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COURSE OF LECTURES

NATURAL PHILOSOPHY,

VOLUME II. PLATES.

COURSE OF LECTURES

NATURAL PHILOSOPHY

MECHANICAL ARTS.

BY THOMAS YOUNG, M.D.,

A NEW EDITION,, WITH REFERENCES AND NOTES,

BY THE

REV. P. KELLAND, M.A., F.R.S., LOND. & EDINB.,

frliustratftr tn> Itumerous (Engrabtngs on

IN TWO VOLUMES.
VOLUME II. - PLATES.

LONDON:
PRINTED FOR TAYLOR AND WALTON,

UPPER GOWER STREET.
1845.

IIM'O # (Kf I H '] J A H f T A

Printed by J. & H. COX, BROTHERS (LATB COX & SONS),
74 & 75, Great Queen Street, Lincoln's-Inn Fields.

FESCEIPTION OF PLATES.

M363J23

nroaans

DESCRIPTION OF PLATES.

PLATE I.

Fig. 1. The point A being supposed to move in a right line to B, A B is the direc-
tion of its motion. P. 15.

Fig. 2. The lines AB, BC, CD, are the successive directions of the point A,
moving from A to D in the figure A B C D. P. 15.

Fig. 3. The tangent A B is the direction of the motion of the point C, moving in
the curve C D, when it arrives at E. P. 15.

Fig. 4. The square A B, moving on the board C D, so that the points E, F, de-
scribe the parallel lines EG, F H, with equal velocities, the plane A E F B is in rec-
tilinear motion with respect to the surface CD. P. 18.

Fig. 5. The cycloid ABC, and the trochoid D E F are the results of the rotatory
motion of the points B and E round the centre of the wheel, combined with the pro-
gressive motion of the wheel along the base A C. P. 18, 34.

Fig. 6. A B is a fixed bar, C D an arm which slides on it, E C F a thread pass-
ing round the pulley at C, and either fixed to the pin on the slider F, or passed over
the pulley G, and fixed again at H. The arm turns round the same axis that carries
the pulley at C, and may be fixed by means of the screw which is cut on the axis,
while two other screws keep it steady by pressing on the slider below it. The point
I describes, by its compound motion, the oblique line K I. P. 18.

Fig. 7. The diagonal A B of the parallelogram C D is the joint result of the mo-
tions represented by its sides AC, AD. P. 19.

Fig. 8. The line A B may be either simply drawn in the direction A B, or it may
be traced by the equal motions A C and A D of the arm and its slider, or by the un-
equal motions A E and A F. P/19.

Fig. 9. The body A, moving uniformly along the line A B, first approaches to
the point C, and then recedes from it, as if repelled. P. 21.

Fig. 10. When A B and A C approach each other, and coincide, the diagonal A D
becomes equal to their sum. P. 23.

Fig. 11. Atwood's machine. The boxes A, B, containing equal weights, are
connected by the thread A C B, passing over the pulley C, which is supported either
on friction wheels, or by the points of screws, one of which is seen at D, The box A
is made to descend either by a flat weight placed on it, or by the bar E, which is
intercepted by the ring F, and the box continues to descend till it strikes the stage
G ; the space being measured on the scale H I, and the time by the pendulum K,
which may be kept in motion by a clock scapement with a weight. The machine is
levelled by the screws L, M. P. 23, 41.

Fig. 12. The time of the descent of a falling body being represented by any por-
tion A B of the base of a triangle, the velocity will be proportional to B C, which is
equal to A B, and the space described during the time D E, supposed infinitely
short, will be proportional to the area D E F G, which is expressed by the product of
B C and D E ; consequently the whole area A E F will represent the space described
in the time A E, and A H I the space described in the time AH; but A H I is half
of the square H K, and A E F of EL: the space is therefore always as the square of
the time, and is equal to half the space which would be described in the same time
with the final velocity. P. 24.

Fig. 13. The whirling table. The arms A B, C D, are made to revolve on the
axes E F, G H by the string passing over the wheel I, the upper or under pulley of
either axis being employed at pleasure : the stages K, L, with their weights, are
placed at certain distances from the centre, by means of the racks or teeth below
them ; they move along the arms by means of friction wheels resting on wires, and
they raise the weights M, N, by means of threads passing each over two pullies.
P. 27.

Fig. 14. If a body revolving in a curve ABC, by means of a force directed to
D, describe the portions A E, B F, C G in equal times, the areas A D E, B D F,
C D G, will be equal, and the velocities in A, B, and G, will be inversely as the per-
pendiculars D H, D I, and D K. P. 28.

b'2

DESCRIPTION OF PLATES.

PLATE II.

Fig. 15. The ball A, revolving round the point B, and being drawn towards it
by means of the thread B C, with a force variable at pleasure, its velocity may be
observed to vary according to its distance from the point B. P. 29.

Fig. 16. The curve A B C D E is an ellipsis ; F and G are its foci, A D its greater
axis, and C E its lesser axis. P. 29.

Fig. 17. The horizontal range, A B, of a body projected at an elevation of 45°,
is greater than A C or A D, the ranges of bodies projected with the same velocity at
a greater or less elevation. If the parallel lines E F, G H, be always as the squares
of A E, A G, the curve A F H will be a parabola ; and such is the path of a pro-
jectile. P. 31.

Fig. 18. The path of a ball moving swiftly through the atmosphere nearly resem-
bles the curve A B. P. 31.

Fig. 19. The ball A, having descended along the groove A B, describes the para-
bola B C, passing through the rings D, E. P. 31.

Fig. 20. The cylinder A, loaded at the axis, descends along an inclined plane
more rapidly than the cylinder B, loaded with an equal weight at the circumference.
P. 33.

Fig. 21. The balls A, B, C, descend along the planes A D, B E, C F, of equal
height, in times proportional to their lengths. The upper surfaces of the slips A D,
B E, C F, are slightly grooved. P. 33.

Fig. 22. The balls A, B, C, descend in equal times along the chords A D, B D,
CD. P. 33.

Fig. 23. The same ball, descending from equal heights, at A, B, or C, by differ-
ent paths, will rise to the same height at D on the opposite side of E. P. 34. ,'. •

Fig. 24. The thread A B, playing between the cycloidal cheeks A C, A D, de-
scribes the cycloid C E D ; and the balls B, F, descending from any two points of the
curve, will meet at E, in the same time that the ball G falls from a point nearly £ of
A E above A. The space described by the pendulum in descending is always pro-
portional to the height HI, to which a body setting out from E, and revolving
uniformly in a circle, will rise in the same time. The circle E I lies without the
cycloid C E D, and is somewhat less inclined to the horizon at equal distances from
E. P. 35.

Fig. 25. The ball A, descending from B in the curve B A, arrives at C before the
ball D, moving in a right line on the plane B C. P. 36.

Fig. 26. The balls A, B, C, being made to revolve by means of the whirling table,
they are always found in the same horizontal plane. The joint connecting them with
the axis is represented at D, as seen from above. P. 36.

Fig. 27. The equal vibrations, represented by A B, C D, compose, when united,
the circular revolution, A E B : the unequal vibrations A B, F G, compose the ellipsis
A H B ; the place of the body being always ascertained by combining the versed sines
of two circular arcs increasing uniformly. P. 37.

Fig. 28. The balls A, B, as" their revolution becomes more rapid, fly out, and the
point C is depressed. P. 37.

Fig. 29. The mass of the body A being 1, and that of B 2, and A C being twice
B C, C is the centre of inertia [gravity]. P. 40.

Fig. 30. The balls A and B are suspended by long threads, which allow them to
move in the arcs AC, B D ; the ball A is perforated in a horizontal direction, and
contains a spiral spring, which is confined by the thread E, and being set at liberty
by burning this thread, strikes the ball B, so as to cause each of the balls to move
through an arc, of which the chord is proportional to the weight of the other ball.
P. 40.

DESCRIPTION OF PLATES.

PLATE III.

Fig. 31. The centre of inertia of the bodies A, B, C, D, may be determined,
either by finding E the centre of inertia of A and B, and supposing a body equal to
their sum to be placed in it, then determining F from E and C ; and G, the point
required, from F and D ; or by finding first H and I from A, C, B, D, taken in pairs,
and dividing H I in due proportion in the same point G. P. 41.

Fig. 32. The point A being the centre of inertia of the bodies B, C, D, E, the
products obtained by multiplying B by B F, C by C G, D by D H, and E by E I,
are equal, when added together, to the product of the masses of all the bodies by the
distance A K ; all the lines drawn to the plane F I being parallel. P. 42.

Fig. 33. The weights ABC will remain at rest when they are in the same pro-
portion to each other as the respective sides of the triangle D E F ; D F being parallel
to E G. P. 47.

Fig. 34. The bodies A, B, remain in equilibrium when their centre of inertia C
is immediately below the point of suspension D. P. 47.

Fig. 35. The system of bodies A, B, C, is at rest when the centre of inertia D is
immediately below the point of suspension E. P. 47.

Fig. 36. The bodies A, B, remain at rest when the centre of inertia C is imme-
diately above the point of support D. P. 47.

Fig. 37. The bodies A, B, remain at rest when the centre of inertia C coincides
with the fulcrum, or point of support. P. 47.

Fig. 38. The irregular body A B remains at rest when the centre of inertia C is
immediately below the point of suspension D. P. 47.

Fig. 39. A being the centre of gravity of the board B, C, the point of suspension
being D, E, or F, the position of the vertical line will be D A, E A, or F A. P. 47.

Fig. 40. The equilibrium of the vessel A is stable, that of the vessel B tottering ;
the path of the centre of gravity having its concavity upwards in the first, and down-
wards in the second. P. 48.

Fig. 41. Paths of the centre of gravity of an oval. P. 48.

Fig. 42. Paths of the centre of gravity of a body resting on a sphere. P. 48.

Fig. 43. A, the path of the centre of gravity of a body standing on a flat basis ;
B, the tottering equilibrium of the same body inclined. P. 48.

Fig. 44. The effects of a certain inclination of a waggon, loaded with light and
heavy materials, are represented at A and B respectively. P. 48.

Fig. 45. The suspension of a weight from a rod projecting over a table. P. 49.

Fig. 46. A shows the path of the centre of gravity of a loaded cylinder on an
inclined plane, B that of the centre of gravity of a double cone moving towards the
more elevated end of a triangular surface. C is an elevation of the double cone.
P. 49.

Fig. 47. A B is a lever of the first kind, the forces acting on different sides of the
fulcrum C ; D E of the second kind, the forces being applied at D and F, on the
same side of E. P. 50.

Fig. 48. A force applied at A may be held in equilibrium by a triple force, applied
in the direction B C, either at B or at C, or in a direction perpendicular to the arm
C D at E, D E and D B being each one third of A D. P. 51.

