

10. Such observations, vague, scattered, and inexact as they
were, supplied, however, data by which, in several cases, it was
possible to compute the real motion of these bodies through space,
their positions in relation to the sun, the earth, and the planets,
and the paths they followed in moving through the system, with
sufficiently approximate accuracy to conclude with certainty that
they were one or other of the conic sections, the place of the sun
being the focus.

This was sufficient to bring these bodies under the general
operation of the attraction of gravitation.

It still remained, however, to determine more exactly the
specific character of these orbits. Are they ellipses more or less
eccentric ? or parabolas ? or hyperbolas ? — Any of the three classes
of orbits would, as has been shown, be equally compatible with
the law of gravitation.

11. It might be supposed that the same course of observation
as that by which the orbit of a planet is traced would be applicable
equally to comets. Many circumstances, however, attend this
latter class of bodies, which render such observations impossible,
and compel the astronomer to resort to other means to determine
their orbits.

A spectator stationed upon the earth keeps within his view each
of the other planets of the system throughout nearly the whole of
its course. Indeed, there is no part of the orbit of any planet in
which, at some time or other, it may not be seen from the earth.
Every point of the path of each planet can therefore be observed ;
and, although without waiting for such observation, its course
might be determined, yet it is material here to attend to the fact,
that the whole orbit may be submitted to direct observation. The
different planets, also, present peculiar features by which each
may be distinguished. Thus, as has been explained, they are
observed to be spherical bodies of various magnitudes. Their
surfaces are marked by peculiar modes of light and shade, which,
although variable and shifting, still, in each case, possess some
prevailing and permanent characters by which the identity of the
152

COMETARY ORBITS DETERMINED.

object may be established, even were there no other means of
determining it.

Unlike planets, comets do not present to us those individual
characters above mentioned, by which their identity may be
determined. None of them have been satisfactorily ascertained
to be spherical bodies, nor indeed to have any definite shape.
It is certain that many of them possess no solid matter, but are
masses consisting of some nearly transparent substances ; others
are so surrounded with this apparently vaporous matter, that it
is impossible, by any means of observation which we possess, to
discover whether this vapour enshrouds within it any solid mass.
The same vapour which thus envelopes the body (if such there
be within it) also conceals from us its features and individual
character. Even the limits of the vapour itself, if vapour it be,
are subject to great change in each individual comet. Within a
few days they are sometimes observed to increase or diminish
some hundred- fold. A comet appearing at distant intervals
presents, therefore, no very obvious means of recognition. A
like extent of surrounding vapour would evidently be a fallible
test of identity ; and not less inconclusive would it be to infer
diversity from a different extent of nebulosity.

If a comet, like a planet, revolved round the sun in an orbit
nearly circular, it might be seen in every part of its path, and
its identity might thus be established independently of any
peculiar characters in its appearance. But such is not the course
which comets are observed to take.

In general a comet is visible only throughout an arc of its
orbit, which extends to a certain limited distance on each side
of its perihelion. It first becomes apparent at some point of its
path, such as g, g' or g", fig. 1 ; it approaches the sun and dis-
appears after it passes a corresponding point //, g' or g" in departing
from the sun. The arc of its orbit in which alone it is visible
would therefore be g a g, g' a g', or g" a g".

If this arc, extending on either side of perihelion, could always
be observed with the same precision as are the planetary orbits,
it would be possible, by the properties of the conic sections, to
determine not only the general character of the orbit, whether it
be an ellipse, or parabola, or an hyperbola, but even to ascertain
the individual curve of the one kind or the other in which the
comet moves, so that the course it followed before it became visible,
as well as that which it pursues after it ceases to be visible, would
be as certainly and precisely known as if it could be traced by
direct observation throughout its entire orbit.

12. If it be ascertained that the arc in which the comet moves
while it is visible is part of an hyperbola, such as g a g, it will be

153

COMETS.

inferred that the comet coming from some indefinitely distant
region of the universe, has entered the system in a certain direction r
h' A, which can be inferred from the visible arc g a g, and that it
must depart to another indefinitely distant region of the universe
following the direction h A', which is also ascertained from the
visible arc gag.

If, on the other hand, it be ascertained that the visible arc,
such as g' a g', be part of a parabola, then, in like manner by the
properties of that curve, it will follow that it entered the system
coming from an indefinitely distant region of the universe in a
certain direction, p' p, which can be inferred from the visible
arcg' ag', and that after it ceases to be visible, it will issue from
the system in another determinate direction, p p', parallel to that
by which it entered.

The comet, in neither of these cases, would have a periodic
character. It would be analogous to one of those occasional
meteors which are seen to shoot across the firmament never again
to reappear. The body arriving from some distant region, and
coming, as would appear, fortuitously within the solar attraction,
is drawn from its course into the hyperbolic or parabolic path,
which it is seen to pursue, and escapes from the solar attraction,
issuing from the system never to return. The phenomenon would
in each case be occasional, and, in a certain sense,' accidental,
and the body could not be said properly to belong to the system.
So far as relates to the comet itself, the phenomenon would consist
in a change of the direction of its course through the universe,
operated by the temporary action of solar gravitation upon it.

13. But the case is very different, the tie between the comet
and the system much more intimate, and the interest and physical
importance of the body transcendently greater when the arc, such
as g" a g", proves to be part of an ellipse. In that case, the invisible
part of the orbit being inferred from the visible, the major axis
a a' would be known. The comet would possess the periodic
character, making successive revolutions like the planets, and
returning to perihelion a after the lapse of its proper periodic
time, which could be inferred by the harmonic law from the
magnitude of its major axis.

Such a body would then not be, like those which follow hyper-
bolic or parabolic paths, an occasional visit to the system, con-
nected with it by no permanent relation, and subject to solar
gravitation only accidentally and temporarily. It would, on the
contrary, be as permanent, if not as strictly regular, a member of
the system as any of the planets, though invested, as will pre-
sently appear, with an extremely different physical character.

It will therefore be easily conceived with what profound interest
154

SH
it n

PERIODIC COMETS.

comets were regarded before the theory of gravitation had been
yet firmly established or generally accepted, and while it was, so
to speak, upon its trial. These bodies were, in fact, looked for as
the witnesses whose testimony must decide its fate.

14. Difficulties, however, which seemed almost insurmountable,
opposed themselves to a satisfactory and conclusive analysis of
their motions. Many causes rendered the observations upon their
apparent places few in number and deficient in precision. The
arcs fj a g, g' a y', and g" a g" of the three classes of orbit in
any of which they might move without any violation of the law
of gravitation were very nearly coincident in the neighbourhood
of the place of perihelion a. It was, for example, in almost all
the cases which presented themselves, possible to conceive three
different curves, an eccentric ellipse, such as a b a' b', a parabola,
such as p' p a, and an hyperbola, such as hf h a, so related that
the arcs gag, g' a g', and g" a g", would not deviate one from
another to an extent exceeding the errors inevitable in cometary
observations. Thus any one of the three curves within the limits of
the visible path of the comet might with equal fidelity represent
its course. In such cases, therefore, it was impossible to infer,
from the observations alone, whether the comet belonged to the
class of hyperbolic or parabolic bodies, which have no periodic
character, or to the elliptic, which has.

15. The character of periodicity itself, which belongs exclusively
to elliptic orbits, supplied the means of surmounting this diffi-
culty. If any observed comet have an elliptic motion, it must
return to perihelion after completing its revolution, and it must
have been visible on former returns to that position. Not only
ought it to be expected, therefore, that such a comet would re-
appear in future after absences of equal duration (depending on
its periodic time), but that its previous returns to perihelion
would be found by searching among the recorded appearances of
such objects for any, the dates of whose appearance might cor-
respond with the supposed period, and whose apparent motions, if
observed, might indicate a real motion in an orbit, identical or
nearly so with that of the comet in question.

If the motion of such a body were not affected by any other
force except the solar attraction, it would re-appear after each
successive revolution at exactly the same point; would follow,
while visible, exactly the same arc g" a g" ; would move in the
same plane, inclined at the same angle to the ecliptic, the nodes-
retaining the same places ; and would arrive at its perihelion at
exactly the same point a, and after exactly equal intervals.

Now, although the disturbing actions of the planets near which
it might pass, in departing from and returning to the sun, must

155

COMETS.

be expected to he much more considerable than when one planet
acts upon another, as well because of the extreme comparative
lightness of the comet, as of the great eccentricity of its orbit,
which sometimes actually or nearly intersects the paths of several
planets, and especially those of the larger ones, yet still such
planetary attractions jire only disturbances, and cannot be sup-
posed to efface that character which the orbit receives from the
predominant force of the immense mass of the sun. . While there-
fore we may be prepared for the possibility, and even the pro-
bability, that the same periodic comet on the occasion of its
successive re-appearances, may follow a path g" a g" in passing
to and from its perihelion, differing to some extent from that
which it had followed on previous appearances, yet in the main
such differences cannot, except in rare and exceptional cases, be
very considerable, and for the same reason the intervals between
its successive periods, though they may differ, cannot be subject
to any very great variation.

16. If then, on examining the various comets whose appearances
have been recorded, and whose places while visible have been
observed, and on computing from the apparent places the arc of
the orbit through which they moved, it be found that two or more
of them, while invisible, moved in the same path, the presumption
will be that these were the same body re-appearing after having
completed its motion in an elliptic orbit ; nor should this pre-
sumption of identity be hastily rejected because of the existence
of any discrepancies between the observed paths, or any inequality
of the intervals between its successive re-appearances, so long
as such discrepancies can fairly be ascribed to the possible
disturbances produced by planets which the comet might have
encountered in its path.

17. Many comets, however, have been recorded , but not observed.
Historians have mentioned, and even described, their appearances,
and in some cases have indicated the chief constellations through
which such bodies passed, although no observations of their appa-
rent places have been transmitted by which any close approximation
to their actual paths could be made. Nevertheless, even in these
cases, some clue to their identification is supplied. The intervals
between their appearances alone is a highly probable test of
identity. Thus if comets were regularly recorded to have
appeared at intervals of fifty years (no circumstance affording
evidence of the diversity of these objects), they might be assumed,
with a high degree of probability, to be the successive returns of
an elliptic comet having that interval as its period.

18. The appearances of about 400 comets had been recorded in
the annals of various countries before the end of the seventeenth

156

COMETS INCLUDED WITHIN SATURN'S ORBIT,

century, the epoch signalised by the discoveries and researches of
Newton. In most cases, however, the only circumstance recorded
was the appearance of the object, accompanied in many instances
with details bearing evident marks of exaggeration respecting its
magnitude, form, and splendour. In some few cases, the con-
stellations through which the object passed successively, with the
necessary dates, are mentioned, and in some, fewer still, obser-
vations of a rough kind have been handed down. From such
scanty data, eagerly sought for in the works preserved in different
countries, sufficient materials have been collected for the com-
putation, with more or less approximation, of the elements of the
orbits of about sixty of the 400 comets above mentioned.

Since the time of Newton, Halley, and their contemporaries,
observers have been more active, and have had the command of
instruments of considerable and constantly increasing power ; so
that every comet which has been visible from the northern hemi-
sphere of the earth since that time, has been observed with
continually increasing precision, and data have been in all cases
obtained, by which the elements of the orbits have been calculated.
Since the year 1700, accordingly, about 140 have been observed,
the elements of the orbits of which have been ascertained with
great precision.

It appears, therefore, that of the entire number of comets which
have appeared in the firmament, the orbits of about 200 have
been ascertained. Of this number forty have been ascertained,
some conclusively, others with more or less probability, to revolve
in elliptic orbits.

Seven have passed through the system in hyperbolas, and con-
sequently will not visit it again, unless they be thrown into other
orbits by some disturbing force.

One hundred and sixty have passed through the system either
in parabolic orbits, or in ellipses of such extreme eccentricity as
to be undistinguishable from parabolas by any data supplied by
the observations.

II. — ELLIPTIC COMETS REVOLVING WITHIN THE ORBIT OF
SATURN.

19. In 1818, a comet was observed at Marseilles, on the 26th
of November, by M. Pons. In the following January, its path
being calculated, M. Arago immediately recognised it as identical
with one which had appeared in 1805. Subsequently, M. Encke
of Berlin succeeded in calculating its entire orbit — inferring the
invisible from the visible part — and found that its period was

157

COMETS.

about 1200 days. This calculation was verified by the fact of its
return in 1822, since which time the comet has gone by the name
of Enckes comet, and returned regularly.

It may be asked, How it could have happened that a comet
which made its revolution in a period so short as three years and
a quarter, should not have been observed until so recent an epoch
as 1818 ? This is expfained by the fact that the comet is so small,
and its light so feeble even when in the most favourable position,
that it can only be seen with the aid of the telescope, and not
even with this except under certain conditions which are not ful-
iilled on the occasion of every perihelion passage. Nevertheless,
the comet was observed on three former occasions, and the general
elements of its path recorded, although its elliptic, and con-
sequently periodic character, was not recognised.

On comparing, however, the elements then observed with those
of the comet now ascertained, no doubt can be entertained of their
identity.

20. In the following table are given the elements of the orbit
of this comet, as computed from the observations made upon it at
each of its three appearances in 1786, 1795, and 1805, before its
periodic character was discovered, and at its eleven subsequent
appearances up to 1852.

TABLE I.

Elements of the Orbit of Encke's Comet to 1852.

Mean

Peri-

Longitude

Longitude

Distance,
Earth's

Eccen-
tricity.

helion
Distance.