Fig. 49. A force, acting at A on the lever A B, has a great mechanical advantage
in turning the lever C D ; but when the levers are in the position BE, D F, the force
acts with a similar disadvantage. P. 51.

Fig. 50. The diameter of the cylinder A being three times as great as that of B,
the weight C, or an equivalent force applied to the winch D, will support a triple
weight at E. P. 51.

DESCRIPTION OF PLATES.

PLATE IV.

Fig. 51. The weight A, acting on the double cylinder B, supports the weight C
by the pulley running in the angle of the rope D C E, which is wound on the larger
cylinder at D, while it is uncoiled from the smaller at E, and the force is the same as
if the weight C were attached to the line C F, acting on the axis F, of which the
diameter is equal to the difference of the radii of the double cylinder. P. 52, 158.

Fig. 52. A single fixed pulley, supporting two equal weights. P. 52.

Fig. 53. A single moveable pulley, by means of which a weight supports another
twice as great. P. 52.

Fig. 54. The arrangement of pullies in ships' tackles, with a force of six to one.
P. 53.

Fig. 55. An arrangement of pullies in a vertical line, with a force of six to one.
P. 53.

Fig. 56. Mr. Smeaton's blocks, giving a force of twenty to one, the rope being
applied in the middle of the outer series, and following the order of the figures from
1 to 21. P. 53, 159.

Fig. 57. A system of pullies fixed on one axis hi each block ; having a power of
8 to 1. P. 53.

Fig. 58. A system of pullies, each of which doubles the effect ; having a power of
8 to 1. P. 53.

Fig. 59. A system of pullies with each rope fixed to the weight, having a force of
7 to 1. P. 53.

Fig. 60. Two systems of pullies, of the kind denominated Spanish bartons, in
which two of the pullies are suspended by the same rope ; the one has a power of 4,
the other of 5. P. 53.

Fig. 61. A. The depression of the middle weight being one third of its distance
from the pullies, it sustains two equal weights, which are together three times as
great as itself. B. The depression of the smaller weight being one fourth of its
distance from the pulley, it supports a weight twice as great as itself. P. 53.

Fig. 62. A joiner's saw, stretched by twisting a double cord, by means of a lever
passing through it. P. 54.

Fig. 63. The weight A, resting on an inclined plane of which the height is to the
oblique length as 3 to 5, is sustained by a weight B three fifths as great as itself; and
if for the resistance of the plane we substitute the action of the weight C, reduced to
the direction A D perpendicular to the plane, this weight must be four fifths of the
weight A, the horizontal length of the wedge being four fifths of its oblique length.
P. 54.

Fig. 64. The weights A, B, and C, acting, by means of threads passing over
pullies, which are fixed to any required part of a horizontal table, on the rollers which
press against the sides of a wedge, proportional in length to the respective weights,
retain each other in equilibrium, when their directions meet in one point. In order
that the threads may pass on each side of the wedge, it may be supported by three or
more balls. P. 54.

DESCRIPTION OF PLATES. vii

PLATE V.

Fig. 65. By means of the moveable inclined plane A B, of which the height A C
is one third of the horizontal length B C, the weight D, acting horizontally, sustains
a triple weight E, acting in a vertical direction. P. 55.

Fig. 66. A B being one fourth of B C, the rope AB must exert a force of tension
equal to one fourth of the weight C, in order to support it, supposing the surfaces to
be without friction. But if the friction of the end of the beam A C were equal to
one fourth of the pressure, it would support the weight C without any other force,
whatever might be its magnitude. P. 55.

Fig. 67. A B being half of B C, or one fourth of C D, the force extending the
rope C D each way is equal to the weight E. P. 55.

Fig. 68. The thin wedge A B, of which the height is one fifth of the length, being
rolled round the cylinder C, makes the screw D, by means of which the weight E is
capable of supporting a weight five times as great as F. P. 55.

Fig. 69. A is a screw, and B the nut belonging to it. P. 55.

Fig. 70. The endless screw A B acts on the teeth of the wheel C D. P. 55.

Fig. 71. The distance of the threads of the interior screw is four fifths of that of
the exterior or perforated screw, and this distance is one thirtieth of the circumference.
Hence the weight A is capable of sustaining a weight B 150 times as great as itself.
P. 56.

Fig. 72. The apparatus for experiments on collision. Those balls which are not
employed may be left behind the graduated arc, as at A and B ; some of the strings
have balls of half the weight of the rest, others have a small dish C, on which
balls of clay, or of wax softened with one fourth its weight of oil, may be supported.
P. 58.

Fig. 73. If the ball A strike the ball B in the oblique direction A C, the ball B
will be impelled in the direction C D perpendicular to the surface of contact ; and
the velocity E C being resolved into E F and F C, the part F C will continue un-
altered ; and if the balls are exqual, the part E F will be destroyed, so that the ball
A will move after the stroke in the direction C G, excepting the effect of any accidental
disturbance which may be derived from the resistance of the surrounding bodies. If
we imagine a ball at C in contact with B, in the direction D B, we may aim a blow
at the centre of this ball, in order to drive the ball B to D ; and if B happen to be
situated any where in the semicircle D C G, the motion of A after the impulse will
be in the direction B G or G B, if there be no resistance. When the ball H is re-
flected by a fixed obstacle, as by the cushion of a billiard table, at I, its velocity K I
may be resolved into the parts K L, LI; the part K L continues, and may be repre-
sented by L M equal to K L, the part L I is converted into I L in a contrary direc-
tion, which when combined with L M makes IM, the angle LIM being equal to
L I K. We may find the proper direction for striking any ball by reflection if we
suppose a ball N in contact with the nearest point of the cushion, and making N O
equal to M N, aim at a ball supposed to be at O. In the same manner if we wish to
impel the ball P in the direction P Q by a stroke of the ball R after reflection at S,
we first place a ball at T behind P, and determine the direction R S by aiming at a
ball U, as if we wished to strike a ball at T with a direct impulse. But in the case
of a billiard ball, the rotation of the ball round its axis, which is not destroyed by
the collision, will cause the ball to move, on account of the friction of the table, in a
direction different from its first direction : thus the ball C will not go on to G, but
will strike the cushion between C and D ; and the ball H, after reflection at I, will
proceed in a direction a little nearer to N than I M ; so that the imaginary ball O
ought perhaps to be placed as far from the cushion itself as M, in order that the ball
may be struck after reflection. P. 62.

Fig. 74. Mr. Smeaton's apparatus for experiments on rotatory motion. P. 64.

Fig. 75. The moveable centre of suspension being fixed at the distance of 5
inches from one of the balls, and 7 from the other, the vibration is performed at the
same time as that of a pendulum 37 inches long. P. 65.

Fig. 76. The three weights, supported on wheels, being drawn up the three in-
clined planes at the same time, by the action of three other equal weights, the
middle weight arrives first at the top, the length of its plane being twice the height.
P. 67.

viii DESCRIPTION OF PLATES.

PLATE VI.

Fig. 77. The proportions of the diameters of the different parts of the double
pullies being 3 to 2, 5 to 2, and 8 to 2, the middle weight may be observed to rise
the most rapidly. P. 67.

Fig. 78. A wheel supposed to be capable of producing a perpetual motion ; the
descending balls, acting at a greater distance from the centre, but being fewer
in number, than the ascending. In the model, the balls may be kept in their
places by a plate of glass covering the wheel. P. 70.

Fig. 79. A, the inclination of cross lines generally most convenient for pro-
ducing the effect of a tint, in drawing. B shows the effect of lines crossing each
other perpendicularly, and C that of lines crossing too obliquely. Where the sur-
face to be shaded is large, the separate lines or hatches should begin and end with
a point, in order that the junction of the different portions may escape observation.
P. 72.

Fig. 80. Dr. Hooke's telegraph, in which the characters are arranged behind a
screen, and drawn out as they are required. P. 77.

Fig.' 81. Dr. Hooke's alphabet, with some other arbitrary characters for his tele-
graph. P. 77.

Fig. 82. A beam compass, with a scale. P. 78.

Fig. 83. ..85. Instruments for drawing arcs of large circles. P. 78.

Fig. 86. A pair of triangular compasses. P. 78.

Fig. 87. Marquois's scales, for drawing parallel lines. P. 79.

Fig. 88. A pen for ruling musical lines. P. 79.

Fig. 89. A pantograph. A being the centre of motion, B the tracing point,
and C the describing point. A B is always to AC as A D to A E, and the copy F is
similar to the original G. P. 79.

Fig. 90. A pair of proportional compasses. P. 79.

DESCRIPTION OF PLATES.

PLATE VII.

Fig. 91. A Sector. The scale of equal parts is marked L. As A B is to A C,
so is B D to C E ; and if any line B D be placed with its extremities in the third
division of the scale on each leg, the distance C E between the seventh divisions will
contain 7 equal parts, of which B D contains 3 ; and the same is true of any other
numbers. P. 80.

Fig. 92. A vernier, indicating 38$, of the divisions of its scale. P. 81.

Fig. 93. A sliding rule. The slider being drawn out, so that the division marked
1 is opposite to 3 on the rule ; all the other figures on the rule are triple of those
which stand opposite to them. P. 82.

Fig. 94. A circular logarithmic instrument. The inner circle slides within the
outer, and as it is represented in the figure, each number stands opposite to another
which is twice as great. P. 82.

Fig. 95. A steel chain, made by Ramsden. A, the screw for bringing the mark
B precisely to the point required ; C, a joint between the adjoining links ; D, a
cross joint at every tenth link ; E, a pulley and weight for stretching the chain.
P. 86.

Fig. 96. A micrometrical scale made by Troughton. The compound microscopes
A and B are fixed nearly at the required distance on the scale C ; A is then made to
point exactly to a division of the standard scale D by means of the screw E, and B
to another division, at the required distance, by means of the screw F, the fractional
parts being added by the turns of the crew G. The scale D is then removed, and
the object to be compared with it is put in its place. P. 86.

Fig. 97. A diagonal scale. The line A B contains 274 parts, of which the units
of the scale contain 100. P. 86.

Fig. 98. The statuary's compass, seen sideways. The pin AB is forced.down,
till it is stopped by the moveable stud C ; the screw D fixes it in its angular posi-
tion. It is also capable of motion round the axis E F, which is fixed by the screw
G. P. 87.

Fig. 99. An instrument for making drawings in perspective ; the perforated
sight may be drawn out to any required distance. The dotted lines show how a
second frame may be applied instead of the sight, so as to answer the same purpose.
P. 88.

Fig. 100. Illustration of the principles of perspective. A being the place of
the eye, and B C the plane of projection, if A D be parallel to E F, G H, and I K,
D will be their vanishing point, and E D, G D, and I D, their whole images : A L
being parallel to E M and I N, L will be their vanishing point, and EL, I L, their
whole images : and A O being parallel to P Q, O will be its vanishing point.
P. 89.

Fig. 101. A being the centre of the picture, A B the horizontal vanishing line,
A C the vertical line, and D the point of distance, if a ground plan E F G H of any
figure on the horizontal plane be placed in its true position with respect to I K, the
bottom of the picture, the vanishing points of all its lines will be found by drawing
DL, DM, D N, and D O, parallel to those lines respectively; and the whole
images of the lines will be P L, Q M, K N, and I O, determining, by their inter-
sections, the figure R S T U, which will be the projection of E F G H. The plan
may also be drawn, in an inverted position, below the line I K, and the point of
distance taken above A instead of below it. P. 89.

Fig. 102. A B being the whole image of the line represented by A C as a ground
plan, and D the point of distance, we may find E, the image of the point C, by
drawing C D ; or we may make B F=B D and A G=A C, F G will then also cut
A B in the point E. P. 89.

DESCRIPTION OF PLATES.

PLATE VIII.

Fig. 103. The heights of the houses, windows, doors, and figures are determined
by lines directed to the centre of the picture ; the true height being measured on the
lines AB, CD, where the objects are supposed to touch the plane of projection.
The distance E F, and all other parts of lines perpendicular to the picture, are
measured by laying off the lengths of the originals, as G H, on the line AC, and
drawing I E G, I F H, from I, the point of distance ; which, in most cases, will be
more remote from the centre of the picture than it is here made. The line KL,
and others parallel to A C, may be measured by the assistance of any point M in the
horizontal line, the distances, NO, OP, being laid off on AC, or simply by reduc-
ing the scale in the proportion of M P to M L. P. 89.

Fig. 104. A circle thrown into perspective, by means of the circumscribed square,
the points of contact being found by bisecting the sides. P. 89.

Fig. 105. Two perspective delineations, and two orthographical projections of a
cube, in different positions. For the orthographical projection, the ground plan
being A B C D, the image of any point A, B, may be found by drawing A E, B F,
perpendicular to the ground line ; E G, F H, parallel to the line assumed for the di-
rection of the centre of the picture, and A G, B H parallel to the line of direction of
the point of distance ; the intersections G and H will then be the points correspond-
ing to A and B. P. 90.

Fig. 106. A is the orthographical projection of a sphere, with some of its circles;
B the stereographical projection of the same circles. P. 90.

Fig. 107. A balance made by Fidler for the Royal Institution, nearly resembling
those of Ramsden and Troughton. The middle column A is raised at pleasure by
the cock B, and carries the round ends of the axis in the forks at its upper part, in
order to remove the pressure on the sharp edges of the axis within the forks. The
scales are occasionally supported by the pillars C and D, which are elevated or de-
pressed by turning the handle E. The screw F serves for raising or lowering a weight
within the conical beam, by means of which the place of the centre of gravity is
regulated. The extent of the vibrations is measured on the graduated arc G.
P. 97.

Fig. 108. A balance for the illustration of different kinds of equDibrium. When
the scales are hung on the middle pins, A, B, which are in the same horizontal line
with the support of the beam, the equilibrium is neutral, the weights acting as if the
centre of gravity coincided with the point of suspension. If the scales be hung on
the lowest pins C, D, the centre of gravity will be nearly in the line C D, and its
path the curve E, which has its concavity upwards ; but if the scales are hung on the
pins F, G, the path of the centre of gravity will be convex upwards, and the beam
will overset. In reality the true paths of the centre of gravity would be nearly in
the curves H and I, situated between the weights in the scales : but these are similar
to the other curves. P. 97.

Fig. 109. When the equilibrium of a balance is tottering, the lower weight acts
with the greatest advantage : thus the effect of the weight A is reduced in the pro-
portion of B C to D C, by the obliquity of the arm C A, while the weight E acts on
the whole length of its arm C F. P. 97.

Fig. 110. If A B C be a semicircle, and B D represent a given weight, and A D
its counterpoise in one of the scales of an unequal balance, D C will be the counterpoise
in the other scale. It is obvious that A C is more than twice as great as B D.
P. 97.

DESCRIPTION OF PLATES. xi

PLATE IX.

Fig. 111. A weighing machine. The platform supporting the weight rests on
the pins A, B, C, D, at equal distances from the fulcra E, F, G, H ; so that
wherever the weight may be placed, it presses equally on the lever I K, at L, and is
counterpoised by a much smaller weight placed in the scale M. P. 97.

Fig. 112. A steelyard resembling that of Mr. Paul, in which different weights
may be employed. A, a loop to check the vibrations ; B, a scale to be suspended by
the hook C. If great delicacy be required in the weights, the fractional parts may
be expressed by the turns of a micrometer screw D, furnished with an index. P. 97.

Fig. 113. A bent lever balance. P. 98.

Fig. 114. A spring steelyard : half the case being removed, to show the spring.
P. 98.

Fig. 115. AB, the path of the centre of gravity of the human body, such as it
would be described in walking, if the legs were inflexible. C D, the path described
in running, on the same supposition. P. 100.

Fig. 116. The actual path of the centre of gravity, as it is usually described.
P. 100.

Fig. 117. An elastic column, compressed by a weight acting at the distance of
one third of its depth from the concave surface ; the compression being every where
as the distance of the lines A B, A C. P. 107.

Fig. 118. An elastic column, extended by a weight acting at the distance of one
third of its depth from the convex surface, the extension being every where as the
distance of A B, A C. P. 107.

Fig. 119. An elastic column, compressed by a weight acting immediately on the
concave surface : the compression extends only to the line AB, the parts beyond this
line being extended. P. 107.

Fig. 120. A column bent, by a weight acting longitudinally, into the form of a
harmonic curve : the line A B C D is the limit between the parts which are com-
pressed, and those which are extended. P. 107.

Fig. 121. An elastic plate or rod, considerably bent by a weight acting at its ex-
tremity. P. 107.

Fig. 1 22. An elastic rod fixed at one end, and bent by its own weight. P. 108.

Fig. 123. An elastic rod supported at each end, and bent by its own weight.
P. 108.

xii DESCRIPTION OF PLATES.

PLATE X.

Fig. 124. The manner in which a prismatic column is crushed by pressure, sup-
posing the lateral adhesion to be simply proportional to the surface concerned. P. 112.

Fig. 125. The manner in which a column is crushed, supposing the lateral ad-
hesion to be increased by pressure. P. 112.

Fig. 126. The circle is as strong as the circumscribing square, supposing the
adhesion proportional to the surface, the relative force of all its chords being equal.
P. 113.

Fig. 127. The three circles are as strong as the circumscribing parallelogram. P. 1 13.

Fig. 128. A, the strongest form for a beam, cut out of a plank of uniform depth,
for resisting a longitudinal force ; B, the form into which it is bent ; both curves
being circular. P. 116.

Fig. 129. A, the strongest form for a beam, cut out of a plank of equable
breadth, for resisting a longitudinal force which bends it into the cycloidal curve
seen at B. P. 116.

Fig. 130. A, the strongest form for a square or turned beam or column, slightly
bent by a longitudinal force : B, the form into which it is bent by such a force. P. 116.

Fig. 131. The strongest form of a beam cut out of a horizontal plank, fixed at
one end, and supporting a weight at the other. P. 116.

Fig. 132. The strongest form of a beam cut out of a vertical plank, fixed at one
end, and supporting a weight at the other ; the outline being parabolic. In practice
the best method in such a case would be simply to reduce the depth at the end to
one half of the whole, keeping the outline straight ; in this manner one fourth of
the timber would be saved. P. 116.

Fig. 133. The strongest form of a square or turned beam, fixed at one end, and
supporting a weight at the other; the outline being a cubic parabola. P. 116.

Fig. 134. The strongest form for the outline of a compound spring, supporting
a weight at the end. P. 116.

Fig. 135. The strongest form for a beam cut out of a horizontal plank, fixed at
one end, and supporting a weight equally distributed throughout its length ; the
outline being a parabola. P. 116.

Fig. 136. The strongest form for a beam cut out of a vertical plank, fixed at
one end, and supporting a weight equally distributed throughout its length. P. 116.

Fig. 137. The strongest form for a square or turned beam, fixed at one end,
and supporting a weight equally distributed throughout its length ; the outline being
a semicubic parabola, in which the cube of the thickness is as the square of the
distance from the end. P. 116.

Fig. 138. The strongest form for a beam cut out of a vertical plank, for sup-
porting its own weight ; the outline being a parabola. P. 116.

Fig. 139. The strongest form for a turned beam, for supporting its own weight ;
the outline being parabolic. P. 116.

Fig. 140. The strongest form of a beam calculated to resist the pressure of its
own weight by lateral adhesion only. The outline is a logarithmic curve, which
never comes into contact with the axis, and in order that the condition of equal
strength may be possible, the beam must be loaded with a weight at its extremity,
equal to that of the portion which is wanting to complete the figure. P. 116.

Fig. 141. The strongest form for a beam cut out of a horizontal plank, sup-
ported at both ends, and bearing a weight at the middle. P. 116.

Fig. 142. The strongest form for a beam cut out of a horizontal plank, sup-
ported at both ends, and bearing a weight equally distributed throughout its length ;
the outline being parabolic. P. 116.

Fig. 143. The strongest form for a beam cut out of a vertical plank, supported
at both ends, and bearing a weight equally distributed throughout, the outline being
elliptic. P. 116.

Fig. 144. The strongest form for a beam cut out of a horizontal plank, firmly
fixed at both ends, and supporting a weight at the middle. P. 116.

Fig. 145. The strongest form for a beam cut out of a vertical plank, firmly
fixed at both ends, and supporting a weight at the middle, the curves being para-
bolic. P. 116.

Fig. 146. The strongest form for a beam cut out of a vertical plank, and sup-

DESCRIPTION OF PLATES. xiii

porting every where a weight proportional to the distance from the extremity ; the
outline being a cubic parabola. P. 116.

Fig. 147. The strongest form for a square or turned beam, supporting every
where a weight proportional to the distance from the extremity, and represented by
the section of the same figure, which is a pyramid or a.cone. P. 116.