Aphelion
Distance.

Perihelion.

of
Ascending

Inclination.

= 1.

Node.

Time of Perihelion

d'=ax

d"=ax

Passage.

a

c

(l-«)

(1+0

!7

'

i

o / ,,

i n

, ,t

h. m.

1786

2-2080

0-8484

0-3348

4-0812

156 38 0

334 8 0

13 36 0

Jan. 30. 21 7

1795

2-2130

0-8489

0-3344

4-0916

156 41 20

334 39 22

13 42 30

Dec. 21. 10 44

1805

2-2131

0-8462

0-3404

4-0860

156 47 24

334 20 10

13 33 30

Nov. 21. 12 9

1819

2-2141

0-8486

0-3353

4-0929

156 59 12

334 33 19

13 36 54

Jan. 27. 6 18

18-22

2-2244

0-8445

0-3460

4.1028

157 11 44

334 25 9

13 20 17

May 23. 23 16

1825

2-2233

0-8449

0-3449

4-1017

157 14 31

334 27 30

13 21 24

Sept. 16. 6 43

18-29

2-2239

0-8446

0-3455

4-1023

157 17 53

334 29 32

13 20 34

Jan. 9. 18 3

1S32

22219

0-8454

0-3435

4-1003

157 21 1

334 32 9

13 22 9

May 23. 23 34

1835

2-2227

0-8450

0-3444

4-1010

157 23 29

334 34 59

13 21 15

Aug. 26. 8 49

1838

2-2222

0-8452

0-3440

4-1004

157 27 4

334 36 41

13 21 28

Dec. 19. 0 27

1842

2-2229

0-8448

0-3450

4-0998

157 29 27

334 39 10

13 20 26

April 12. 0 35

1845

2-2215

0-8474

0.3381

4-1049

157 44 21

334 19 34

13 7 34

Aug. 9. 15 11

1848

2-2147

0-8478

0-3371

4-0923

157 47 8

334 22 12

Nov. 26. 3+0

1852

2-2152

0-8477

0-3374

4-0930

157 51 2

334 23 21

13 7 55

March 14. 20

M. T. B.

The motion of this comet is direct ; and its period in 1852 was
3-29616 years, which is subject to a slight variation.
It is evident that between 1786 and 1795 there were two,
153

ENCKE'S COMET.

between 1795 and 180,3 two, and, in fine, between 1805 and 1819
three, unobserved returns to perihelion.

It appears, therefore, that, excepting the oval form of the orbit,
the motion of this body differs in nothing from that of a planet
whose mean distance from the sun is that of the nearest of the
planetoids. Its eccentricity is such, however, that when in
perihelion it is within the orbit of Mercury, and when in aphelion
it is outside the most distant of the planetoids, and at a distance
from the sun equal to four-fifths of that of Jupiter.

21. A fact altogether anomalous in the motions of the bodies of
the solar system, and indicating a consequence of the highest
physical importance, has been disclosed in the observation of the
motion of this comet. It has been found that its periodic time,
and consequently its mean distance, undergoes a slow, gradual,
and apparently regular decrease. The decrease is small, but not
at all uncertain. It amounted to about a day in ten revolutions,
a quantity which could not by any means be placed to the account
either of errors of observation or of calculation ; and, besides,
this increase is incessant, whereas errors would affect the result
sometimes one way and sometimes the other. The period of the
comet between 1786 and 1795 was 12081 days; between 1795 and
1805 it was 1207& days ; between 1805 and 1819 it was 1207T4S
days ; in 1845 it was 1205^ days ; and, in fine, in 1852 it was
1204 days.

The magnitude of the orbit thus constantly decreasing (for the
cube of its greater axis must decrease in the same proportion as
the square of the period), the actual path followed by the comet
must be a sort of elliptic spiral, the successive coils of which are
very close together, every successive revolution bringing the
comet nearer and nearer to the sun.

Such a motion could not arise from the disturbing action of the
planets. These forces have been taken strictly into account in
the computation of the ephemerides of the comet, and there is
still found this residual phenomenon, which cannot be placed to
their account, but which is exactly the effect which would arise
from any physical agency by which the tangential motion of the
comet would be feebly but constantly resisted. Such an agency,
by diminishing the tangential velocity, would give increased
efficacy to the solar attraction, and, consequently, increased cur-
vature to the comet's path ; so that, after each revolution, it
would revolve at a less distance from the centre of attraction.

22. It is evident that a resisting medium, such as the lumini-
ferous ether * is assumed to be in the hypothesis which forms

* See "Hand-Book of Astronomy," § 1225.

159

COMETS.

the basis of the undulatory theory of light, would produce just
such a phenomenon, and accordingly the motion of this comet
is regarded as a strong evidence tending to convert that hypo-
thetical fluid into a real physical agent.

It remains to be seen whether a like phenomenon will be
developed in the motion of other periodic comets. The discovery
of these bodies, and the observation of their motions, are as yet
too recent to enable astronomers, notwithstanding their greatly
multiplied number, to pronounce decisively upon it.

23. If the existence of this resisting medium should be estab-
lished by its observed effects on comets in general, it will follow,
that, after the lapse of a certain time (many ages, it is true, but
still a definite interval), the comets will be successively absorbed
by the sun, unless, as is not improbable, they should be previously
vaporised by their near approach to the solar fires, and should
thus be incorporated with his atmosphere.

In the efforts by which the human mind labours after truth, it
is curious to observe how often that desired object is stumbled upon
by accident, or arrived at by reasoning which is false. One of
Newton's conjectures respecting comets was, that they are "the
aliment by which suns are sustained ; " and he therefore con-
cluded that these bodies were in a state of progressive decline
upon the suns, round which they respectively swept; and that
into these suns they from time to time fell. This opinion appears-
to have been cherished by Newton to the latest hours of his life :
he not only consigned it to his immortal writings, but, at the age
of eighty-three, a conversation took place between him and his
nephew on this subject, which has come down to us. " I cannot
say," said Newton, "when the comet of 1680 will fall into the
sun ; possibly after five or six revolutions ; but whenever that
time shall arrive, the heat of the sun will be raised by it to such
a point, that our globe will be burnt, and all the animals upon it
will perish. The new stars observed by Hipparchus, Tycho, and
Kepler, must have proceeded from such a cause, for it is impos-
sible otherwise to explain their sudden splendour." His nephew
then asked him, " Why, when he stated in his writings that
comets would fall into the sun, did he not also state those vast fires
they must produce, as he supposed they had done in the stars ? "
— "Because," replied the old man, "the conflagrations of the
sun concern us a little more directly. I have said, however,"
added he, smiling, " enough to enable the world to collect my
opinion."

160

Fig. 19.— Jan. 24, 1836.

Fig. 21.— Jan. 26, 1S36.

Fig. 20.— Jan. '25, 1836. Fig. 22.— Jan. 27, 1S36.

HALLEY'S COMET DEPARTING FROM THE SUN IN 1836.

COMETS.

CHAPTEE II.

24. Why like effects are not manifested in the motion of the planets. — 25.
Corrected estimate of the mass of Mercury.— 26. Biela's comet. — 27. Pos-
sibility of the collision of Biela's comet with the earth. — 28. Resolution
of Biela's comet into two. — 29. Changes of appearance attending the
separation. — 30. Faye's comet. — 31. Reappearance in 1850-1 calculated
by M. Le Verrier. — 32. De Vice's comet. — 33. Brorsen's comet. — 34.
D' Arrest's comet. — 35. Elliptic comet of 1743. — 36. Elliptic comet
of 1766. — 37. Lexell's comet. — 38. Analysis of Laplace applied to
Lexell's comet.— 39. Its orbit before 1767 and after 1770 calculated
by his formulae. — 40. Revision of these researches by M. Le Verrier.
— 41. Process by which the identification of periodic comets may be
decided. — 42. Application of this process by M. Le Verrier to the
comets of Faye, Be Vico, and Brorsen, and that of Lexell — their
diversity proved. — 43. Probable identity of De Vico's comet with the

LARDNER'S MUSEUM OP SCIENCE. M 161

No. 138.

COMETS.

comet of 1678. — 44. Blainplan's comet of 1819. — 45. Pons's comet of
1819.— 46. Pigott's comet of 1783.— 47. Peters's comet of 1846.—
48. Tabular synopsis of the orbits of the comets which revolve within
Saturn's orbit. — 49. Diagram of the orbits. — 50. Planetary character
of their orbits. — III. ELLIPTIC COMETS, WHOSE MEAN DISTANCES ARE
NEARLY EQUAL TO THAT OP URANUS : 51. Comets of long periods first
recognised as periodic. — 52. Newton's conjectures as to the existence
of comets of long periods. — 53. Halley's researches. — 54. Halley
predicts its re-appearance in 1758-9.

24. IT may be asked, If the existence of a resisting medium be
admitted, whether the same ultimate fate must not await the
planets ? To this inquiry it may be answered that, within the
limits of past astronomical record, the ethereal medium, if it
exist, has had no sensible effect on the motion of any planet.
That it might have a perceptible effect upon comets, and yet not
upon planets, will not be surprising, if the extreme lightness of
the comets compared with their bulk be considered. The effect
in the two cases may be compared to that of the atmosphere upon
a piece of swan's down and upon a leaden bullet moving through
it. It is certain that whatever may be the nature of this resisting^
medium, it will not, for many hundred years to come, produce the
slightest perceptible effect upon the motions of the planets.

25. The masses of comets in general are, as will be explained,,
incomparably smaller than those of the smallest of the planets ;
so much so, indeed, as to bear no appreciable ratio to them. A
consequence of this is, that while the effects of their attraction
upon the planets are altogether insensible, the disturbing effects
of the masses of the planets upon them are very considerable.
These disturbances, being proportional to the disturbing masses,
may then be used as measures of the latter, just as the movement
of the pith-ball in the balance of torsion supplies a measure of
the physical forces to which that instrument is applied.

Encke's comet, near its perihelion, passes near the orbit of
Mercury ; and when that planet, at the epoch of its perihelion,
happens to be near the same point, a considerable and measurable
disturbance is manifested in the comet's motion, which being
observed, supplies a measure of the planet's mass.

This combination of the motions of the planet and comet took
place under very favourable circumstances, on the occasion of the
perihelion passage of the comet in 1838, the result of which,
according to the calculations of Professor Encke" , was the discovery
of an error of large amount in the previous estimates of the mass
of the planet. After making every allowance for other planetary
attractions, and for the effects of the resisting medium, the
existence of which it appears necessary to admit, it was inferred
162

BIELAS COMET.

that the mass assigned to Mercury by Laplace was too great in
the proportion of 12 to 7.

This question is still under examination, and every succeeding
perihelion passage of the comet will increase the data by which
its more exact solution may be accomplished.

26. On February 28, 1826, M. Biela, an Austrian officer,
observed in Bohemia a comet, which was seen at Marseilles at
about the same time by M. Gambart. The path which it pursued,
was observed to be similar to that of comets which had appeared
in 1772 and 1806. Finally, it was found that this body moved
round the sun in an oval orbit, and that the time of its revolution
was about 6 years and 8 months. It has since returned at its
predicted times, and has been adopted as a member of our system,
under the name of Biela's comet.

Biela's comet moves in an orbit whose plane is inclined at a
small angle to those of the planets. It is but slightly oval, the
length being to the breadth in the proportion of about 4 to 3.
When nearest to the sun, its distance is a little less than that of
the earth ; and when most remote from the sun, its distance
somewhat exceeds that of Jupiter. Thus it ranges through the
solar system, between the orbits of Jupiter and the earth.

This comet had been observed in 1772 and in 1806 ; but the
elliptic form of its orbit, and consequently its periodicity, was
not discovered. Its return to perihelion was predicted and
observed in 1832, in 1846, and in 1852 ; but that which took
place in 1838 escaped observation, owing to its unfavourable
position and extreme faintness,

A Table, showing the elements of this comet during each of its
appearances from 1772 to 1846 inclusive, may be seen by refer-
ence to my " Hand-Book of Astronomy."

27. One of the points at which the orbit of Biela's comet
intersects the plane of the ecliptic, is at a distance from the earth's
orbit less than the sum of the semi-diameters of the earth and the
comet. It follows, therefore (2905), that if the comet should
arrive at this point at the same moment at which the earth
passes through the point of its orbit which is nearest to it, a
portion of the globe of the earth must penetrate the comet.

It was estimated on the occasion of the perihelion passage of
this comet in 1832, that the semi-diameter of the comet (that
body being nearly globular, and having no perceptible tail) was
21000 miles, while the distance of the point at which its centre
passed through the plane of the ecliptic, on the 29th October in
that year, from the path of the earth was only 18600 miles. If
the centre of the earth happened to have been at the point of its
orbit nearest to the centre of the comet on that day, the distance

ai 2 163

COMETS.

between the centres of the two bodies would have been only
18600 miles, while the semi- diameter of the comet was 21000
miles ; and the semi-diameter of the earth being in round
numbers 4000 miles, it would follow that in such a contingency
the earth would have plunged into the comet to a depth of

21000 + 4000 — 18600 = 6400 miles,

a depth exceeding three-fourths of the earth's diameter.

The possibility of such a catastrophe having been rumoured,
great popular alarm, was excited before the expected return of the
comet in 1832. It was, however, shown that on the 29th October
the earth would be about five millions of miles from the point of
danger, and that, on the arrival of the earth at that point, the
comet would have moved to a still greater distance.