PLATE XL

Fig. 148. A machine for examining the strength of materials. The force is
applied by means of the winch A, which winds up the rope B C, passing over the
first pulley, and under the second, which is directly under the point D, at which the
force acts on the piece E F to be broken ; the pullies slide on two parallel bars,
fixed in a frame, which is held down by a point projecting at G, from the lever G H,
which is graduated like a steelyard, and measures the force. The piece to be broken
is held by a double vice, I, K, with four screws, two of them hiding the other two
in the figure : if a wire is to be torn, it may be fixed to the cross bar L M ; and
a substance to be crushed must be placed under the lever N O, the end N receiving
the rope, and the end O being held down by the click, which acts on the double
ratchet O P. The lever is double from O to Q, and acts on the substance by a loop,
fixed to it by a pin. P. 116.

Fig. 149. The outline of a column diminished one fifth of its diameter, in two
different ways : the side A being an arc of an ellipsis, of which the semidiameter
A B is the lesser semiaxis, joined at A to a right line A C, of one third of the length
of the column, the part A D being cylindrical ; the side D E is a cubic parabola,
and may be drawn mechanically by fixing a straight ruler E F, in such a position
that D F may be twice the diminution at E, and then bending it to D : the dimi-
nution being every where as the cube of the distance from D. These two methods
are compared in a contracted scale at G : the outer line represents the first method,
and the next line the second ; the third, which is nearest to G, the conchoid of
Nicomedes, recommended by Chambers, said to be found in the columns of the
Pantheon ; the curve beginning at the base. Palladio fixes the rule at A, and bends
it to H, which makes the curvature abruptly greater at H. P. 122.

Fig. 150. A section of Mr. Smeaton's light house at the Eddystone. P. 122.

Fig. 151. Mr. Smeaton's mode of uniting tiers of stones by wooden pins and
wedges. P. 123.

Fig. 152. A string of beads, suspended in equilibrium from two points, and re-
maining in equilibrium in an inverted position. The ends are supported by two
pieces, which slide backwards and forwards, and are fixed by screws : the string is
also tightened by turning a pin. P. 154.

Fig. 153. A system of bars, hanging in equilibrium, and supporting each other
in the same form when inverted. P. 154.

Fig. 154. A, a chain loaded, at equal distances, with other chains of such a
length, as to represent the depth of the materials pressing on an arch of the form
shown by the first chain, and holding it in equilibrium. B, an arch of a similar
form. P. 154.

Fig. 155. A comparison of the curves which have various advantages for the
construction of an arch supporting a horizontal road. The full line is an elliptic
arc, somewhat less than half the ellipsis. The outside curve, which is also con-
tinued furthest down, is that which is calculated for resisting the pressure of materials
acting like a fluid, or in the manner of wedges; the second dotted curve, for sup-
porting the pressure of the materials above each part, supposed to act in a vertical
direction only ; the third is a circular arc, making one third of a whole circle ; the
fourth is part of a logarithmic curve, which is nearly of equal strength with re-
spect to the tendency of the materials to give way for want of lateral adhesion ; and
the fifth is composed of parabolic curves, showing the outline which would be
strongest for supporting any additional weight placed on the middle of the arch.
If the height were greater in proportion to the span, as usually happens in practice,
there would be less difference between the curves. The radius of curvature at the
summit being A B, the horizontal thrust is equal to the weight of the portion
A B C D of the materials. P. 125.

xiv DESCRIPTION OF PLATES.

PLATE XII.

Fig. 156. The middle arch of Black Friars Bridge. P. 126.

Fig. 157. A spherical dome, of which the lower parts are made thicker, in order
that they may be of equal stability throughout. From A to B the dome is of
equable thickness : below C and D the thickness cannot be increased sufficiently to
procure an equilibrium, without the application of a chain or hoop, of which the
section is represented at C, D. If the thickness were not at all increased, a hoop
would be required at E, F, or still higher. P. 127.

Fig. 158. A section of the roof of St. Paul's Cathedral. The section of the
dome consists of two circular arcs, of which the centres are a little beyond the axis :
it is supported by carpentry, resting on a cone of brickwork. The internal dome is
of brickwork only, and is open at the summit. P. 127.

Fig. 159. A section of the dome of the Pantheon at Rome. P. 127.

Fig. 160. A Tuscan column, with its pedestal, capital, and entablature. P. 127.

Fig. 161. A Doric column. P. 127.

Fig. 162. An Ionic column. P. 127.

Fig. 163. A Corinthian column. P. 127.

Fig. 164. A Composite column. P. 127.

Fig. 165. An elevation of the end of King's College Chapel, Cambridge ; show-
ing on one side the buttresses, the tower being supposed to be removed, and on the
other the tower, which not only supplies the place of a buttress at the end, but
assists also in supporting a considerable portion of the thrust in the direction of the
length of the chapel ; the roof, which is of stone, being vaulted in this direction as
well as transversely. There is also a roof of carpentry, covered with lead above the
stone roof. P. 128.

DESCRIPTION OF PLATES. xv

PLATE XIII.

Fig. 166. Joints for a tie beam. The joints at A and B cannot be more than
half as strong as the entire beam, supposing the adhesion, produced by the pressure
of the bolts, as strong as could be required. The joint at C is called a dovetail
joint ; its strength is a little less than that of A and B, but the adhesion is more
easily secured, since a force tending to separate the beams must tighten the joint.
P. 128.

Fig. 167. Joints for a tie beam. The joint A, if sufficiently tight, may possess

of the strength of the beam. The joint B may be as strong as the beam, if the

adhesion were great enough, but it would be difficult to apply sufficient pressure to

create such an adhesion, and if the beam were subject to be much shaken, the joint

would be a very bad one. P. 128.

Fig. 168. A good joint for a tie beam ; the adhesion being secured by a slight
diminution of the strength. P. 128.

Fig. 169. A, a simple scarfed joint, which may be tightened by a wedge at the
centre ; it is not strong. B, a scarfed joint, which is much stronger. P. 129.

Fig. 170. A joint for a beam supporting a weight by its transverse strength.
The junction might be made, if it were necessary, by means of a third piece, of
which the limits are marked by the dotted line. The strength is but little diminished
by the joint. P. 129.

Fig. 171. A beam supporting a weight by its transverse strength, joined to
another by means of a third piece of half the depth, spliced or fished on below the
beam, and secured by pins, and by blocks or joggles. The strength is a little
greater than that of the original beam. The dotted lines show the proportion in
which the strata are extended or compressed, the lower part of the original beam
remaining in its natural state, without sustaining any pressure, as far as one fourth
of the depth, and a little further. P. 129.

Fig. 172. A joint for a beam pressing obliquely against another. The dotted
lines show the form of the tenon, which may occupy a considerable part of the
breadth of the beam. The upper strap, A, is in the most usual situation, but the
lower one, B, appears to afford greater strength, as it presses the beams more closely
together, yet without any danger of crippling them ; besides the advantage of having
a firmer hold of the lower beam. P. 130.

Fig. 173. A joint for a horizontal beam suspended from a vertical one : the end
of the tenon being dilated by wedges, and the whole secured by a strong strap.
The tenon ought not to be wide, since it diminishes the strength of the horizontal
beam. P. 130.

Fig. 174. The straps, bent so as to deviate from the right lines joining their
extremities in the degree that is here represented, have their strength reduced to
about one seventh of that which they would have if straight. Thus, A B is only
one seventh as strong as C D, supposing the substance inflexible. P. 130.

Fig. 175. The simplest form of a roof. A B, A C, are the rafters, and B C the
tie beam ; the weight of each half being represented by A B, or A C, the thrust in
the direction of the rafters will be A D, and the horizontal thrust each way B D or
CD. It is obvious that A D will be least when B A C is a right angle. P. 130.

Fig. 176. A common roof, with braces. AB is the king post, and B C, BD
the braces. P. 130.

Fig. 177. A kirb or mansard roof, the rafters of which hold each other in equili-
brium. A B and C D are queen posts helping to support the tie beam. The piece
A C acts as a strut, in supporting the pressure occasioned by the weight of the tie
beam. The heads of the queen posts are not much thickened, in order to avoid the
change arising from the unequal contraction of the wood. P. 130.

xvi DESCRIPTION OF PLATES.

PLATE XIV.

Fig. 178. Three sketches for wooden bridges ; the last requires no abutments.
P. 131.

Fig. 179. The centering used for building one of the arches of Black Friars
Bridge. It was struck, or removed, by forcing back the compound wedges A, B,
by the impulse of a battering ram. P. 131.

Fig. 180. Modes of supporting a series of rods, for communicating alternate
motion. A is the best and most common method, the rods being suspended from
a centre above them : at B the centre of motion is below the rods. Where
there is a declivity, the arrangement at C may be useful. The mode shown at D is
also recommended in some cases. P. 133.

Fig. 181. A lever strengthened by a projecting frame. P. 133.

Fig. 182. A bent lever strengthened by a cross bar. P. 133.

Fig. 183. Hooke's universal joint. P. 133.

Fig. 184. A wheel with a crank, for producing alternate motion in a rod.
P. 134, 257.

Fig. 185. A wheel with an inclined and undulated surface, for producing alternate
motion in a rod, with the interposition of a friction wheel. P. 134, 257.

Fig. 186. A frame for guiding the motion of a point A in a direction nearly rec-
tilinear, A B being to CD as C D to BE. [It is termed a parallel motion, Watt
obtained a patent for it on the 28th of April, 1784.] The dotted line shows the
path of the point A. P. 134, 257.

Fig. 187. A frame for producing a motion nearly rectilinear in the point A. It
may be applied to a pump rod B C, worked by a crank, or otherwise. P. 134, 257.

Fig. 188. A compound frame, for keeping two rods AB, C D, in a direction
very nearly parallel. E F is 36 parts of the scale, F G, 64, G H and H I each 80,
EH and H K 20, GL $ GK, or 106§, KM 33£, and LM and MN each 133£.
P. 134.

DESCRIPTION OF PLATES. xvii

PLATE XV.

Fig. 189. The form of a wheel or pulley, on which a broad strap runs, the sur-
face being convex : the wheel which drives it is of a similar form, but its upper part
only is shown in the figure. P. 135.

Fig. 190. The teeth of two wheels, formed into epicycloidal curves, acting on
planes : the dotted lines show the effective magnitude of the wheels. P. 135.

Fig. 191. The teeth of two wheels, formed into involutes of circles, described by
uncoiling a thread from the dotted circles ; the point of contact of the teeth being
always in the straight line which touches both circles. P. 135.

Fig. 192. Two surfaces formed into involutes of circles, revolving in contact with
each other, the equidistant lines, drawn on them, continuing to meet each other
throughout the revolution. P. 135.

Fig. 193. The pinion A is of the kind called a spur wheel ; B is a crown wheel,
or a contrate wheel. P. 136.

Fig. 194. The wheel and pinion are both bevilled ; the faces of the teeth being
directed to the point A. P. 136.

Fig. 195. Two wheels a little eccentric, acting on each other. P. 137.