28. Resolution of Biela's comet into two. — One of the most
extraordinary phenomena of which the history of astronomy
affords any example, attended the appearance of this comet in
1846. It was on that occasion seen to resolve itself into two
distinct comets, which, from the latter end of December, 1845, to
the epoch of its disappearance in April, 1846, moved in distinct
and independent orbits. The paths of these two bodies were
in such optical juxtaposition that both were always seen together
in the field of view of the telescope, and the greatest visual angle
between their centres did not amount at any time to 10', the
variation of that angle arising principally from the change of
direction of the visual line, relatively to the line joining their
centres, and to the change of the comet's distance from the earth.
M. Plantamour, director of the Observatory of Geneva, calcu-
lated the orbits of these two comets, considered as independent
bodies ; and found that the real distance between their centres
was, subject to but little variation while visible, about thirty-
nine semi-diameters of the earth, or two-thirds of the moon's
distance. The comets moved on thus side by side, without
manifesting any reciprocal disturbing action ; a circumstance
no way surprising, considering the infinitely minute masses of
such bodies.

29. The original comet was apparently a globular mass of
nebulous matter, semi-transparent at its very centre, no appear-
ance of a tail being discoverable. After the separation, both
cornets had short tails, parallel in their direction, and at right
angles to the line joining their centres ; both had nuclei. From
the day of their separation the original comet decreased, and the
companion increased, in brightness, until (on the 10th February)
they were sensibly equal. After this the companion still increased
164

FAYE S COMET.

in brightness, and from the 14th to the 16th was not only greatly
superior in brightness to the original, but had a sharp and star-
like nucleus, compared to a diamond spark. The change of
brightness was now reversed, the original comet recovering its
superiority, and acquiring on the 18th the same appearance as
the companion had from the 14th to the 16th. After this the
companion gradually faded away, and disappeared previously to
the final disappearance of the original comet on 22nd April.

It was observed also that a thin luminous line or arc was
thrown across the space which separated the centres of the two
nuclei, especially when one or the other had attained its greatest
brightness, the arc appearing to emanate from that which for the
moment was the brighter.

After the disappearance of the companion, the original comet
threw out three faint tails, forming angles of 120° with each
other, one of which was directed to the place which had been
occupied by the companion.

It is suspected that the faint comet which was observed by
Professor Secchi to precede Biela's comet in 1852, may have been
the companion thus separated from it, and if so, the separation
must be permanent, the distance between the parts being greater
than that which separates the earth from the sun.

30. On the 22nd November, 1843, M. Faye, of the Paris Obser-
vatory, discovered a comet, the path of which soon appeared to be
incompatible with the parabolic character. Dr. Goldschmidt
showed that it moved in an ellipse of very limited dimensions,
with a period of 7| years. It was immediately observed as being
extraordinary, that, notwithstanding the frequent returns to peri-
helion which such a period would infer, its previous appearances
had not been recorded. M. Faye replied by showing that the
aphelion of the orbit passed very near to the path of Jupiter, and
that it was possible that the violent action of the great mass of
that planet, in such close proximity with the comparatively light
mass of the comet, might have thrown the latter body into its
present orbit, its former path being either a parabola or an ellipse,
with such elements as to prevent the comet from coming within
visible distance. M. Faye supported these observations by refer-
ence to a more ancient comet, which we shall presently notice, to
which a like incident is supposed with much probability, if not
certainty, to have occurred.

31. The observations which had been made in 1843, at several
observatories, but more especially those made by M. Struve at
Pultowa, who continued to observe the comet long after it ceased
to be observed elsewhere, supplied to M. Le Yerrier the data neces-
sary for the calculation of its motion in the interval between its

165

COMETS.

perihelion in 1843 and its expected re-appearance in 1850-1,
subject to the disturbing action of the planets, and he predicted its
succeeding perihelion for the 3rd of April, 1851.

Aided by the formula) of M. Le Terrier, Lieutenant Stratford
calculated a provisional ephemeris in 1850, by which observers
might be enabled mor£ easily to detect the comet, which was the
more necessary as the object is extremely faint and small, and not
capable of being seen except by means of the most perfect tele-
scopes. By means of this ephemeris, Professor Challis, of Cam-
bridge, found the comet on the night of the 28th November very
nearly in the place assigned to it in the tables. Two observations
only were then made upon it, which, however, were sufficient to
enable M. Le Verrier to give still greater precision to his formula?,
by assigning a definite numerical value to a small quantity which
before was left indeterminate. Lieutenant Stratford, with the
formula thus corrected, calculated a more extensive and exact
ephemeris, extending to the last day of March, and published it
in January, 1851, in the Nautical Almanack.

The comet, though extremely faint and small, and consequently
difficult of observation, continued to be observed by Professor
Challis, with the great Northumberland telescope, at Cambridge,
and by M. Struve at Pultowa, and it was found to move in exact
accordance with the predictions.

32. On the 22nd August, 1844, M. de Yico, of the Roman
Observatory, discovered a comet whose orbit was soon afterwards
proved by M. Faye to be an ellipse of moderate eccentricity, with
a period of about 5| years. It arrived at its perihelion on the
2nd of September, and continued to be observed until the 7th
of December.

33. On the 26th of February, 1846, M. Brorsen, of Kiel, dis-
covered a faint comet, which was soon found to move in an elliptic
orbit, with a period of about 5£ years. Its position in the heavens
not being favourable, the observations upon it were few, and the
resulting elements, consequently, not ascertained with all the pre-
cision that might be desired. Its re-appearance on its approach
to the succeeding perihelion, was expected from September to
November, 1851. It escaped observation, however, owing to its
unfavourable position in relation to the sun. Its next perihelion
passage will take place in 1857.

34. On the 27th of June, 1851, Dr. d' Arrest, of the Leipsic
Observatory, discovered a faint comet, which M. Yillarceaux
proved to move in an elliptic orbit, with a period of about 6|
years. The next perihelion passage of this comet will take place
in the end of 1857, or the beginning of 1858.

35. A revision of the recorded observations of former comets by
166

OTHER ELLIPTIC COMETS.

the more active and intelligent zeal of modern mathematicians and
computers, has led to the discovery of the great probability of
several among them having revolved in elliptic orbits, with periods
not differing considerably from those of the comets above men-
tioned. The fact that these comets have not been re-observed on
their successive returns through perihelion, may be explained, either
by the difficulty of observing them, owing to their unfavourable
positions, and the circumstance of observers not expecting their
re-appearance, their periodic character not being then suspected ;
or because they may have been thrown by the disturbing action
of the larger planets into orbits such as to keep them continually
out of the range of view of terrestrial observers.

Among those may be mentioned a comet which appeared in 1743,
and was observed by Zanetti at Bologna ; the observations indicate
an elliptic orbit, with a period of about 5^ years.

36. This comet, which was observed by Messier, at Paris, and
by La Xux, at the Isle of Bourbon, revolved, according to the cal-
culations of Burckhardt, in an ellipse with a period of 5 years.

37. The history of astronomy has recorded one singular example
of a comet which appeared in the system, made two revolutions
round the sun in an elliptic orbit, and kthen disappeared, never
having been seen either before or since.

This comet was discovered by Messier, in June 1770, in the
constellation of Sagittarius, between the head and the northern
extremity of the bow, and was observed during that month. It
disappeared in July, being lost in the sun's rays. After passing
through its perihelion, it re-appeared about the 4th of August,
and continued to be observed until the first days of October, when
it finally disappeared.

All the attempts of the astronomers of that day failed to deduce
the path of this comet from the observations, until six years later,
in 1776, Lexell showed that the observations were explained, not,
as had been assumed previously, by a parabolic path, but by an
ellipse, and one, moreover, without any example at that epoch,
which indicated the short period of 5^ years.

It was immediately objected to such a solution, that its admis-
sion would involve the consequence that the comet, with a period
so short, and a magnitude and splendour such as it exhibited in
1770, must have been frequently seen on former returns to peri-
helion ; whereas no record of any such appearance was found.

To this Lexell replied, by showing that the elements of its orbit,
derived from the observations made in 1770, were such, that at
its previous aphelion, in 1767, the comet must have passed within
.a distance of the planet Jupiterfifty-eighttimes less thanits distance
from the sun ; and that consequently it must then have sustained

167

COMETS.

an attraction from the great mass of that planet, more than three
times more energetic than that of the sun ; that consequently it
was thrown out of the orbit in which it previously moved, into the
elliptic orbit in-which it actually moved in 1770 ; that its orbit
previously to 1767 was, according to all probability, a parabola ;
and, in fine, that consequently moving in an elliptic orbit from
1767 to 1770, and having the periodicity consequent on such
motion, it nevertheless moved only for the first time in its new
orbit, and had never come within the sphere of the sun's attraction
before this epoch.

Lexell further stated, that since the comet passed through its
aphelion, which nearly intersected Jupiter's orbit, at intervals
of somewhat above 5| years, and it encountered the planet near
that point in 1767, the period of the planet being somewhat
above eleven years, the planet after a single revolution, and the
comet after two revolutions, must necessarily again encounter
each other in 1779 ; and, that since the orbit was such that the
comet must in 1779 pass at a distance from Jupiter 500 times less
than its distance from the sun, it must suffer from that planet
an action 250 times greater than the sun's attraction, and that
therefore it would in all probability be again thrown into a
parabolic or hyperbolic path; and if so, that it would depart for
ever from our system to visit other spheres of attraction. Lexell,
therefore, anticipated the final disappearance of the comet, which
actually took place.

In .the interval between 1770 and 1779, the comet returned
once to perihelion ; but its position was such that it was above the
horizon only during the day, and could not in the actual state of
science be observed.

38. At this epoch analytical science had not yet supplied a
definite solution of the problem of cometary disturbances. At a
later period the question was resumed by Laplace who, in his
celebrated work, the " Mecanique celeste," gave the general
solution of the following problem : —

u The actual orbit of a comet being given, what was its orbit
before, and what will be its orbit nfter being submitted to any
given disturbing action of a planet near which it passes ? "

39. Applying this to the particular case of LexelPs comet, and
assuming as data the observations recorded in 1770, Laplace
showed that, before sustaining the disturbing action of Jupiter in
1767, the comet must have moved in an ellipse of which the semi-
axis major was 13-293, and consequently that its period, instead
of being 5^ years, must have been 48| years ; and that the
eccentricity of the orbit was such that its perihelion distance
would be but very little less than the mean distance of Jupiter,

168

LEXELL'S COMET.

and that consequently it could never have been visible. It
followed also that, after suffering the disturbing action of Jupiter
in 1779, the comet passed into an elliptic orbit whose semi-axis
major was 7 '3, that its period was consequently twenty years, and
that its eccentricity was such that its perihelion distance was more
than twice the distance of Mars, and that in such an orbit it
could not become visible.

40. This investigation was afterwards revised by M. Le Terrier,*
which showed that the observations of 1770 were not suffi-
ciently definite and accurate to justify conclusions so absolute.
He has shown that the orbit of 1770 is subject to an uncer-
tainty comprised between certain definite limits ; that tracing
the consequences of this to the positions of the comet in 1767
and 1779, these positions are subject to still wider limits of
uncertainty. Thus he shows that, compatibly with the obser-
vations of 1770, the comet might in 1779 pass either consi-
derably outside, or considerably inside Jupiter's orbit, or might,
as it was supposed to have done, have passed actually within the
orbits of his satellites. He deduces in fine the following general
conclusions : —

1 . That if the comet had passed within the orbits of the satellites,
it must have fallen down upon the planet and coalesced with it ;
an incident which he thinks highly improbable, though not
absolutely impossible.

2. The action of Jupiter may have thrown the comet into a
parabolic or hyperbolic orbit, in which case it must have
departed from our system altogether, never to return, except by
the consequence of some disturbance produced in another sphere
of attraction.

3. It may have been thrown into an elliptic orbit, having a
great axis and long period, and so placed and formed that the
comet could never become visible ; a supposition within which
comes the solution of Laplace.

4. It may have had merely its elliptic elements more or less
modified by the action of the planet, without losing its character
of short periodicity ; a result which M. Le Yerrier thinks the
most probable, and which would render it possible that this
comet may still be identified with some one of the many comets of
short period, which the activity and sagacity of observers are
every year discovering.

To facilitate such researches, M. Le Yerrier has given a Table,
including all the possible systems of elliptic elements of short
period which the comet could have assumed, subject to the dis-

* See Mem. Acad. des Sciences, 1847, 1848.

169

COMETS.

turbing action of Jupiter in 1779, and taking the observations of
1770 within their possible limits of error.

He further demonstrates, that the orbit in which the comet
moved antecedently to the disturbing action of Jupiter upon it in
1767, not only could not have been a parabola or hyperbola, but
must have been an ettipse, whose major axis was considerably
less than that which Laplace deduced from the insufficient
observations of Messier. He shows that, before that epoch,
the perihelion distance of the comet could not, under any
possible supposition, have exceeded three times the earth's
mean distance, and most probably was included between 1^ and
•2 times that distance; and that the semi-axis major of the
orbit could not have exceeded 4^ times the earth's mean distance,
a magnitude 3 times less than that assigned to it by the calcula-
tions of Laplace.