Fig. 196. An eccentric contrate wheel, acting on a long pinion. P. 137.

Fig. 197. A machine for cutting the teeth of wheels. A is the wheel, of which
the teeth are formed by the revolving saw B, turned by the wheel and pinion C, D,
by means of the handle E, while the frame which holds the saw, moving on hinges,
and resting on a spring, is depressed by the handle F, its place having been pre-
viously adjusted by the screw G. The large plate H I contains a number of concen-
tric circles, variously divided by points, into which the end of the spring I sinks at
each step, so as to fix the apparatus in the required position. P. 137.

Fig. 198. A chronometer for measuring minute portions of time. The axis A B
being turned, either by the handle A or by the weight C, the balls D, E fly out, and
carry the weights F, G further from the axis ; in consequence of which the increased
effect of friction retards the motion, when it becomes too rapid. The barrel H is
turned in the mean time, with the axis, and is allowed to descend as the thread at I
is uncoiled, so that the point K, which is pressed against it by a spring, describes on
it a spiral, which is interrupted whenever the pin K is touched. P. 147.

Fig. 199. The fusee of a watch or clock, the general outline of which forms part
of the hyperbola AB, in which the distance of each point from the axis CD is
inversely as its distance from the line D E. P. 148.

xviii DESCRIPTION OF PLATES.

PLATE XVI.

Fig. 200. A fusee with an auxiliary spring, for continuing the motion when the
watch is wound up. The action of the main spring turns the fusee in the direction
A B ; the fusee acts on the ratchet wheel A B C by means of the click B, and this
wheel impels the toothed wheel D E by the spring C B A, which is supposed to be
seen through it. When the watch is wound up, this spring forces back the wheel
ABC against the click F, which serves as a fixed point, while the other end continues
to act on D E, and to maintain the motion. P. 148.

Fig. 201. The scape wheel A B, moving in the direction A C B, impels the pal-
lets D,E of the crutch or anchor, alternately in contrary directions. P. 149.

Fig. 202. A is the scape wheel, B and C the pallets of the common watch scape-
ment. P. 149.

Fig. 203. The dead beat scapement. The teeth are first received on the flat or
rather cylindrical surfaces A, B, on which they rest until the pendulum arrives near
the middle of its vibration, when the teeth begin to act on the inclined surfaces
terminating the pallets. P. 150.

Fig. 204. The horizontal scapement, for a watch. The tooth A rests first on the
external surface of the cylinder, B C, and then impels it by its inclined face, in the
direction B C ; it afterwards falls on the concave surface D E, and lastly impels the
cylinder in the contrary direction. P. 150.

Fig. 205. The duplex scapement. A B is the pallet, through which the cylinder,
and the tooth which rests on it, are supposed to be seen, the point of the tooth being
about to escape from the notch towards C. The short tooth D next impels the point
of the pallet, and the long tooth E falls on the cylinder. It first rests on the convex
surface, and then drops into the notch, which causes a slight recoil in the wheel, and
passes by, the tooth F being beyond the r^ach of the pallet ; but on its return, the
tooth falls again into the notch ; and when it escapes, the pallet is impelled as before.
P. 150.

Fig. 206. Mr. Mudge's watch scapement. A, the scape wheel, and one of the
subsidiary springs, seen from above ; B, a general view of the balance, with both the
subsidiary springs, seen from one side. The point of one of the teeth rests at C on
the end of the pallet, which is bent so as to detain it until the pin D, which is
attached to the balance, sets it a liberty, by striking against the arm E ; this arm is
then carried on by the balance, to the end of its vibration, and impels it in its return,
until the pallet meets the next tooth. The other spring acts alternately in the same
manner, but in a contrary direction. P. 151.

Fig. 207. An improvement on Mr. Cumming's scapement for a clock. The
tooth A is seen resting on a flat surface at the end of the pallet B : it is disengaged
by the descent of the opposite pallet into the position in which it is represented, the
pallet B being impelled by it at C. This pallet continues resting on the flat end of
the tooth, until the pin D of the pendulum strikes against the arm E, which is
carried before it, and impels the pendulum in its descent, until the pallet B acquires
the situation in which the opposite pallet is represented, and sets that pallet at liberty
from the tooth E, which has raised it. The situation and magnitude of the weights
G, H, may be adjusted at pleasure. P. 151.

Fig. 208. Mr. Arnold's watch scapement. The pin A, projecting from the verge
or axis of the balance, moving towards B, carries before it the spring B, and with it
the stifFer spring C, so as to set at liberty the tooth D, which rests on a pallet pro-
jecting from the spring. The angle E of the principal pallet has then just passed the
tooth F, and is impelled by it until the tooth G arrives at the detent. In the return
of the balance, the pin A passes easily by the detent, by forcing back the spring B.
The screw H serves to adjust the position of the detent, which presses against it.
P. 151.

Fig. 209. Mr. Earnshaw's scapement. A is the unlocking pallet, B the spring
on which it acts, C the detent, holding the tooth D by a pin ; E is the point of the
principal pallet first impelled by the tooth F, G is the tooth next locked, and H the
adjusting screw. P. 151.

Fig. 210. A gridiron pendulum, consisting of three bars of iron, and two of a
mixture of zinc and silver. P. 154.

Fig. 211. A compensation balance, as employed by Arnold. The outside of the

DESCRIPTION OF PLATES. xix

hoops A, B is of brass, the inside of steel : the weights C, D are screwed backwards
and forwards, in order to obtain the requisite degree of compensation. The weights
E, F are employed to regulate the mean rate of the watch, and G, H, and I, for
adjusting it to all positions with respect to the horizon. P. 154.

Fig. 212. The compound plate A B rests on two supports, which are adjusted to
a proper distance by turning the double screw C, the flexure of the plate by heat
raising the bar D, which supports the pendulum, while its effective length is deter-
mined by a fixed clip, which is seen below the plate. P. 154.

PLATE XVII.

Fig. 213. A jack for raising weights by the alternate motions of a lever, the
clicks on each side being detained in the teeth of the ratchets by the assistance of the
springs in which they terminate, and which are connected together. P. 157.

Fig. 214. The mode of supporting a tackle for raising stones in building; the
summit of the triangle, which is composed of three poles, being raised or lowered by
means of a rope and pullies. P. 159.

Fig. 215. A method of raising weights obliquely, by means of a rope passing over
a pulley, which is drawn along horizontally. P. 159.

Fig. 216. A B, a section of an inclined plane, belonging to the Duke of Bridg-
water's canal ; the boats are drawn into the locks at A, which are then filled with
water ; C is the plan of the windlass, by which the descending and ascending boats
are connected together, and which is turned by a winch ; D and E are the locks.
P. 159.

Fig. 217. A crane, with an oblique walking wheel, for oxen or horses. The
wheel is taken from a mill of Leupold. P. 161.

Fig. 218. A crane with a wheel and break like Mr. White's. The man walks at
any required distance from the axis of motion, and pushes forwards the lever A, which
moves the bar B C, connected to the same axis, and removes the break C D from the
circumference of the wheel. P. 161.

Fig. 219. A lewis, for raising stones. P. 161.

Fig. 220. When the centre of gravity A is twice as far from one of the porters B,
as from the other C, the first bears one third of the weight, the other two thirds.
P. 162.

Fig. 221. When the centre of gravity A is above the line joining the points of
support B, C, the load is divided in the ratio of the segments CD, B D, terminated
by the vertical line A D ; but it may be supported by two equal forces in the direc-
tions BE, C F, found by making G H equal to B G, and joining C H ; the angle
G B E being equal to G H F ; the forces and the weight may then be represented by
the lines C I, I K, and C K. P. 162.

Fig. 222. A roller with two wheels fixed on its ends, by means of which the slab
resting on it may be moved to a considerable distance without leaving the roller
behind. P. 164.

Fig. 223. Mr. Garnet's rollers for diminishing friction : their axes being loosely
connected by a ring, in order to keep them in their places. P. 164.

c2

DESCRIPTION OF PLATES.

PLATE XVIII.

Fig. 224. A pair of friction wheels, supporting one end of the axis of a wheel.
P. 164.

Fig. 225. The centre of the wheel A B, passing over the obstacle C, describes
the path DE; that of the larger wheel FG, the path HI, which is less steep.
P. 164.

Fig. 226. The centre of the wheel A B describes the curved path C D, in passing
over the obstacle E, while that of the larger wheel F G has an angle at H. P. 164.
Fig. 227. The wheel A B, moving on a soft road towards B, has to overcome
the resistance of the earth at C. P. 165.

Fig. 228. A section of the wheel of a carnage, a little dished, or inclined out-
wards. P. 166.

Fig. 229. A B and C D being the straps or braces by which a coach is suspended,
if the centre of gravity be at E, F, or G, it must move, when the carriage swings, in
the curve passing through the respective point. P. 167.

Fig. 230. The mode of harnessing two horses, so as to make them draw conve-
niently together : when either horse advances so far that the bar A B assumes the
position C D, the foremost horse has the disadvantage of acting on a lever equivalent
only to E F, while the other horse acts on EC. P. 167.

Fig. 231. A sugar mill. The axis A is turned either by animal force or by
water : the liquor is collected in the trough B, and runs off in the channel C. The
openings D are for the purpose of adjusting the axes of the rollers. The canes are
supplied by the hands of the workmen. P. 170.

Fig. 232. A glazier's vice. The vacuity in the middle shows the form of the
section of the lead which is drawn through it. P. 171.

Fig. 233. A forge hammer, elevated by the plugs, projecting from an axis,
either at A, or more conveniently, at B, and thrown forcibly against the wooden
spring C. P. 171.

Fig. 234. An engine for driving piles, on Vauloue's construction. The horses,
drawing at A, B, raise the weight C, held by the tongs D, fixed in the follower E,
which are opened, when they reach the summit, by being pressed between the
inclined planes F, G, so as to let the weight fall. At the same time the lever H is
raised by the rope I, and presses on the pin K L, so as to depress the lever
M N, and draw the pin O out of the drum P Q ; the follower then descends, and
uncoils the rope, its too rapid motion being prevented by the counterpoise R,
acting on the spiral barrel Q. The motion is regulated by the fly S, the pinion of
which is turned by the great wheel T. P. 173.

Fig. 235. The rollers of the slitting mill. P. 1 74.

Fig. 236. A simple plough. A is the coulter, for dividing the ground ; B the
share, fixed on the mould board C, for turning it to the right hand ; D is the rest,
and E, F, the handles. P. 175.

Fig. 237. Section of a threshing mill. The corn is drawn in by the rollers or
feeders A, B : it is beaten by the revolving beaters C, D, and the straw is drawn
out by the rakes E, F, which discharge it at G ; the grain falling through the arched
bottoms H I, I G, which are formed like sieves. P. 178.

Fig. 238. A corn mill, with some of the improvements made in America, by Mr.
Ellicott and Mr. Evans. The corn being poured into the funnel A, is conveyed by the
revolutions of a spiral B C, to C, whence it is raised, by the chain of buckets C D,
to be cleaned by the revolving sieve E, and the fan F ; it is then deposited in the
granary G, which supplies the funnel or mill hopper H ; this being perpetually agi-
tated by the iron axis of the upper mill stone, shakes it by degrees into the perfora-
tion of the stone ; it escapes, when ground, at I, and is conveyed, by means of the
carrier K L, and the elevator LM, to the cooler N, where it is spread on a large
surface : it passes afterwards to the bolter O, and is received in the binn P, from
whence it is taken to be packed in sacks or barrels. Q, represents the surface of a
mill stone, cut into furrows, in order to make it act more readily on the corn.
P. 179.

DESCRIPTION OF PLATES. xxi

PLATE XIX.

Fig. 239. The surfaces of the fluid in the bent tube A B remain on the same
level, in the same manner as if the tube were absent, and the fluid made a part of
that which is contained in the reservoir CD. P. 197.

Fig. 240. The bucket A being suspended by the rope B, and made to revolve
rapidly round its axis, the surface of the water assumes a parabolic form. P. 198.

Fig. 241. A heavier fluid being contained in the upper part of the bent tube A B,
which is immersed in the lighter fluid, filling the vessel C D, the fluid in the tube
remains in a state of tottering equilibrium, when its surfaces are in the same level.
P. 198.

Fig. 242. The fluid ABC presses on the bottom of the vessel B C with the same
force as if the vessel were of the form B C D E. P. 199.

Fig. 243. The portion A B C D of the fluid being supposed to be congealed, and
then to form a part of the vessel, the pressure on the bottom would remain unaltered.
P. 199.

Fig. 244. The weight A may be supported by the pressure of a small quan-
tity of fluid, either by making the surface of the vessel BC very large, and the
height of the tube D E moderate, or, while the vessel F remains of a moderate size,
by making the height of the tube G H very great. P. 199.

Fig. 245. The pressure on any small part of the side of the vessel A B, at C or D
may be represented by the line C E, D F, and the whole pressure on the side by the
triangle B G, of which the centre of gravity is at H ; and if the side A I be sup-
ported by a single prop, it must be placed at the point K, the height of which is
equal to that of H. P. 200.

Fig. 246. If the height of the surface A above B be to B C as the specific gravity
of the fluid in B C to that of the fluid in A B, the fluids will support each other.
P. 201.

Fig. 247. Two square beams floating at the depths shown at A and B, will have
a certain degree of stability, but if they sink, as at C, they will overset. But a beam
of the breadth shown at D will always float securely. P. 202.

Fig. 248. A jar containing images of fishes, with bubbles of air in them, which
sink when the cover of the jar is pressed with the hand. P. 202.

Fig. 249. Dr. Hooke's semicylindrical counterpoise, by means of which a vessel
is kept always full. P. 203.

Fig. 250. The form into which the flexible bottom of a cistern would be bent by
the pressure of the water ; the curve is the same as that into which an elastic rod
would be bent by forces acting at A and B. P. 203.

Fig. 251. The bottle A, containing air and mercury, has the tube A B fitted into
it: and when the jar CD, in which it is enclosed, is exhausted by means of the air
pump, the elasticity of the air in the bottle forces the mercury up the tube.
P. 204.

Fig. 252. An instrument for showing the buoyant effect of the air, called by
Boyle a statical baroscope ;* the index A shows, on the scale B C, the degree in
which the ball D is obliged to descend, by the diminution of the weight of the air.
P. 206.

Fig. 253. The line 0 denoting the natural density of the air, the line 1 A next
above it shows the degree in which the air is expanded at the height of a mile, and 1 B
the density of the air at the same height ; in the same manner IOC shows the expan-
sion of the air at the height of 10 miles, and 10 D its density ; and 5 E, below the line,
the density which it would acquire at the depth of 5 miles below the earth's surface.
The lines A C, D B E, are of the kind called logarithmic curves. P. 206.

Fig. 254. The box or bason, in which the mercury of the common barometer is
contained : A is a float for adjusting the height, by means of the screw B, operating
on the leather which forms the bottom of the cavity. P. 209.

* Ph. Tr. 1665, p. 231.

xxii DESCRIPTION OF PLATES.

PLATE XX.

Fig. 255. A jet or vein of a fluid, passing through an orifice in a thin plate in
any direction, and contracted after its escape, in consequence of the lateral motions
of the particles which flow towards the stream, nearly in the directions of the lines
here drawn. P. 212.

Fig. 256. A stream flowing through a short cylindrical pipe, compared with
another flowing through a diverging conical pipe, the directions of the motions of
the particles appearing to be nearly similar in both cases. P. 213.

Fig. 257. In an experiment of D. Bernoulli, the water flowing through the
conical pipe A drew up water through the tube B from the vessel C ; in another of
Venturi, the water flowing through the cylindrical pipe D raised water through the
tube E. P. 213.

Fig. 258. A siphon, through which a fluid runs from the higher vessel into the
lower one. P. 215.

Fig. 259. A fluid flowing through a vertical pipe, and filling a vessel to a height
nearly equal to the length of the pipes, while it is discharged through a similar
horizontal pipe. P. 216.

Fig. 260. Subterraneous cavities, with outlets in the form of siphons, through
which they do not begin to discharge any water till they are nearly full ; the lower
one will then continue to run till it be empty. In the mean time either of them may
keep up a constant stream by other passages. P. 217.

Fig. 261. A tube turned up and contracted, so as to throw out the fluid con-
tained in it, in a jet, which rises very nearly to the height of the fluid in the tube.
P. 217.

Fig. 262. The forms of jets issuing from various parts of a reservoir, the ampli-
tude A B being twice C D, and A E four times F G. P. 217.

Fig. 263. A series of waves, moving in the direction A B, and reflected by the
obstacle B, lose the appearance of progressive motion, and vibrate up and down
within the limits of the curves A C D E B, and F G H I K ; the elevation and
depression become however twice as great as before reflection. P. 219.

Fig. 264. A series of waves diverging from a centre A, and striking a fixed
obstacle B C, are reflected by it into the same form as if they proceeded from the
centre D, at an equal distance on the opposite side of the surface B C. P. 219.

Fig. 265. An apparatus for observing the motions of waves excited in a fluid
poured into the trough A B, by the vibrations of the elastic wire C, loaded with a
moveable weight D ; the shadow of the waves being thrown on a screen E by the
lamp F, through the bottom of the trough, which is of glass. P. 220.

Fig. 266. A series of waves, diverging from the centre A, and passing through
the aperture B C, extend themselves on each side so as to fill the space B C D E,
while they affect the parts without this space much less sensibly. P. 220, 360.

Fig. 267. Two equal series of waves, diverging from the centres A and B, and
crossing each other in such a manner, that in the lines tending towards C, D, E, and
F, they counteract each other's effects, and the water remains nearly smooth, while
in the intermediate spaces it is agitated. P. 220, 364.

DESCRIPTION OF PLATES. xxiii

PLATE XXI.

Fig. 268. A stream of air being forced through the pipes A and B, the mercury
in the barometer C D falls from C to D. P. 225.

Fig. 269. A stream entering the reservoir A, by the pipe B, carries with it all
the water C, which stands above the level of its upper surface. P. 226.

Fig. 270. The ball A is permanently supported by the jet B, because when it
falls into the position here represented, the centrifugal force of the water at A carries
it back to the middle of the jet. P. 226.

Fig. 271. A plate, bent into the form ABC, turning on the centre B, is
impelled by a stream of air D in the direction C D. P. 226.

Fig. 272. A cylinder moveable on an axis, with two curved pipes inserted in its
lower part, seen from above. The stream A enters at the top of the cylinder, and is
discharged by the orifices B, C, so as to turn the vessel in the direction B D.
P. 229.

Fig. 273. A jet of a fluid, striking on an obstacle of equal diameter, and separated
by it so as to continue its motion obliquely. P. 229.

Fig. 274. The whole resistance directly opposed to the surface A B being repre-
sented by B C, the portion which, according to the principles of the resolution of
forces, ought to act on the wedge A B D, is represented by B E ; and in the same
manner the resistance on A B F is to the whole as B G to B C. P. 230.

Fig. 275. The form of the dead water moving before an obtuse body is nearly
like that of A B C ; and the form adapted for moving through the water with the
least possible resistance like A B D C. P. 231.

Fig. 276. The direction in which the particles of a fluid are supposed to move
when they strike against a concave surface. P. 232.

Fig. 277. A hydrostatic balance. P. 235.

Fig. 278. Mr. Nicholson's hydrometer, to be employed with weights, for finding
the specific gravity of fluids or solids. P. 236.

Fig. 279. A spirit level. P. 237.

Fig. 280. An overflowing lamp. The hemispherical counterpoise, which is so
loaded, that its centre of gravity is at A, raises the surface of the heavy fluid B the
higher as it is more exhausted, so that the oil C is always forced up nearly to the
level of the wick at D. The oil is poured in by a pipe, in the middle of the
cylindrical column. The air holes may be made wherever it is most convenient.
P. 237.

Fig. 281. A section of an embankment, of a proper form to be opposed to the sea,
with a drain passing through it, and a valve at its opening. P. 238.

Fig. 282. The form recommended for the section of a river or canal. P. 238.

Fig. 283. A B shows the strongest form for a vertical beam, fixed above and
below, and calculated to resist the pressure of a fluid ; the greatest thickness being
at C ; and D E is the outline of a series of horizontal planks, of such a thickness as
to afford equal strength throughout the sluice or floodgate. P. 239.

Fig. 284. A box, with a valve supported by a hollow ball, for letting out air from
pipes, when it is below the level of the reservoir. P. 241.

Fig. 285. Two methods of letting out air from pipes, when it is above the level of
the reservoir ; A a valve with a stopcock near it ; B a vessel of water, screwed on for
receiving the air; to be replenished with water as it becomes empty. P. 242.

Fig. 286. A section of a compound stopcock, which receives a fluid from either
of the pipes A, B, or C, into a cavity which descends a little in the direction of the
axis, and communicates with the pipe D, by means of one of the bores represented
by dotted lines, according to the position into which the moveable cylinder is turned.
P. 242.