41. It must not, however, be supposed that it is sufficient to
compare the actual elements of each periodic comet thus dis-
covered, with the elements given in the table of M. Le Yerrier,
and to infer the absence of identity from their discordance. Such
an inference would only be rendered valid by showing, that in
past ages, the comet in question had suffered no serious disturbing
action by which the elements of its orbit could be considerably
changed. To decide the question a much more laborious and
difficult process must be encountered: a process from which the
untiring spirit of M. Le Yerrier has not shrunk. It is necessary,
in fine, to the satisfactory and conclusive solution of such a
problem, that the periodic comet in question should be traced
%ack through all its previous revolutions up to 1779, that all
the disturbances which it suffered from the planets which it
•encountered in that interval be calculated and .ascertained, and
that by such means the orbit which it must have had previous to
such disturbances, in 1779, be determined. Such orbit would
then be compared with the table of possible orbits of Lexell's
•comet, as given by M. Le Yerrier ; and if it were found to be
identical with any of them, the identity of the comet in ques-
tion with that of Lexell, would be inferred with the highest
degree of probability; but if, on the other hand, such discre-
pancies were found to prevail as must exceed all supposable
errors of observation or calculation, the diversity of the comets
would follow.

42. M. Le Yerrier has applied these principles to the comets of
Faye, De Yico, and Brorsen ; tracing back their histories during
their unseen motions for three-quarters of a century, and ascer-
taining the effects of the disturbing actions which they must
severally have sustained from revolution to revolution, until he

170

GENERAL PLAN OF ELLIPTIC COMETS.

brought them to the epoch of 1779. On comparing- the orbits
thus determined with those of the table of possible orbits of
LcxelPs comet, he has shown that none of them can be identical
with it, however strongly some of the elements of their present
orbits may raise such a presumption.

43. The comet of De Yico having presented striking analogies
with a comet which was observed by Tycho Brahe and Hoth-
mann in 1585, and one observed by La Hire in 1678, M. Le
Verrier has applied like principles to the investigation of these
questions.

MM. Laugier and Mauvais observed that the elements of De
Vico's comet presented such a resemblance to that of Tycho Brahe,
as almost to decide the question of their identity. M. Le Terrier
tracing back the comet of De Vico to 1585, has shown that its
orbit at that epoch was so different from that of the comet of
Tycho, as to be incompatible with any plausible inference of their
identity.*

He has shown, however, by like reasoning, that there is a high
degree of probability that the comet of De Vico is identical with
that observed by La Hire in 1678.

44. M. Blainplan discovered a comet at Marseilles on the 28th
November, 1819, which was observed at Milan until 25th
January, 1820. The observations reduced and calculated by
Prof. Encke gave an elliptic orbit with a period a little short of
five years. Clausen conjectures'that this comet may be identical
with that of 1743. It has not been seen since 1820.

45. A comet was discovered by M. Pons on June 12th, 1819,
which was observed until July 19th. Prof. Encke assigned to it
an elliptic orbit, with a period of 5^ years.

46. A comet, discovered by Mr. Pigott at New York in 1783,
was shown by Burckhardt to have an elliptic orbit, with a period
of 5 1 years.

47. On the 26th June, 1846, a comet was discovered at Naples
by M. Peters, which was subsequently observed at Rome by De
Yico, and continued to be seen until 21st July. An elliptic orbit
is assigned to this comet, with a period of from thirteen to sixteen
years, some uncertainty attending the observations. The re-
appearance of this comet may be expected in 1859, 1860.

48. A synoptical Table, showing the elements of the elliptic
comets above described, may be seen by a reference to my
" Hand-Book of Astronomy."

49. In fig. 2 (page 172), the orbits of these thirteen comets
brought to a common plane, are represented roughly but in their

* Mem. Acad. des Sciences, 1847.

171

ELLIPTIC COMETS.

proper proportions and relative positions, so as to exhibit to the
eye their several ellipticities, and the relative directions of their
axes.* All these bodies, without one exception, revolve in the
common direction of the planets.

50. It is not alone, however, in the direction of their motions
that the orbits of these bodies have an analogy to those of the
planets. Their inclinations, with one exception, are within the
limits of those of the planets. Their eccentricities, though incom-
parably greater than those of the planets, are, as will presently
appear, incomparably less than those of all other comets yet dis-
covered. Their mean distances and periods (with the exception
of the last two in the Table just referred to) are within the limits
of those of the planetoids.

The comparison of the numbers given in this Table with those
in the Tables of the elements of other elliptic comets, and the
comparison of the diagrams of their orbits with those of others,
will show in a striking manner, to how great an extent the orbits
of this group of comets possess the planetary character. Besides
moving round the sun in the common direction, their inclinations,
with a single exception, are within the limits of those of the
planets. It is true that their eccentricities have an order of
magnitude much greater ; but on the other hand, it will be seen
presently that they are incomparably less than the eccentricities
of all other periodic comets yet discovered. Their mean distances
and periods place them in direct analogy with the planetoids.

Moderate as are the eccentricities as compared with those of other
comets, they are sufficiently great to impart a decided oval form
to the orbits, and to produce considerable differences between the
perihelion and aphelion distances, as will be apparent by inspecting
the Table. It appears that while the perihelion of Encke's comet
lies within the orbit of Mercury, its aphelion lies outside the orbit
of the most remote of the planetoids, and not far within that of
Jupiter. The perihelion of Biela's comet, in like manner, lies
between the orbits of the earth and Yenus, while its aphelion
lies outside that of Jupiter. In the case of Faye's comet, the
least eccentric of the group, the perihelion lies near the orbit of
Mars, and the aphelion outside that of Jupiter.

It must be remembered that the elliptic form of these orbits
has only been verified by observations on the successive returns
to perihelion of the first five comets in the Table. The elliptic
elements of the others may, so far as is at present known, have
been effaced by disturbing causes.

* In the diagram, to prevent confusion, the orbits of the different comets
are indicated by dotted or broken lines of different kinds.

17

COMETS.

The angular motions at the mean and extreme distances from
the sun have been computed by the formulae

1,296000 a2 a-

'

In which a represents ^:he mean angular velocity, a the greatest,
and a", the least ; d' the perihelion, and d" the aphelion distances.
P is the periodic time of the comet, and a the mean distance.
The same numbers which express these angular motions, also
express in all cases the intensities of solar light and heat in the
several positions of the comet ; and also the apparent motion of
the sun, as seen from the comet ; and a comparison of these with
the corresponding numbers related to any of the planets, will
illustrate in a striking manner how different are the physical
conditions by which these two classes of bodies are affected ; and
this will be more and more striking, when the other groups of
comets have been noticed.

Taking the comet of Encke as an example, it appears that
while its mean daily motion is 1076" or 18', its motion in aphe-
lion is only 5', and in perihelion nearly 13°. Its motion in
perihelion, the light and heat it receives from the sun, and
the apparent motion of the sun as seen from it, are therefore
severally more than 150 times greater in perihelion than in.
aphelion.

III. — ELLIPTIC COMETS, WHOSE 3IEA!ST DISTANCES ARE NEARLY
EQUAL TO THAT OF URANUS.

51. It might be expected, that comets moving in elliptic orbits
of small dimensions, and consequently having short periods, would
have been the first in which the character of periodicity would be
discovered. The comparative frequency of their returns to those-
positions near perihelion, where alone bodies of this class are
visible from the earth, and the consequent possibility of verifying
the fact of periodicity, by ascertaining the equality of the intervals
between their successive returns to the same heliocentric position,
to say nothing of the more distinctly elliptic form of the arcs of
their orbits in which they can be immediately observed, would
afford strong ground for such an expectation ; nevertheless in this
case, as has happened in so many others in the progress of phy-
sical knowledge, the actual results of observation and research
have been directly contrary to such an anticipation ; the most
remarkable case of a comet of large orbit,: long period, and
rare returns, being the first, and those of small orbits, short
174

HALLEY'S COMET.

periods, and frequent returns, the last whose periodicity has been
discovered.

52. It is evident that the idea of the possible existence of
comets with periods shorter than those of the more remote planets,,
and orbits circumscribed within the limits of the solar system,
never occurred to the mind either of Xewton or any of his con-
temporaries or immediate successors.

In the third book of his PRINCIPIA, he calls comets a species of
planets, revolving in elliptic orbits of a very oval form. But he
continues, "I leave to be determined by others the transverse
diameters and periods, by comparing comets which return after
long intervals of time to the same orbits."

It is interesting to observe the avidity with which minds of a
certain order snatch at such generalisations, even when but slen-
derly founded upon facts. These conjectures of Newton were soon
after adopted by Yoltaire : " II y a quelque apparence," says he,
in an essay on comets, " qu'on connaitra un jour un certain
nonibre de ces autres planetes qui sous le nom de cometes tournent
comme nous autour du soleil, mais il ne faut pas esperer qu'on le&
connaissent toutes."

And again, elsewhere, on the same subject: —

" Cometes, que 1'on craint a 1'egal du tonnerre,
Cessez d'epoitvanter les peuples de la terre ;
Dans une ellipse immense achevez votre cours,
Remontez, descendez pres de 1'astre des jours."

53. Extraordinary as these conjectures must have appeared at
the time, they were soon strictly realised. Halley undertook
the labour of examining the circumstances attending all the
comets previously recorded, with a view to discover whether
any, and which of them, appeared to follow the same path.
He found that a comet which had been observed by himself,
by Newton, and their contemporaries in 1682, followed a path
while visible, which coincided so nearly with those of comets
which had been observed in 1607, and in 1531, as to render it
extremely probable, that these objects were the same identical
comet, revolving in an elliptic orbit of such dimensions, as to
cause its return to perihelion at intervals of 75 — 76 years,

The comet of 1682 had been well observed by La Hire, Picard,
Hevelius, and Flamstead, whose observations supplied all the
data necessary to calculate its path while visible. That of 1607
had been observed by Kepler and Longomontanus ; and that of
1531, by Pierre Apian at Ingolstadt, the observations in both
cases being also sufficient for the determination of the path of the
body, with all the accuracy necessary for its identification.

175

COMETS.

' 54. Of the identity of the paths while visible on each of these
appearances Halley entertained no doubt, and announced to the
world the discovery of the elliptic motion of comets, as the result
of combined observation and calculation, and entitled to as much
confidence as any other consequence of an established physical
law ; and predicted the» re-appearance of this body, on its suc-
ceeding return to perihelion in 1758-9. He observed, however,
that as in the interval between 1607 and 1682 the comet passed
near Jupiter, its velocity must have been augmented, and conse-
quently its period shortened by the action of that planet. This
period, therefore, having been only seventy-five years, he inferred
that the following period would probably be seventy- six years or
upwards ; and consequently that the comet ought not to be expected
to appear until the end of 1758, or the beginning of 1759. It is
impossible to imagine any quality of mind more enviable than that
which, in the existing state of mathematical physics, could have
led to such a prediction. The imperfect state of science ren-
dered it impossible for Halley to offer to the world a demonstra-
tion of the event which he foretold. " He therefore," says M. de
Pontecoulant, " could only announce these felicitous conceptions of
a sagacious mind as mere intuitive perceptions, which must be
received as uncertain by the world, however he might have felt
them himself, until they could be verified by the process of a
rigorous analysis."

Subsequent researches gave increased force to Halley's predic-
tion ; for it appeared from the ancient records of observers, that
comets had been seen in 1456 and 1378, whose elements were
identical with those of the comet of 1682.

176

Fig. 26.— Feb. 7, 1836.

Fig. 28.— Feb. 10, 1836.

HALLKY'S COMET DEPARTING FROM THE SUN IN 1836.

COMETS.

CHAPTEE III.

Halley's prediction (continued). — 55. Great advance of mathematical
and physical sciences between 1682 and 1759. — 56. Exact path of
the comet on its return, and time of its perihelion, calculated and
predicted by Clairaut and Lalande. — 57. Remarkable anticipation
of the discovery of Uranus. — 58. Prediction x>f Hal ley and Clairaut
fulfilled by reappearance of the comet in 1758-9. — 59. Disturbing
action of a planet on a comet explained. — 60. Effect of the perturbing
action of Jupiter and Saturn on Halley's comet between 1682 and
1758. — 61. Calculations of its return in 1835-36. — 62. Predictions
fulfilled.— 53. Elements of the orbit of Halley's comet.— 64. Pons's
comet of 1812. — 65. Olbers's comet of 1815.— 66. De Vice's comet
of 1846.— 67. Brorsen's comet of 1847.— 68. Westphal's comet of
1852. — 69. Data necessary to determine the motions of these six
comets. — 70. Diagram of their orbits. — 71. Planetary characters are
nearly effaced in these orbits. — IV. ELLIPTIC COMETS, WHOSE MEAN

DISTANCES EXCEED THE LIMITS OP THE SOLAR SYSTEM : 72. Synopsis

of twenty-one elliptic comets of great eccentricity and long period. —
73. Plan of the form and relative magnitude of the orbits. — V. 74.
HYPERBOLIC COMETS : VI. 75. PARABOLIC COMETS. — VII. PHYSICAL
CONSTITUTION OP COMETS : 76. Apparent form — head and tail. — 77.

LIRDNER'S MUSEUM OF SCIENCE.
No. 140.

177

COMETS.

Nucleus. — 78. Coma. — 79. Origin of the name. — 80. Magnitude of
the head.— 81. Magnitude of the nucleus.— 82. The tail.— 83. Mass,
density, and volume of comets.

THE appearance of the comet in 1456, was described by con-
temporary authorities to have been an object of " unheard-of
magnitude ; " it was accompanied by a tail of extraordinary length,
which extended over sixty degrees (a third of the heavens), and
continued to be seen during the whole of the month of June. The
influence which was attributed to this appearance, renders it pro-
bable that in the record there exists more or less of exaggeration.
It was considered as the celestial indication of the rapid success of
Mohammed II., who had taken Constantinople, and struck terror
into the whole Christian world. Pope Calixtus II. levelled the
thunders of the Church against the enemies of his faith, terres-
trial and celestial, and in the same bull exorcised the Turks and
the comet ; and in order that the memory of this manifestation of
his power should be for ever preserved, he ordained that the bells
of ail the churches should be rung at midday — a custom which is
preserved in those countries to our times. It must be admitted
that, notwithstanding the terrors of the Church, the comet pur-
sued its course with as much ease and security, as those with which
Mohammed converted the church of St. Sophia into his principal
mosque.

The extraordinary length and brilliancy which was ascribed to
the tail upon this occasion, have led astronomers to investigate
the circumstances under which its brightness and magnitude would
be the greatest possible ; and, upon tracing back the motion of
the comet to the year 1456, it has been found that it was then
actually under the circumstances of position with respect to the
earth and sun most favourable to magnitude and splendour. So
far, therefore, the results of astronomical calculation corroborate
the records of history.

55. In the interval of three-quarters of a century* which elapsed
between the announcement of Halley's prediction and the date of
its expected fulfilment, great advances were made in mathematical
science ; new and improved methods of investigation and calcu-
lation were invented ; and, the theory of gravitation was pur-
sued with extraordinary activity and success through its conse-
quences in the mutual disturbances produced upon the motions
of the planets and satellites, by the attraction of their masses one
upon another. As the epoch of the expected return of the comet
to its perihelion approached therefore, the scientific world resolved
to divest, as far as possible, the prediction, of that vagueness
which necessarily attended it owing to the imperfect state of
science at the time it was made, and to calculate the exact effects
178

HALLBY'S COMET.

of those planets whose masses were sufficiently great, in accele-
rating or retarding its motion while passing near them.

56. This inquiry, which presented great mathematical diffi-
culties and involved enormous arithmetical Jabour, was undertaken
by Clairaut and Lalande : the former, a mathematician and
natural philosopher, who had already applied with great success
the principles of gravitation to the motions of the moon, undertook
the purely analytical part of the investigation, which consisted
in establishing certain general algebraical formulae, by which the
disturbing actions exerted by the planets on the comet were
expressed ; and Lalande, an eminent practical astronomer, under-
took the labour of the arithmetical computations, in which he
was assisted by a lady, Madame Lepaute, whose name has thus
become celebrated in the annals of science.

"When it is considered that the period of Halley's comet is about
seventy-five years, and that for two successive periods, it was
necessary to calculate every portion of its course separately in
this way, some notion may be formed of the labour encountered
by Lalande and Madame Lepaute. " During six months," says
Lalande, " we calculated from morning till night, sometimes
even at meals ; the consequence of which was, that I contracted
an illness which changed my constitution for the remainder
of my life. The assistance rendered by Madame Lepaute was
such, that without her we never could have dared to undertake
this enormous labour, in which it was necessary to calculate the
distance of each of the two planets, Jupiter and Saturn, from
the comet, and their attraction upon that body, separately, for
every successive degree, and for 150 years."

The name of Madame Lepaute does not appear in Clairaut' s
memoir; a suppression which Lalande attributes to the influence
exercised by another lady to whom Clairaut was attached.
Lalande, however, quotes letters of Clairaut, in which he speaks
in terms of high admiration of "la savante calculatrice." The
labours of this lady in the work of calculation (for she also
assisted Lalande in constructing his " Ephemerides ") at length
so weakened her sight that she was compelled to desist. She
died in 1788, while attending on her husband, who had become
insane. See the article on comets, by Prof, de Morgan, in the
" Companion to the British Almanac" for the year 1833.

These elaborate calculations being completed, Clairaut presented
the result of their joint labours, in a memoir to the Academy of
Sciences of Paris, in which he predicted the next arrival of the
comet at perihelion, on the 18th of April, 1759 ; a date, however,
which, before the re-appearance of the comet, he found reason to
change to the llth of April; and assigned the path which the

N 2 179

COMETS.

comet would follow while visible as determined by the following
data : — •

Longitude Longitude of Perihelion

Inclination. ofiiode,, perihelion. distance. Direction.

17° 3?' 53° 5°' 3°3° 10/ °'S8 retrograde.

•

57. In announcing his prediction Clairaut stated, that the
time assigned for the approaching perihelion might vary from the
actual time to the extent of a month ; for that independently of
any error either in the methods or process of calculation, the
event might deviate more or less from its predicted occurrence,
by reason of the attraction of an undiscovered planet of our
system revolving beyond the orbit of Saturn. In twenty-two
years after this time this conjecture was realised, by the dis-
covery of the planet Uranus, by the late Sir William Herschel,
revolving round the sun one thousand millions of miles beyond
the orbit of Saturn !

58. The comet, in fine, appeared in December 1758, and followed
the path predicted by Clairaut, which diifered but little from that
which it had pursued on former appearances. It passed through
perihelion on the 13th of March, within twenty-two days of
time, and within the limit of the possible errors assigned by
Clairaut.

59. The general effects of a planet in accelerating or retarding
the motion of a comet are easily explained, although the exact
details of the disturbances are too complicated to admit of any
exposition here.

Let P, fig. 3, represent the place of the disturbing planet, and
c that of the comet. The attraction of the planet on the comet
will then be a force directed from c
towards P, and by the principle of the
composition of forces is equivalent to
two components, one c m in the direc-
tion of the comet's path, and the other
c n perpendicular to that path. If the
motion of the comet be directed from c
towards m, it will be accelerated ; and
if it be directed from c towards m', it
will be retarded by that component of
the planet's attraction which is directed from c to m. The other
component c n being at right angles to the comet's motion, will
have no direct effect either in accelerating or retarding it.

It appears, therefore, in general that, if the direction of the
comet's motion c m make an acute angle with the line c P drawn
to the planet, the planet's attraction will accelerate it ; and if
180

HALLEYS COMET.

Fig. 4.

its direction c m' make an obtuse angle with the line c P, it will
retard it.

This being understood, the disturbing action of a planet such
as Jupiter or Saturn on a comet such as Halley's may be easily
comprehended. In fig. 4, the orbit of the comet is represented at
A c P c" in its proper proportions,
A P being the major axis, P the place
of perihelion, A that of aphelion,
and s that of the focus in which the
sun is placed. The small circle de-
scribed round s represents in its
proper proportions the orbit of the
earth, whose distance is about twice
that of the comet when the latter is
at perihelion. The circle p p' p" re-
presents in its proper proportions
the orbit of Jupiter which, for illus-
tration, we shall consider as the
disturbing planet.

It will be apparent on the mere
inspection of the diagram, that lines
drawn from the planet whatever be
its place, to any point whatever of
the comet's path between its aphe-
lion A, and the point m' where it
arrives at the orbit of the planet
in approaching the sun, will make
acute angles with the direction of
the comet's motion ; and that, con-
sequently, the comet will be accele-
rated by the action of the planet.
In like manner, it is apparent that
lines drawn from the planet what-
ever be its place, to any point
whatever of the comet's path between m and aphelion A, will
make obtuse angles with the direction of the comet's motion ; and,
consequently, the comet will be retarded by the action of the
planet, in departing from the sun, from m to A.

In that part of the comet's path which lies within the planet's
orbit, the action of the planet alternately accelerates and retards
it, according to their relative position. If the planet be at p,
suppose p o drawn so as to be at right angles to the path of the
comet. Between m' and o the action of the planet at p will
accelerate the comet, and after the comet passes o it will retard
it. In like manner if the planet be at p", it will first retard

181

COMETS.

the motion of the comet proceeding from m' through p towards A,
and will continue to do so until the line of direction becomes-
perpendicular to that of the comet's motion, after which it will
accelerate it.

It appears, therefore, that during the period of the comet,
the disturbing action of tSe planet is subject to several changes-
of direction, owing partly to the change of position of the comet
and partly to that of the planet ; and the total effect of the dis-
turbing action of the planet on the comet's period is found by
taking the difference between the total amount of all the accele-
rating and all the retarding actions.

In the case of the planet Jupiter and Halley's comet, the
former makes nearly seven complete revolutions in a single period
of the comet ; and consequently its disturbing action is not only
subject to several changes of direction, but also to continual vari-
ation of intensity, owing to its change of distance from the comet.

Small as the arc m' p m of the comet's path is which is included
within the orbit of Jupiter, the fraction of the period in which
this arc is traversed by the comet is much smaller, as will be
apparent by considering the application of the principle of equable
areas * to this case. The time taken by the comet to move over
the arc m' p m is in the same proportion to its entire period, as the
area included between the arc m' P m and the lines m1 s and m s-
is to the entire area of the ellipse A r.

To simplify the explanation the orbit of the comet has here
been supposed to be in the plane of that of the disturbing planet.
If it be not, the disturbing action will have another component
at right angles to the plane of the comet's orbit, the effect of
which will be a tendency to vary the inclination.

60. The result of the investigation by Clairaut showed, that the
total effect of the disturbing action of Jupiter and Saturn on
Halley's comet between its perihelions in 1682 and in 1759, was-
to increase its period by 618 days as compared with the time of
its preceding revolution, of which increase, 100 days were due to
the action of Saturn, and 518 to that of Jupiter.

Clairaut did not take into account the disturbing action of the
earth, which was not altogether inconsiderable, and could not
allow for those of the undiscovered planets, Uranus and Neptune.
The effects of the action of the other planets, Mars, Yenus, Mer-
cury, and the planetoids, are in these cases insignificant.

61. In the interval of three quarters of a century which pre-
ceded the next re-appearance of this comet, science continued to
progress, and instruments of observation and principles and

* See "Handbook of Astronomy," chap. xii. § 2599.
182

H ALLEY 8 COMET.

methods of investigation were still further improved ; and, above
all, the number of observers was greatly augmented. Before the
epoch of its return in 1835, its motions, and the effects produced
upon them by the disturbing action of the several planets, were
computed by MM. Damoiseau, Pontecoulant, Rosenberger, and
Lehmann, who severally predicted its arrival at perihelion : —

Damoiseau . . .4th Nov. 1835.
Pontecoulant . . . 7th ,,

Kosenberger . . . llth ,,
Lehmann . . . . 26th ,,

62. These predictions were all published before July, 1835.
The comet was seen at Rome on the 5th August, in a position
within one degree of the place assigned to it for that day, in the
ephemeris of M. Rosenberger. On the 20th August, it became
visible to all observers, and pursued the course with very little
deviation, which had been assigned to it in the ephemerides,
arriving at its perihelion on the 16th Nov., being very nearly a
mean between the four epochs assigned in the predictions.

After this, passing south of the equator, it was not visible in
northern latitudes, but continued to be seen in the southern
hemisphere until the 5th of May, 1836, when it finally disappeared,
not again to return until the year 1911.

63. A synoptical Table of the elements of the orbit of this
comet, deduced from the observations made on each of its seven
successive returns to perihelion, between 1378 and 1835 in-
clusive, may be seen by a reference to the " Hand-Book of
Astronomy."

It appears that the mean distance of this comet is about
eighteen times that of the earth, and that it is consequently at a
little less than the mean distance of Uranus. "When in peri-
helion, its distance from the sun is about half the earth's
distance, while its distance in aphelion is above thirty-five times
the earth's distance, and therefore seventy times its perihelion
distance.

64. On the 20th of July, 1812, a comet was discovered by
M. Pons, whose orbit was calculated by Professor Encke, and was
found to be an ellipse of such dimensions as to give a period of
75| years, equal to that of Halley's comet.

65. On the 6th of March, 1815, Dr. Olbers discovered at
Bremen, a comet whose orbit, calculated by Professor Bessel,
prcrved to be an ellipse, with a period of 74 years. The next
perihelion passage of this comet is predicted for the 9th of
•February, 1887.

183

COMETS.

66. On the 28th of February, 1846, M. de Vico discovered a
comet at Rome, whose orbit, calculated by MM. van Deinse
and Pierce, appears to be an ellipse, with a period of 72-73
years.

67. A comet was discovered by M. Brorsen at Altona, on the
20th of July, 1847 ; the o*bit of which, calculated by M. d' Arrest,
appears to be an ellipse, with a period of 75 years.

68. On the 27th of June, 1852, a comet was discovered by
M. Westphal at Gottingen, and was soon afterwards observed by
M. Peters at Constantinople. The calculation of its orbit proves
it to be an ellipse, with a period of about 70 years.

69. A synoptical Table, presenting the data necessary to deter-
mine the motions of these six comets, may be seen by a reference
to the " Hand-Book of Astronomy."

70. In fig. 5 (p. 145), is presented a plan of the orbits, brought
upon a common plane, and drawn according to the scale indicated.
This figure shows, in a manner sufficiently exact for the purposes
of illustration, the relative magnitudes and the forms of the six
orbits, as well as the directions of their several axes with relation
to that of the first point of Aries.