Fig. 287. Valves of different kinds ; A the common clack valve ; B a double
clack valve, consisting of two semicircular valves ; C a pyramidical valve, consisting
of four triangular pieces ; D a circular valve turning on an axis ; E a steam valve
of metal, sometimes called a T valve ; F, a valve of oiled silk or bladder, supported
by a grating, for air. P. 242.

xxiv DESCRIPTION OF PLATES.

PLATE XXII.

Fig. 288. Mr. Woltmann's hydrometrical fly. The plates A, B, are so adjusted
by experiment, as to move exactly or very nearly with the velocity of the wind, a few
degrees being allowed as a compensation for the retardation of friction. The cord C
is drawn up, and the wheel D is caused to revolve, at a time observed by a stop
watch ; and its surface is graduated so as to number the revolutions of the fly.
P. 243.

Fig. 289. An apparatus for measuring a ship's way, resembling Captain Hamil-
ton's. A is a funnel partly covered, B a part of the ship's keel, C the upper part of
the pipe D, in which the smaller pipe E F slides in a collar of leathers, so as to have
the orifice F level with the surface of the water. This pipe has a small aperture at
the bottom, which limits the magnitude of the stream discharged into the vessel G,
the end F being considerably larger. The tube H serves as a gage, to measure the
velocity at any given time. P. 243.

Fig. 290. An overshot wheel, on which the water is admitted in a retrograde
direction, so as to run off hi a continued stream ; at the lower part of the wheel it is
retained in the buckets partly by the assistance of a sweep. P. 245.

Fig. 291. A breast wheel, with a sweep. P. 246.

Fig. 292. An undershot wheel. P. 246.

Fig. 293. A the form of the sail of a windmill : B the best inclination for each
part of the sail A, according to Smeaton's experiments. P. 247.

Fig. 294. A kite supported by the wind, of which the force acts nearly in the line
A B, perpendicular to the surface of the kite ; and this, compounded with the force
of the cord A C, produces the result A D, which sustains the weight of the kite.
P. 247.

Fig. 295. A ship working against a wind ; the force of the wind acting nearly in
the direction A B, perpendicular to the sails, the ship's real course is B C, the angle
C B D being the lee way. P. 249.

Fig. 296. The anoria, or noria, used in Spain, for drawing water, by a series of
earthen pitchers, connected by ropes, and passing over a sprocket wheel. P. 250.

Fig. 297. An undershot waterwheel, carrying fixed buckets, which raise a portion
of water, and deliver it into a trough, furnished with a projection, which stands
under the buckets, at the upper part of the wheel. P. 250.

Fig. 298. A throwing wheel, for draining fens, worked by a windmill or other-
wise, and carrying the water upon a sweep from a lower to a higher level. P. 250.

Fig. 299. The rope pump of Vera, for raising water by means of friction : the
rope is kept stretched by a pulley under the water, which is loaded with a weight,
and slides in a groove. P. 251.

Fig. 300. The screw of Archimedes, nearly as described by Vitruvius. P. 251.

Fig. 301. The screw of Archimedes, as recommended by D. Bernoulli. P. 251.

Fig. 302. A waterscrew, revolving within a fixed cylinder. P. 252.

Fig. 303. The spiral pump of Wirtz. P. 253.

DESCRIPTION OF PLATES. xxv

PLATE XXIII.

Fig. 304. A centrifugal pump. The machine is first filled through the funnel A,
and when it is made to revolve, the water is discharged into a circular trough, of
which a section is seen at B and C. The valve at D remains shut while the pump is
filling. P. 253.

Fig. 305. A pump consisting of two plungers, continued nearly to the height at
which the water is delivered. P. 254.

Fig. 306. Lahire's double forcing pump. When the piston is depressed, the
water enters the barrel at the valve A, and goes out at B ; when it is elevated, it
enters at C and escapes at D. P. 254.

Fig. 307. The common piston, coated with leather. P. 254.

Fig. 308. Mr. Bramah's press. The pump A forces the water through the pipe
B into the barrel C, in which it acts very powerfully on the large piston D, and
raises the bottom of the press E. P. 254.

Fig. 309. The common sucking pump. P. 254.

Fig. 310. A bag pump, the bag or puff A being extended and contracted by the
motion of the piston. P. 255.

Fig. 311. A lifting pump, the piston rod AB being drawn up by a frame.
P. 255.

Fig. 312. A sucking pump, converted, by the addition of a collar of leathers at A,
into a forcing pump. P. 255.

Fig. 313. A fire engine, on a construction similar to some machines described by
Ramelli. AB is the piston, working within a cylindrical barrel, and moved by the
handles C D. When the end C is depressed, the water enters through the valves E
and F, and is discharged at G and H ; when D is depressed, the water enters at I
and K, and is discharged at L and M, into the air vessel N, whence it is expelled by
the pipe O. The pipes P and Q, may be united, if it be required. P. 255.

Fig. 314. From Ramelli. The wheel AB, revolving in the direction B A, carries
a portion of water C between itself and the sweep D E, which is intercepted by the
slider F, and forced up the pipe,E G. P. 256.

Fig. 315. From Ramelli. The roller A, revolving within the reservoir B C, which
is nearly cylindrical, carries with it the slider D E, which is made to sweep the internal
surface of the cylinder from C to F, by means of a projecting surface acting on the
end D, so that the water G is forced through the pipe F. P. 256.

Fig. 316. From the cabinet of Mr. Serviere. The wheels A and B carry, during
their revolution, a quantity of water from C to D, or from D to C, according to the
direction in which they are turned. P. 256.

Fig. 317. Mr. Gwynn's patent water engine. The valve A is kept, partly by
means of the spring B, but still more by the pressure of the water, in contact with
the roller or piston C, which revolves within the box D E, and sweeps it from E to
F, so that the portion of water G is forced, during each half of a revolution, into the
pipe F ; or is drawn from F to E, when the roller revolves in a contrary direction.
P. 256.

Fig. 318. A chain pump. P. 257.

Fig. 319. The mechanism of Holl's acting pump. In the position of the stopcock
A B, here represented, the water flows out of the barrel C, and the piston D is
allowed to descend. The rod E then turns the stopcock, and the barrel C
communicates only with the pipe F, which fills it, and forces up the piston, until the
stopcock is turned back to its former position. P. 257.

Fig. 320. The hydraulic air vessels of Schemnitz. The reservoir A being filled
with water, and B with air, and water being poured into the funnel C, the air in B
acts by the pipe D on the water in A, and forces it up the pipe E. P. 258.

Fig. 321. A being the high water mark, and B the low water mark, the vessels C
and D are filled at high water from below, the air being suffered to escape by a
stopcock, which is opened by the fall of the ball F ; at low water the air will enter
the vessel D at B ; and before the next high water, the water C will be forced up the
pipe E. P. 258.

Fig. 322. The fountain of Hero. Its operation resembles that of the hydraulic
air vessels, fig. 320 ; but the pipe D here ascends. P. 258.

Fig. 323. The hydraulic ram of Montgolfier. When the water in the pipe A B has
acquired a sufficient velocity, it raises the valve B, which stops its passage, so that a
part of it is forced through the valve C, into the air vessel D, whence it rises through
the pipe E. P. 259.

xxvi DESCRIPTION OF PLATES.

PLATE XXIV.

Fig. 324. The cupping instrument of Hero. The cavity A was partly exhausted
by applying the mouth repeatedly to the pipe B, the stopcock B being turned after
each application. When the stopcock C was opened, the air at D in contact with the
skin was also rarefied, and the effect of suction was produced. P. 260, 276.

Fig. 325. Mr. Cuthbertson's air pump. When the piston rod A is depressed, it
leaves the piston B a little behind it, so as to make an opening between two conical
parts which are ground together, and the air escapes from the lower part of the barrel
into the upper part ; when it is elevated, the whole piston is raised, and a wire, which
slides through the axis of the rod, raises a small valve at the bottom of the barrel,
which leads to the receiver C, by the tube D E : the air is forced from the upper
part of the barrel through a valve in the oil vessel F, whence the oil runs back, when
it overflows, by a tube leading to the mouth of the barrel ; and if this tube be stopped
by turning its cock, the air may be condensed into a receiver fixed at G. At H is a
long gage, with a barometer immersed in the same bason of mercury. The piston rod,
which is hollow, has a perforation a little above A, to admit the oil, in order that the
wire may work freely in it. P. 261.

Fig. 326. The two flies A and B being caused to revolve with equal velocities by
the descent of the weight C, they continue to move for an equal length of time in the
vacuum of the air pump. P. 261.

Fig. 327. The air in the bottle A expands, when the receiver B is exhausted, and
causes the water to rise in a jet. P. 261.

Fig. 328. A pear gage ; to be suspended in a receiver by a hook like that which
is shown in fig. 325. P. 262.

Fig. 329. A condenser, with screws, for confining the receiver. A is a gage for
showing the degree of condensation ; B the piston of the syringe, with a valve of the
best kind, which is conical, and is confined by a spiral spring. But in common,
the valves are made of leather, with a plate of metal to strengthen it. P. 262.

Fig. 330. A diving bell. A is the forcing pump, B a stopcock for letting out the
heated air, C a strong glass for giving light, D a float for the security of the diver.
P. 262.

Fig. 331. Laurie's hydraulic bellows. When the vessel A is raised, the air enters
at the valve B ; when it is depressed, the valve B shuts, and the air is forced through
the pipe C D, which conducts it to the reservoir E, where it is confined by the valve
F, and forced by the pressure of the water through the pipe G. P. 263.

Fig. 332. Mr. Watt's gasometer. The pressure is regulated by the magnitude
of the weights A and B, which act by the spiral fusees C, D, so as to sustain a part
of the weight of the inverted vessel, represented by the exterior dotted line. The gas
is admitted at E or F, and is delivered at G. G H is a gage for showing the height
of the water within and without the moveable vessel. I is a cock for letting off the
water. P. 263.

Fig. 333. The shower bellows. The stream A, passing through the strainer B,
carries with it a quantity of air through the pipe C, which rises to the upper part of
the air vessel D, and is discharged by the pipe E. P. 263.

Fig. 334. The centrifugal bellows. By the revolution of the fly, the air is caused
to enter at A, and is discharged at B. P. 264.

Fig. 335. The original steam engine of Savery. The vessel A being filled with
steam from the boiler B, and the stopcock being turned, the steam cools and is con-
densed, and water is forced into its place by the pressure of the atmosphere, through
the valve C : the steam is then readmitted, and forces the water to ascend through
the valve D and the pipe D E. The vessel F acts alternately with A. P. 266.

Fig. 336. The common steam engine of Newcomen and Beighton. The steam
being admitted into the cylinder A below the piston, the weight B is allowed to
descend : a jet of water is then admitted by the pipe C, which condenses the steam,
and the pressure of the atmosphere then depresses the piston : a part of this water is
admitted by the pipe D into the boiler, in order to keep it sufficiently full. P. 266.