71. By comparing the elements given in the table referred to
above, and the forms and magnitudes of the orbits shown in the
diagram, with those of the first group of elliptic comets given in
Table III, " Hand-Book of Astronomy," chap. XVIII, and drawn
in fig. 2 (p. 172), it will be perceived that the planetary charac-
teristics noticed in the latter group, are nearly effaced. Five of
the six comets composing the second group, revolve in the common
direction of the planets, and this is the only planetary character
observable among them. The inclinations, no longer limited to
those of the planetary orbits, range from 18° to 74°. The eccen-
tricities are all so extreme, that the arc of the orbit near perihelion
approximates closely to the parabolic form, and the most remark-
able body of the group, the comet of Halley, revolves in a direction
contrary to the common motion of the planets.

But it is more than all in the elongated oval form of their
orbits, that this group of comets differs, not only from the planets,
but from the first group. While their perihelia are at distances
from the sun, between those of Mars and Mercury, their aphelia
are from two to five hundred millions of miles outside the orbit
of Neptune. Thus, the comet of Halley, for example, in perihe-
lion, is at a distance from the sun less than that of Venus; but
at its aphelion, its distance exceeds that of Neptune by a space
greater than Jupiter's distance from the sun. The mean angular
motion of this comet is nearly the same as that of Uranus ; but
its angular motion in perihelion is three times that of Mercury ;
184

OTHER ELLIPTIC COMETS.

while its angular motion in aphelion is little more than half that
of Neptune.

The corresponding variations of solar light and heat, and of the
apparent magnitude and motion of the sun as seen from the comet,
may be easily inferred.

IV.— ELLIPTIC COMETS WHOSE MEAN DISTANCES EXCEED THE
LIMITS OF THE SOLAK SYSTEM.

72. Although the periodicity of this class of comets has not yet
in any instance been certainly established by observations made
upon their successive returns to perihelion, the observations made
upon them during a single perihelion passage indicate an arc of
their orbit, which exhibits the elliptic form so unequivocally, as
to supply computers and mathematicians with the data necessary
to obtain, with more or less approximation, the value of the eccen-
tricity, which, combined with the perihelion distance, gives the
form and magnitude of the comet's orbit.

By calculations conducted in this manner, and applied to the
observations made on various comets which have appeared since
the latter part of the seventeenth century, the elliptic orbits of
twenty-one of these bodies have been computed, and are given in
the order of the dates of their perihelion passages in a table which
will be found in my " Hand-Book of Astronomy."

Of this group the least eccentric is No. 15, which passed its
perihelion in 1840. This comet was discovered at Berlin by M.
Bremiker, and its orbit was calculated by Gotze, and proved to
be an ellipse, having the elements given in the table, subject to
no greater uncertainty than ith of the value assigned to the mean
distance. The eccentricity, and consequently the form of the
orbit, is similar to that of Halley, but the major axis is 2| times,
and the period nearly five times greater. Its perihelion distance
is equal to that of Mars, and its aphelion distance more than three
times that of Neptune.

No. 6, which passed its perihelion in 1793, has an orbit, accord-
ing to the calculations of D' Arrest, nearly similar both in form and
magnitude, as will be seen by comparing the numbers given in
the table. More uncertainty, however, attends the estimation of
these elements.

The comets which approached nearest to the sun were the great
comets of 1680 and 1843, Nos. 1 and 16 in the table, both memor-
able for their extraordinary magnitude and splendour.

The elements of that of 1680, given in the table, are those

185

COMETS.

which have resulted from ' the calculations of Professor Enck6r
based on all the observations of the comet which have been re-
corded. The elements of the great comet of 1843 have resulted
from the computations of Mr. Hubbard. Both are subject to-
considerable uncertainty, and must be accepted only as the best
approximations that carf be obtained.

What is not subject, however, to the same uncertainty, is the
extraordinary proximity of these bodies to the
Fig- 6. sun at their respective perihelia. The peri-

helion distance of the comet of 1680 was about
576000 miles, and that of 1843, 538000 miles.
Now the semidiameter of the sun being 441000-
miles, it follows that the distance of the centres
of those comets respectively from the surface
of the sun at perihelion must have been only
235000 and 97000; so that if the semi-
diameter of the nebulous envelope of either of
them exceeded this distance, they must have
actually grazed the sun.

The velocity of the orbital motion of these-
bodies in perihelion appears by the table to be
such, that the comet of 1680 would have re-
volved round the sun in a minute, and that of
1843 in a little less than two minutes, if they re-
tained the same angular motion undiminished.
The distance to which the comet of 1680
recedes in its aphelion is 28| times greater than
that of Neptune. The apparent diameter of the
sun seen from that distance would be 2", and
the intensity of its light and heat would be
730000 times less than at the earth ; while
their intensity at the perihelion distance would
be 26000 times greater, so that the light and
heat received by the comet in its aphelion
would be 26000 x 730000=18980 minion times
less than in perihelion.

The greatest aphelion distances in the table
are those of Nos. 5, 13, and 17, the comets of 1780, 1830, and 1844,
amounting to from 100 to 140 times the distance of Neptune ; the
eccentricities differing from unity by less than j^. These orbits,
though strictly the results of calculation, must be regarded as
subject to considerable uncertainty.

73. To convey an idea of the form of the orbits of the comets
of this group, and of the proportion which their magnitude
bears to the dimensions of the solar system, we have drawn in
186

HYPEKBOLIC AND PARABOLIC COMETS.

fig. 6, an ellipse, which may be considered as representing the
form of the orbits of the comets Nosk 15, 6, 9, 12, and 1, of
Table VI in the " Handbook of Astronomy."

If the ellipse represent the orbit of the comet No. 15, the circle
a will represent on the same scale the orbit of Neptune.

If the ellipse represent the orbit of the comet No. 6, the circle b
will represent the orbit of Neptune.

If the ellipse represent the orbit of No. 9, the circle c will
represent the orbit of Neptune.

If the ellipse represent the orbit of No. 12, the circle d will
represent the orbit of Neptune.

If the ellipse represent the orbit of No. 1, the circle e will
represent the orbit of Neptune.

V. — HYPERBOLIC COMETS.

74. In a Table in the "Hand-Book of Astronomy " are given
the elements of seven comets which appear by the results of cal-
culations made upon the observations to have passed through the
system in hyperbolic orbits.

VI. — PARABOLIC COMETS.

75. Of all the remaining number of comets which have been
seen in the heavens and recorded in history, one hundred and
sixty-one have been observed with sufficient precision to enable
astronomers to determine, with more or less approximation, their
parabolic orbits. A table giving the elements of these, with the
dates of their appearance, will be found in the eighteenth chapter
of the " Handbook of Astronomy."

VII.— PHYSICAL CONSTITUTION OF COMETS.

76. Comets in general, and more especially those which are
visible without a telescope, present the appearance of a roundish
mass of illuminated vapour or nebulous matter, to which is often,
though not always, attached a train more or less extensive, com-
posed of matter having a like appearance. The former is called
the HEAD, and the latter the TAIL, of the comet.

77. The illumination of the head is not generally uniform.
Sometimes a bright central spot is seen in the nebulous matter
which forms it. This is called the NUCLEUS. .

The nucleus sometimes appears as a bright stellar point, and

187

COMETS.

sometimes presents the appearance of a planetary disc seen
through a nebulous haze. In general, however, on examining
the object with high optical power, these appearances are changed,
and the object seems to be a mere mass of illuminated vapour
from its borders to its centre.

78. When a nucleus R apparent, or supposed to be so, the
nebulous haze which surrounds it and forms the exterior part of
the head is called the coma.

79. These designations are taken from the Greek word Ko/jL-f]
(kome) hair, the nebulous matter composing the coma and tail
being supposed to resemble hair, and the object being therefore
called KofjL-fjr-rjs (kometes), a hairy star.

80. As the brightness of the coma gradually fades away towards
the edges, it is impossible to determine with any great degree of
precision its real dimensions. These, however, are obviously
subject to enormous variation, not only in different comets com-
pared one with another, but even in the same comet during the
interval of a single perihelion passage. The greatest of those
which have been submitted to micrometrical measurement was
the great comet of 1811, the diameter of the head of which was
found to be not less than 1| millions of miles, which would give
a volume greater than that of the sun in the ratio of about 2 to 1 .
The diameter of the head of Halley's comet when departing from
the sun, in 1836, at one time measured 357000 miles, giving a
volume more than sixty times that of Jupiter. These are, how-
ever, the greatest dimensions which have been observed in this
class of objects, the diameter rarely exceeding 200000 miles, and
being generally less than 100000.

81. Attempts have been made where nuclei were perceivable,
to estimate their magnitude, and diameters have been assigned to
them, varying from 100 to 5000 miles. For the reasons, how-
ever, already explained, these results must be regarded as very
doubtful.

Those who deny the existence of solid matter within the coma,
maintain that even the most brilliant and conspicuous of those
bodies, and those which have presented the strongest resemblance
to planets, are more or less transparent. It might be supposed
that a fact so simple as this, in this age of astronomical activity,
could not remain doubtful ; but it must be considered, that the
combination of circumstances which alone would test such a ques-
tion, is of rare occurrence. It would be necessary that the centre
of the head of the comet, although very small, should pass
critically over a star, in order to ascertain whether such star is
visible through it. With comets having extensive comae without
nuclei, this has sometimes occurred ; but we have not had such

PHYSICAL CONSTITUTION OF COMETS.

satisfactory examples in the more rare instances of those which
have distinct nuclei.

In the ahsence of a more decisive test of the occultation of a
star by the nucleus, it has been maintained that the existence of
a solid nucleus may be fairly inferred from the great splendour
which has attended the appearance of some comets. A mere mass
of vapour could not, it is contended, reflect such brilliant light.
The following are the examples adduced by Arago : —

" In the year 43 before Christ, a comet appeared which was
said to be visible to the naked eye by daylight. It was the comet
which the Romans considered to be the soul of Caesar transferred
to the heavens after his assassination.

" In the year 1402 two remarkable comets were recorded. The
first was so brilliant that the light of the sun at noon, at the end
of March, did not prevent its nucleus, or even its tail, from being
seen. The second appeared in the month of June, and was visible
also for a considerable time before sunset.

" In the year 1532, the people of Milan were alarmed by the
appearance of a star which was visible in the broad daylight. At
that time Yenus was not in a position to be visible, and conse-
quently it is inferred that this star must have been a comet.

"The comet of 1577 was discovered on the 13th of November
by Tycho Brahe, from his observatory on the isle of Huene, in the
Sound, before sunset.

" On the 1st of February, 1744, Chizeaux observed a comet
more brilliant than the brightest star in the heavens, which soon
became equal in splendour to Jupiter, and in the beginning of
March it was visible in the presence of the sun. By selecting a
proper position for observation, on the 1st of March it was seen at
one o'clock in the afternoon without a telescope."

Such is the amount of evidence which observation has supplied
respecting the existence of a solid nucleus. The most that can
be said of it is, that it presents a plausible argument, giving some
probability, but no positive certainty, that comets have visited
our system which have solid nuclei, but, meanwhile, this can
only be maintained with respect to a few : most of those which
have been seen, and all to which very accurate observations have
been directed, have afforded evidence of being mere masses of
semi-transparent matter.

82. Although by far the great majority of comets are not
attended by tails, yet that appendage, in the popular mind, is
more inseparable from the idea of a comet than any other attribute
of these bodies. This proceeds from its singular and striking
appearance, and from the fact that most comets visible to the
naked eye have had tails. In the year 1531, on the occasion of

189

COMETS.

one of the visits of Halley's comet to the solar system, Pierre
Apian observed that the comet generally presented its tail in the
direction opposite to that of the sun. This principle was hastily
generalised, and is even at present too generally adopted. It is
true that in most cases the tail extends itself from that part of the
comet which is most remote from the sun ; but its direction rarely
corresponds with the direction which the shadow of the comet
would take. Sometimes it has happened that the tail forms with
a line drawn to the sun a considerable angle, and cases have
occurred when it was actually at right angles to it.

Another character which has been observed to attach to the
tails of comets, which, however, is not invariable, is, that they
incline constantly toward the region last quitted by the comet, as
if in its progress through space it were subject to the action of
some resisting medium, so that the nebulous matter with which
it is invested, suffering more resistance than the solid nucleus,
remains behind it and forms the tail.

The tail sometimes appears to have a curved form. That of
the comet of 1744 formed almost a quadrant. It is supposed that
the convexity of the curve, if it exists, is turned in the direction
from which the comet moves. It is proper to state, however, that
these circumstances regarding the tail have not been clearly and
satisfactorily ascertained.

The tails of comets are not of uniform breadth or diameter ;
they appear to diverge from the comet, enlarging in breadth and
diminishing in brightness as their distance from the comet
increases. The middle of the tail usually presents a dark stripe,
which divides it longitudinally into two distinct parts. It was
long supposed that this dark stripe was the shadow of the body
of the comet, and this explanation might be accepted if the tail
was always turned from the sun; but we find the dark stripe
equally exists when the tail, being turned sideways, is exposed to
the effect of the sun's light.

This appearance is usually explained by the supposition that
the tail is a hollow, conical shell of vapour, the external surface
of which possesses a certain thickness. When we view it, we
look through a considerable thickness of vapour at the edges, and
through a comparatively small quantity at the middle. Thus
upon the supposition of a hollow cone, the greatest brightness
would appear at the sides, and the existence of a dark space in
the middle would be perfectly accounted for.

The tails of comets are not always single ; some have appeared

at different times with several separate tails. The comet of

1744, which appeared on the 7th or 8th of March, had six tails,

each about 4° in breadth, and from 30° to 44° in length. Their

190

TAILS OF COMETS.

sides were well defined and tolerably bright, and the spaces
between them were as dark as the other parts of the heavens.

The tails of comets have frequently appeared, not only of
immense real length, but extending over considerable spaces of
the heavens. It will be easily understood that the apparent
length depends conjointly upon the real length of the tail, and
the position in which it is presented to the eye. If the line of
vision be at right angles to it, its length will appear as great as
it can do at its existing distance ; if it be oblique to the eye, it
will be foreshortened, more or less, according to the angle of
obliquity. The real length of the tail is easily calculated when
the apparent length is observed and the angle of obliquity
known.

In respect of magnitude, the tails are unquestionably the most
stupendous objects which the discoveries of the astronomer have
ever presented to human contemplation.

The following are the results of the observation and measure-
ment of a few of the more remarkable.

Table.

No.

Date of
Appearance.

Greatest observed
Length of Tail.

miles.

VIII

148

1847

5000000

—

73

1744

I 9000000

VI

A

1769

40000000

VIII

46

1618

50000000

VI

I

1680

I 00000000

8

1811

I 00000000

—

Q

iSii

1 30000000

—

16

1843

200000000

The magnitude of these prodigious appendages is even less
amazing than the brief period in which they sometime emanate
from the head. The tail of the comet of 1843, long enough to
stretch from the sun to the planetoids, was formed in less than
twenty days.

83. The masses of comets, like those of the planets, would be
ascertained if the reciprocal effects of their gravitation and those
of any known bodies in the system could be observed. But
although the disturbing action of the planets on these bodies is
conspicuous, and its effects have been calculated and observed,
not the slightest effect of the same kind has ever been ascertained
to be produced by them, even upon the smallest bodies in the
system, and those to which comets have approached most nearly.

In fine, notwithstanding the enormous number of comets,

191

COMETS.

observed and unobserved, which constantly traverse the solar
system in all conceivable directions ; notwithstanding the per-
manent revolution of the periodic comets, whose presence and
orbits have been ascertained ; notwithstanding the frequent visits
of comets, which so thoroughly penetrate the system as almost to
touch the surface of the §un at their perihelion, the motions of
the various bodies of the system, great and small, planets major
and minor, planetoids and satellites, go on precisely as if no
such bodies as the comets approached their neighbourhood.
Not the smallest effects of the attraction of such visitors are
discoverable.

Now since, on the other hand, the disturbing effects of the
planets upon the comets are strikingly manifest, and since the
comets move in elliptic, parabolic, or hyperbolic orbits, of which
the sun is the common focus, it is demonstrated that these bodies
are composed of ponderable matter, which is subject to all the
consequences of the law of gravitation. It cannot, therefore, be
doubted that the comets do produce a disturbing action on the
planets, although its effects are inappreciable even by the most
exact observation. Since, then, the disturbances mutually pro-
duced are in the proportion of the disturbing masses, it follows
that the masses of the comets must be smaller beyond all calcula-
tion than the masses even of the smallest bodies among the planets
primary or secondary.

The volumes of comets in general exceed those of the planets
in a proportion nearly as great as that by which the masses of the
planets exceed those of the comets. The consequence obviously
resulting from this, is that the density of comets is incalculably
small.

Their densities in general are probably thousands of times less
than that of the atmosphere in the stratum next the surface of
the Earth.

192

Fig. 27.— February 10, 1836.

Fig. 29.— February 23, 1836.

HALLEY'S COMET DEPARTING FROM THE SUN IN 1836.

COMETS.

CHAPTEE IV.

84. Light or comets. — 85. Enlargement of magnitude on departing from
the sun. — 86. Professor Struve's drawings of EnckS's comet. — 87.
Remarkable physical phenomena manifested by Halley's comet. —

88. Struve's drawings of the comet approaching the sun in 1835. —

89. Its appearance on the 29th of Sept. — 90. Appearance on Oct. 3.
— 91. Appearance on Oct. 6. — 92. Appearance on Oct. 9. — 93. Ap-
pearance on Oct. 10. — 94. Appearance on Oct. 12. — 95. Appearance
on Oct. 14. — 96. Appearance on Oct. 29.— 97. Appearance on Nov. 5.
— 98. Sir J. Herschel's deductions from these phenomena. — 99. Ap-
pearance of the comet after perihelion. — 100. Observations and
drawings of MM. Maclear and Smith. — 101. Appearance on Jan. 24.
— 102. Appearance on Jan. 25. — 103. Appearance on Jan. 26. — 104.
Appearance on Jan. 27. — 105. Appearance on Jan. 28. — 106. Ap-
pearance on Jan. 30. — 107. Appearance on Feb. 1. — 108. Appearance
on Feb. 7. — 109. Appearance on Feb. 10. — 110. Appearance on
Feb. 16 and 23.— 111. Number of comets.— 112. Duration of the
appearance of comets. — 113. Near approach of comets to the earth.

84. THAT planets are not self-luminous, but receive their light
from the sun, is proved by their phases, and by the shadows of
LARDNER'S MUSEUM OF SCIENCE. o 193

No. 142.

COMETS.

their satellites, which are projected upon them, when the latter
are interposed between them and the sun. These tests are inap-
plicable to comets. They exhibit no phases, and are attended by
no bodies to intercept the sun's light. But, unless it could be
shown that a comet is a solid mass, impenetrable to the solar
rays, the non-existence »f phases is not a proof that the body does
not receive its light from the sun.

A mere mass of cloud or vapour, though not self-luminous, but
rendered visible by borrowed light, would still exhibit no effect
of this kind : its imperfect opacity would allow the solar light
to affect its constituent parts throughout its entire depth — so
that, like a thin fleecy cloud, it would appear not superficially
illuminated, but receiving and reflecting light through all its
dimensions. With respect to comets, therefore, the doubt which
has existed is, whether the light which proceeds from them, and
by which they become visible, is a light of their own, or is the
light of the sun shining upon them, and reflected to our eyes
like light from a cloud. Among several tests which have been
proposed to decide this question, one suggested by Arago merits
attention.

It has been already shown in our Tract on "the Eye" (43),
that the apparent brightness of a visible object is the same at all
distances, supposing its real brightness to remain unchanged.
Now if comets shone with their proper light, and not by light
received from the sun, their apparent brightness would not
decrease as they would recede from the sun, and they would cease
to be visible, not because of the faintness of their light, but
because of the smallness of their apparent magnitude. Now the
contrary is found to be the case. As the comet retires from the
sun its apparent brightness rapidly decreases, and it ceases to be
visible from the mere faintness of its light, while it still subtends
a considerable visual angle.

85. It will doubtless excite surprise, that the dimensions of a
comet should be enlarged as it recedes from the source of heat.
It has been often observed in astronomical inquiries, that the
effects, which at first view seem most improbable, are never-
theless those which frequently prove to be true ; and so it is in
this case. It was long believed that comets enlarged as they
approached the sun ; and this supposed effect was naturally and
probably ascribed to the heat of the sun expanding their dimen-
sions. But more recent and exact observations have shown the
very reverse to be the fact. Comets increase their apparent
volume as they recede from the sun ; and this is a law to which
there appears to be no well-ascertained exception. This singular
and unexpected phenomenon has been attempted to be accounted
194

LIGHT OF COMETS.

for in several ways. Yalz ascribed it to the pressure of the solar
atmosphere acting upon the comet ; that atmosphere being more
dense near the sun, compresses the comet and diminishes its
dimensions ; and, at a greater distance, being relieved from this
coercion, the body swells to its natural bulk. A very ingenious
train of reasoning was produced in support of this theory. The
density of the solar atmosphere and the elasticity of the comet,
being assumed to be such as they might naturally be supposed,
the variations of the comet's bulk are deduced by strict reason-
ing, and show a surprising coincidence with the observed
change in the dimensions. But this hypothesis is tainted by a
fatal error. It proceeds upon the supposition that the comet,
on the one hand, is formed of an elastic gas or vapour ; and, on
the other, that it is impervious to the solar atmosphere through
which it moves. To establish the theory, it would be neces-
sary to suppose that the elastic fluid composing the comet should
be surrounded by a nappe or envelope as elastic as the fluid
composing the comet, and yet wholly impenetrable by the solar
atmosphere.

Several ingenious hypotheses * have been proposed and suc-
cessively rejected for explaining this phenomenon, but it seems
now agreed to ascribe it to the action of the varying temperature
to which the vapour which composes the nebulous envelope is
exposed. As the comet approaches the sun, this vapour is con-
verted by intense heat into a pure, transparent, and therefore
invisible elastic fluid. As it recedes from the sun, the tem-
perature decreasing, it is partially and gradually condensed, and
assumes the form of a semitransparent visible cloud, as steam
does escaping from the valve of a steam boiler. It becomes more
and more voluminous as the distance from the source of heat, and
therefore the extent of condensation, is augmented.

86. Professor Struve made a series of observations on the comet
of Encke, at the period of its reappearance in 1828, and by the
aid of the great Dorpat telescope, made the drawings figs. 7
and 8.

Fig. 7 represents the comet as it appeared on the 7th Novem-
ber, the diameter a b measuring 18'. The brightest part of
the comet extended from k to I, and was consequently eccen-
tric to it, the distance of the centre of brightness from the
centre of magnitude being k K. Between the 7th and the 30th
November, the magnitude of the comet decreased from that
represented in fig. 7, to that represented in fig. 8 ; but the

* For several of these, see Sir J. HerscheFs memoir, •" Proceedings of
Astronomical Society," vol. vi. p. 104.

o 2 195

COMETS.

apparent brightness was so much increased, that at the latter date
it was visible to the naked eye as a star of the 6th magnitude.
The apparent diameter was then reduced to 9'.

On November 7th a star of the llth magnitude was seen
through the comet, so near the centre of brightness that it was
for a moment mistakeif for a nucleus. The brightness of the
star was not in the least perceptible degree dimmed by the mass
of cometary matter through which its light passed.

It was evident that the increase of the brightness of the comet
on the 30th November, must be ascribed to the contraction, and
consequent condensation, of the nebulous matter composing it
in receding from the sun, for its distance from the earth on the
7th November, when it subtended an angle of 18', was 0-515 (the
earth's mean distance from the sun being =1) ; while its distance
on the 30th, when it subtended an angle of 9', was only 0'477.
Its cubical dimensions must, therefore, have been diminished,
and the density of the matter composing it augmented in more
than eight-fold proportion.

87. The expectation so generally entertained, that, on the
occasion of its return to perihelion in 1835, this comet would
afford observers occasion for obtaining new data, for the
foundation of some satisfactory views respecting the physical
constitution of the class of which it is so striking an example,
was not disappointed. It no sooner reappeared than phenomena
began to be manifested, preceding and accompanying the gradual
formation of the tail, the observation of which has been most
justly regarded as forming a memorable epoch in astronomical
history.

Happily, these strange and important appearances were
observed with the greatest zeal, and delineated with the most
elaborate and scrupulous fidelity by several eminent astronomers
in both hemispheres. MM. Bessel at Konigsburg, Schwabe at
Dessau, and Struve at Pultowa, and Sir J. Herschel and Mr.
Maclear at the Cape of Good Hope, have severally published their
observations, accompanied by numerous drawings, exhibiting
the successive transformations presented under the physical
influence of varying temperature, in its approach to and departure
from the sun.

The comet first became visible as a small round nebula, without
a tail, and having a bright point more intensely luminous than
the rest eccentrically placed within it. On the 2nd October, the
tail began to be formed, and, increasing rapidly, acquired a length
of about 5° on the 5th ; on the 20th it attained its greatest length,
which was 20°. It began after that day to decrease, and its
diminution was so rapid, that on the 29th it was reduced to 3°,
196

ENCKES COMET.

Fig. 7.— Encke's Comet, 1828, approaching the Sun ; as it appeared 7th Nov., 1828.
(Telescopic drawing by Strwve.)

Fig. 8.— The same as it appeared 30th November, 1828.

197

COMETS.

and on the oth of November, to 2£°. The comet was observed on
the day of its perihelion by M. Struve, at the Observatory of
Pultowa, when no tail whatever was apparent.

The circumstances which accompanied the increase of the tail
from 2nd October, untjj its disappearance, were extremely remark-
able, and were observed with scrupulous precision, simultaneously
by Bessel at Kouigsburg, by Struve at Pultowa, and by Schwabe
at Dessau, all of whom made drawings from time to time, delineat-
ing the successive changes which it underwent.

On the 2nd, the commencement of the formation of the tail took
place, by the appearance of a violent ejection of nebulous matter
from that part of the comet which was presented towards the sun.
This ejection was, however, neither uniform nor continuous.
Like the fiery matter issuing from the crater of a volcano, it was
thrown out at intervals. After the ejection, which was con-
spicuous, according to Bessel, on the 2nd, it ceased, and no efflux
was observed for several days. About the 8th, however, it re-
commenced more violently than before, and assumed a new form.
At this time Schwabe noticed an appearance which he denominates
a " second tail," presented in a direction opposed to that of the
original tail, and, therefore, towards the sun. This appearance
seems, however, to be regarded by Bessel merely as the renewed
ejection of nebulous matter which was afterwards turned back
from the sun, as smoke would be by a current of air blowing from
the sun in the direction of the original tail.

From the 8th to the 22nd,

Fig. 9.— Sept. 29. 1835. .. „

the form, position, and bright-
ness of the nebulous emana-
tions underwent various and
irregular changes, the last
alternately increasing and de-
creasing.

At one time two, at another
three, nebulous emanations
were observed to issue in
divergent directions. These
directions were continually
varying, as well as their com-
parative brightness. Some-
times they would assume a
swallow- tailed form, resem-
bling the flame issuing from
a fan gas-burner. The prin-
cipal jet or tail was also observed to oscillate on the one side
and the other of a line drawn from the sun through the centre
198

APPEARANCE OF HALLEY S COMET.

•of the head of the comet, exactly as a compass needle oscillates
between the one and the other Fig 10.— Oct. s, 1835.

side of the magnetic meridian.
This oscillation was so rapid,
that the direction of the jets
was visibly changed from hour
•to hour. The brightness of the
matter composing them, being
most intense at the point at
which it seemed to be ejected
from the nucleus, faded away
,as it expanded into the coma,
curving backwards, in the di-
rection of the principal tail,
like steam or smoke before the
wind.

88. These curious pheno-
mena will, however, be more
clearly conceived by the aid
of the admirable drawings of
M. Struve, which we have re-
produced with all practicable
fidelity, in figs. 9, 10, 11, 12,
13, 14, 15, 16, 17, and 18.
These drawings were executed
by M. Kruger, an eminent
artist, from the immediate ob-
servation of the appearances
•of the comet with the great
Fraunhoffer telescope, at the
Pultowa Observatory. The
sketches of the artist were cor-
rected by the astronomer, and
only adopted definitively after
repeated comparisons with the
object. The original drawings
are preserved in the library of
the observatory.

89. Fig. 9 (page 198) repre-
sents the appearance of the
comet on the 29th September.
The tail was difficult to be re-
cognised, appearing to be com-
posed of very feeble nebulous

•matter. The nucleus passed

COMETS.

almost centrically over a star of the tenth magnitude, without in
the slightest degree affecting its apparent brightness. The star was
distinctly seen through the densest part of the comet. Another
transit of a star took place with a like result.

Annexed is the scale according to which this drawing has been
made.

10'

10'

20'

30'

40'

50'

MJIJllilj

90. The comet changed not only its magnitude and form, but also
its position, after September 29. On that day the direction of the

tail was that of the parallel of

Fig. ii.-Oct. 8, 18.35. declination through the head.

On October 3rd it was inclined
from that parallel towards the
north at a small angle, and,
instead of being straight, was
curved, as shown in fig. 10
(page 199). The diameter of
the head was increased in the
ratio of 2 to 3, and the length
of the tail in the ratio of
nearly 1 to 3.

91 . On the 5th, 6th, and 7th
the comet underwent several
changes : the nucleus became
more conspicuous. On the 6th,
a fan-formed flame issued from
it, which disappeared on the
7th, and re-appeared on the 8th with increased splendour, as
represented in fig. 11, which is drawn on the subjoined scale :

10" 0"

20"

40"

60"

80"

100" 120"

I I I

The nucleus appeared like a burning coal of oblong form, and
yellowish colour. The extent of the flame-like emanation was
about 30". The feeble nebula surrounding the nuclei extended
much beyond the limits of the drawing, but, being overpowered
by the moonlight, could not be measured.

92. The comet as it appeared on the following night is shown
in fig. 12, which is on the same scale as fig. 11. The nucleus and
flame-like emanation entirely changed their form and magnitude
200

VARIATIONS OF APPEARANCE.

Fig" 12-Oc

since the preceding night. The tail (not included in the drawing)

measured very nearly 2°.

The flame consisted of two

parts, one resembling that

seen on the 8th, and the

other issuing like the jet

from a hlow-pipe in a di-

rection at right angles to

it. The figure represents

the nucleus and flame as

they appeared at 21h side-

real time, with a magnify-

ing power of 254.

93. The appearance of

the following night is shown

on the same scale in fig.

13. The tail, which still

measured nearly 2°, was

now much brighter, being

visible to the naked eye,

notwithstanding strong

moonlight. The coma was

evidently broader than the

tail. The flaming nucleus

is represented in the draw-

ing as it appeared under a

magnifying power of 86,

with a field of 18' diameter,

the entire of which was

filled with this coma. The

diameter of the latter must, therefore, have been more than 18'.

The drawing was taken at 21'. s. t.

94. The comet is repre-
sented in fig. 14 (page 202),
on the same scale, as it ap-
peared on the night of the
12th. It appeared at Oh 25ra
s. t. for a short interval in
uncommon splendour, the
nucleus and flame, however,
alone being visible, as repre-
sented in the drawing. The
greatest extent of the flame
measured 64"- 7. Its appear-
ance was most beautiful,

]3 _Qct

COMETS.

resembling a jet streaming out from the nucleus, like flame from
a blow-pipe, or the flame from the discharge of a mortar, attended

Tig. 14.— Oct. 12, 1835.

Fig. 15.— Oct. 14, 1835.

with the white smoke driven be-
fore the wind.

95. Its appearance on the 14th
is shown, on the same scale, in
fig. 15. The principal flame was

now greatly enlarged, extending to the apparent length of 134".
Its deflection and curved form were most remarkable.

96. A cloudy sky prevented all observation for 12 days. On
the 27th, the comet appeared to the naked eye as bright as a star
of the third magnitude, the tail being distinctly visible. The
coma surrounding the nucleus appeared as a uniform nebula.
The tail was curved and of great length ; but, owing to the low
altitude at which the observation was taken, it could not be
measured. On the 29th, however, the comet was presented under
much more favourable conditions, and the drawings, fig. 16 and
fig. 17 were made. The former represents the entire comet,
including the whole visible extent of the tail, and is drawn to the
annexed scale of minutes.

10'

20'

ILL

30'

I

40'

The latter represents the head of the comet only, and is drawn to
the annexed scale of seconds.

10" 0" 20" 40" GO" 80" 100" 120"
.11111 Ml! I II I I II I

202

VARIATIONS OP APPEARANCE.

At 20h 30m s. t., the head presented the appearance represented
in fig. 17 (page 204). The chief coma was almost exactly cir-
cular, and had a diameter of
165". With a power of 198,
the nucleus appeared as in
the figure, the diameter being
about !"• 25 to 1"-50. The
flame issuing from the nu-
cleus, curved back like smoke
before the wind, was very
conspicuous. The appearance
•of the formation of the tail as
it issues from the nucleus was
remarkably developed.

97. On the 5th of Novem-
ber the comet appeared as
shown in fig. 18 (page 204).
This drawing represents the
nucleus and flame issuing
from it on the scale of seconds
.given below.

The proper nucleus was
found to measure about 2"-3.
Two flames were seen issuing
from it in nearly opposite
directions, and both curved
towards the same side. The
brighter flame, directed to-
wards the north, was marked
by strongly defined edges.
The other, directed towards
the south, was more feeble and ill-defined.

10'

20"

GO''

180''

120"

ILI1L

I I I

98. Sir J. Herschel, who also observed this comet himself at
the Cape of Good Hope, makes from all these observations the
following inferences.

(1.) That the matter of the comet vaporised by the sun's heat
escapes in jets, throwing the comet into irregular motion by its
reaction, and thus changing its own direction of ejection.

(2.) That this ejection takes place principally from the part
presented to the sun.

(3.) That thus ejected it encounters a resistance from some

203

COMETS.

Fig. 17.-0ct. 29, 1835.

Fig. 18.— Nov. 5, 1835.

204

VARIATIONS OF APPEARANCE.

unknown force by which it is repulsed in the opposite direction,
and so forms the tail. *

(4.) That this acts unequally on the cometary matter, which is
not all vaporised, and of that which is, a considerable portion is
retained, so as to form the head and coma.

(5.) That this force cannot be solar gravitation, being contrary
to that in its direction, and very much greater in its intensity, as
is manifest by the enormous velocity with which the matter of the
tail is driven from the sun.

(6.) That the matter thus repelled to a distance so great from
a body whose mass is so small must, to a great extent, escape
from the feeble influence of the gravitation of the mass composing
the head and coma, and, unless there be some more active agency
in operation, a large portion of such vaporised matter must be
lost in space, never to reunite with the comet. This would
lead to the consequence, that at every passage through its
perihelion the comet would lose more and more of its vaporisable
constituents, on which the production of the coma and tail
depends, so that, at each successive return, the dimensions of
these appendages would be less and less, as they have in fact
been found to be.

99. On receding from the sun after its perihelion, the comet
was observed under very favourable circumstances at the Cape
by Sir J. Herschel and Mr. Maclear. It first reappeared there
on the 24th of January, under an aspect altogether different
from that under which it was seen before its perihelion. It
had evidently, as Sir J. Herschel thinks, undergone some great
physical change, which had operated an entire transformation
upon it.

" Nothing could be more surprising than the total change
which had taken place in it since October. ... A new and
unexpected phenomenon had developed itself, quite unique in the
history of comets. Within the well-defined head, somewhat
eccentrically placed, was a vivid nucleus resembling a miniature
comet, with a bead and tail of its own, perfectly distinct from
and considerably exceeding in intensity the nebulous disc or
envelope which I have above called the ' head.' A minute bright
point, like a small star, was distinctly perceived within it, but
which was never quite so well defined as to give the positive
assurance of the existence of a solid sphere, much less could any
phase be discerned." *

100. The phenomena and changes which the comet presented
from its reappearance on the 24th of January, until its final

* "Cape Observations," p. 397.

205

COMETS.

disappearance, have been described with great clearness by Mr.
Maclear, and illustrated by a beautiful series of drawings by that
astronomer and his assistant, Mr. Smith, in a memoir which
appeared in the tenth volume of the Transactions of the Royal
Astronomical Society, from which we reproduce the series of
illustrations given in figT 19 to fig. 29.

101. On the night of the 24th of January, 1836, the comet
appeared, as in fig. 19, visible to the naked eye as a star of the
second magnitude. The head was nearly circular, and presented
a pretty well-defined planetary disc, encompassed by a coma or
halo of delicate gossamer-like brightness. The diameter of the
head, without the halo or coma, measured 131", and with the
latter 492".

102. On the night of the 25th the comet had the appearance
represented in fig. 20. The circular form was broken, and the
magnitude of the head was increased. Three stars were seen
through the coma and one through the head.

103. On the 26th of January the magnitude of the head was-
further increased, but that of the coma was diminished (fig. 21).

104. On the 27th the comet began to assume a parabolic form,
as shown in fig. 22, and the increasing magnitude continued.

105. On the 28th the coma or halo was quite invisible, but
the nucleus appeared like a faint small star. The magnitude of
the comet continued to increase. The observer fancied he saw the
faint outline of a tail (fig. 23).

Fig. 23.— January 28, 1836.

106. On the 30th the form of the comet became decidedly
parabolic (fig. 24). The breadth across the head was 702", being
greater than on the 24t'i in the ratio of 49 to 70, or 7 to 10,
206

VARIATIONS- OF APPEARANCE.

which corresponds to an increase of volume in the ratio of 1 to 3,
supposing the form to remain unchanged ; but it was estimated

Fig. 24.— January 30, 1836.

that the extension in length gave a superficial increase in the
ratio of 35 to 1, which would correspond to a much greater
augmentation of volume.

107. On the 1st of February a further increase of magnitude-
took place, the figure remaining the same (fig. 25).

Fig. 25.— February 1, 1836.

108. On the 7th of February the comet was rendered faint by
the effect of moonlight (fig. 26).

109. On the 10th a further increase of volume took place, a star
being visible through the body (fig. 27).

207

COMETS.

110. From the 16th, on which it presented the appearance
shown in fig. 28, to the 23rd, when it assumed the appearance
shown in fig. 29, the magnitude went on increasing, while the
illumination became more and more faint, and this continued
until the comet's final disappearance ; the outline, after a short
time, became so faint as to be lost in the surrounding darkness,
leaving a bland nebulous blotch with a bright centre enveloping
the nucleus.

111. According to Mr. Hind, the number of comets which have
appeared since the birth of Christ in each successive century is as
follows : first century, 22 ; second, 23 ; third, 44 ; fourth, 27 ;
fifth, 16 ; sixth, 25 ; seventh, 22 ; eighth, 16 ; ninth, 42 ; tenth,
26; eleventh, 36; twelfth, 26; thirteenth, 26; fourteenth, 29;
fifteenth, 27; sixteenth, 31; seventeenth, 25; eighteenth, 64;
nineteenth (first half), 80. Total, 607.

112. Since comets are visible only near their perihelia, when
their velocity is greatest, the duration of their visibility at any
single perihelion passage is generally short. The longest appear-
ance on record is that of the great comet of 181 1 (No. 8, Table VI.,
" Hand-Book of Astronomy," chap, xviii.), which continued to
be visible for 510 days. The comet of 1825 (No. 2, Table VI.
" Hand-Book of Astronomy," chap, xviii.) was visible for twelve
months, and others which appeared since have been seen for eight
months. In general, however, these bodies do not continue to be
seen for more than two or three months.

113. Considering the vast number of comets which ha\e passed
through the system, such an incident as the collision of one of
them with a planet might seem no very improbable contingency.
Lexell's comet was supposed to have passed among the satellites
of Jupiter ; and, if that was the case, it is certain that the motions
of these bodies were not in the least affected by it. The nearest
approach to the earth ever made by a comet was that of the comet
of 1684 (No. 55, Table VIII., " Hand-Book of Astronomy," chap,
xviii.), which eame within 216 semidiameters of the earth, a
distance not so much as four times that of the moon. We are
not aware of any nearer approach than this being certainly
ascertained.

208

JULY, 1856.

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Chap. I.

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