Fig. 337. Mr. Watt's steam engine. The steam, which is below the piston, is
suffered to escape into the condenser A by the cock B, which is opened by the rod
C, and at the same time the steam is admitted by the cock D into the upper part of
the cylinder ; when the piston has descended, the cocks E and F act in a similar
manner in letting out the steam from above and admitting it below the piston . The

DESCRIPTION OF PLATES. xxvii

jet is supplied by the water of the cistern G, which is pumped up at H from a reser-
voir : it is drawn out, together with the air that is extricated from it by the air pump
I, which throws it into the cistern K, whence the pump L raises it to the cistern M ;
and it enters the boiler through a valve, which opens whenever the float N descends
below its proper place. The pipes O and P serve also to ascertain the quantity of
water in the boiler. The piston rod is confined to a motion nearly rectilinear by the
frame Q ; the fly wheel R is turned by the sun and planet wheel S, T ; and the strap
U turns the centrifugal regulator W, which governs the supply of steam by the valve
or stopcock X. P. 267.

Fig. 338. Mr. Symington's steam boat. A is the boiler, B the cylinder, C the
piston, D the condensation pipe, E the air pump, F stampers for breaking ice.
P. 267.

Fig. 339. An air gun. The air is forced by the syringe A into the cavity sur-
rounding the barrel, whence it is discharged by the valve B, which is opened either
immediately by the action of the trigger C, or by a spring, which is bent by cocking
the gun, and set at liberty by the trigger. P. 269.

PLATE XXV.

Fig. 340. A series of waves or pulses of sound, diverging from one of the foci of
an ellipsis, and reflected towards the other. P. 293.

Fig. 341. Waves diverging from a point near the centre of a circle, and converg-
ing after reflection to a point at an equal distance on the other side of the centre.
P. 293.

Fig. 342. A section of a speaking trumpet and of a hearing trumpet : the lines
representing the direction of the sound before and after its reflections. P. 294.

Fig. 343. A string impelled- by the bow of a violin, and lightly touched at the
same time at a point one third of its length from the end : the small pieces of paper
fly off from the middle of the vibrating portions, while the piece situated at the
remaining point of division retains its situation. P. 299.

Fig. 344. A vibration compounded with another smaller vibration, three times
as frequent, in a transverse direction, the separate vibrations being such that the
points may be always opposite to a point moving uniformly in a circle. Thus the
vibrations in the lines A B and A C compose the complicated figure D E. P. 299.

Fig. 345. A specimen of the manner in which the vibrations of a string are
usually performed when it is struck with a bow. P. 299.

Fig. 346. Specimens of the simplest manner in which sand is collected into lines,
on a plate of glass or metal, which is made to sound by means of the bow of a
violin. P. 300.

Fig. 347. A round plate, performing some of its most complicated vibrations, the
lines of division being indicated by the place of the sand. From Chladni. P. 300.

Fig. 348. A square plate divided into a diversity of vibrating portions. From
Chladni. P. 300.

Fig. 349. The small bones of the left ear, nearly three times the natural size,
supposed to be seen through the membrane of the tympanum, by looking directly
into the auditory canal. AB is the membrane of the tympanum, C the hammer, D
the anvil, E its attachment to the surrounding bone, F the stirrup, G the round
aperture in the bone leading to the cochlea. P. 302.

Fig. 350. A view of the vestibule of the left ear, with the semicircular canals and
the cochlea, seen with the eye a little more depressed than by looking straight through
the canal, and exactly in the direction of the stirrup. AB C is the vestibule, imme-
diately behind the oval aperture, which is covered by the basis of the stirrup, D are
the canals, E the cochlea, the upper spire terminating in the vestibule, the lower in
the round aperture at B. The projection of the membrane of the tympanum is
marked by an oval line. P. 302.

Fig. 351. The structure of the left ear, seen from above, the upper part of the
canal being supposed to be removed. A is the auditory canal, B the membrane of
the tympanum, C the hammer, D the anvil, E the stirrup ; Fthe place of the canals,

xxviii DESCRIPTION OF PLATES.

which are higher than the parts represented, G the place of the cochlea, H the round
aperture. P. 302.

Fig. 352. A, B, C, a representation of the joint effect of two equal vibrations
variously combined, the middle line being always half way between the two outer
ones, and showing the compound vibration reduced to half its real extent : D shows
the mode of finding the joint effect of vibrations, by cutting a surface into sliders,
which are retained in their places by a screw. P. 305.

Fig. 353. The uppermost and lowermost curves represent a series of vibrations,
of which 12 occupy any given period of time : the third and sixth lines two series of
which 15 and 16 occupy respectively the same time : the joint effect of each pair is
shown by the dotted curves which are interposed between them, the middle one
representing the effect denominated a beat. P. 305, 306.

Fig. 354. The proportional lengths of a chord or pipe, constituting the different
notes of the simple diatonic scale, with their mutual relations, shown by their divi-
sions into aliquot parts. P. 307.

Fig. 355. A good practical mode of temperament ; making all the fifths and the
third in the first division perfect concords ; the three remaining fifths equally imper-
fect. P. 309.

Fig. 356. The trumpet of Marigni, with its bridge, which is supported by the
string A B nearly in contact with the sounding board ; this string being either
stretched by a pin at B, or by a cross string B C. The places at which the string is
to be touched, may be marked by frets fixed under them, as they are here shown by
points. At D, the scale of this instrument is exhibited, resembling that of the
trumpet and the French horn. P. 312.

PLATE XXVI.

Fig. 357. The right half of the human larynx. A B C is the outline of the cricoid
cartilage, D E F G H of the thyreoid, and I K L of the arytaenoid cartilage ; M is
the epiglottis, N K the upper ligament of the glottis, O P the lower ligament, and
Q the trachea. P. 313.

Fig. 358. A view of the ligaments of the glottis, seen from above, the larynx
being divided by a horizontal section a little above them. P. 313.

Fig. 359. Sections of the pipes employed by Kratzenstein for producing the
sounds of the different vowels ; in general by means of a larynx resembling the
mouth piece of a reed organ pipe, but in the case of the vowel I by simple inflation
through the tube B. The pipe for U produces the sound O, except when it is very
nearly shut up. P. 313.

Fig. 360. The vox humana organ pipe, with the mouth piece common to reed
pipes in general ; the lower part in contact with the tongue being nearly semicylin-
drical ; the tongue being adjusted to the proper pitch by means of a sliding wire,
which regulates the length of the part that is at liberty to vibrate. P. 314.

Fig. 361. The mouth piece proposed by Kratzenstein, for imitating the human
voice, the tongue A passing freely in and out of the tube, which is more than half of
a cylinder, as is seen at B. P. 314.

Fig. 362. The form of the regal organ pipe. P. 314.

Fig. 363. A front view and section of the open diapason organ pipe of metal. It
is tuned by opening or contracting the upper orifice. P. 314.

Fig. 364. A a front view of the flute organ pipe, of wood, which is tuned by a
plug. B a section of the pipe. P. 314.

Fig. 365. A stopped diapason organ pipe, of metal. It is tuned by altering the
position of the pieces on each side of the mouth. P. 314.

Fig. 366. A chimney pipe. P. 314.

Fig. 367. A spindle shaped organ pipe, contracted above. P. 314.

Fig. 368. A the form of a cromorn pipe, B, of a trumpet pipe, both having reed
mouth pieces. P. 315.

Fig. 369. A ray or pencil of light A B, C B, falling on the surface D E, a por-
tion of the light is reflected, and another portion is transmitted, in the direction
B F, B G, so that B G is equal to B C, and B H to B I, C I K and G H L being

DESCRIPTION OF PLATES. xxix

lines perpendicular to D E at any such distances, that B K may be to B L in a cer-
tain proportion, which is that of the sines of the angles of incidence ABM, C B M,
to those of the angles of refraction FBN, GBN. BO and B P are the reflected
portions of the rays. P. 322.

Fig. 370. A mode of determining the position of a refracted ray, which is parti-
cularly convenient in the case of refractions at spherical surfaces. ABC being any
circle, either touching the refractive surface at A, or being itself a section of the
refracting substance, if another circle D E F be drawn on the same centre, having its
diameter to that of the first as the sine of the angle of incidence to that of refrac-
tion, and a third circle G H I, which is less than the first in the same proportion as
the second is greater ; and if the direction of the incident ray K A be continued to D,
and L D be drawn from the centre cutting G H I in G, A G will be the direction of
the refracted ray ; and if this ray pass again out of the denser medium at B, its
direction B M may be found by drawing L I F, and F B M will be thus truly deter-
mined. P. 322.

Fig. 371. A ray or pencil AB, refracted at B to C, and there reflected by a
perpendicular surface into an opposite direction C B, will return also in the direction
B A, a portion of it being reflected in the first place to D, and in the second to E.
P. 323.

Fig. 372. A pencil AB passing through a substance CD contained between
parallel surfaces, continues its course in the direction E F parallel to A B.
P. 324.

Fig. 373. The ray A B, entering the medium C D through the transparent sub-
stance E F, contained between parallel surfaces, acquires the direction GH, parallel
to IK, into which LI is at once refracted. P. 324.

Fig. 374. The appearance of a prism, of which the lower surface is divided into a
bright and a dark portion, separated by a coloured arch ABC. P. 324.

PLATE XXVII.

Fig. 375. A is an actual focus of diverging rays, B an actual focus both of con-
verging and of diverging rays, C a virtual focus of converging rays, and D a virtual
focus of diverging rays; A and B, B and C, and C and D are foci conjugate to each
other, with respect to the refractions of the three lenses. P. 325.

Fig. 376. The image of the point N, formed by the plane mirror AB, is at an
equal distance behind the mirror ; and in this manner the whole image of the word is
formed in an inverted position. P. 325.

Fig. 377. A BCD represents a pencil of parallel rays falling on the concave
mirror C D, and collected into the principal focus at E, which is half way between
the surface and its centre. F is the principal focus of the convex mirror G ; and H
that of the refracting surface I. P. 326.

Fig. 378. A being the centre of the concave mirror B, the image of an object at
C will be found at D, and the reverse. P. 326.

Fig. 379. A pencil of light, deflected from its path by a prism of a denser sub-
stance, in different positions. P. 326.

Fig. 380. A pencil of light scattered into various directions by a multiplying
glass. P. 326.

Fig. 381. A is a section of a double convex lens, B of a double concave. C is a
planoconvex, D a planoconcave ; and E and F meniscus lenses ; but a meniscus of
the form represented by F is sometimes called a concavoconvex lens. P. 326.

Fig. 382. The pencils of light A, B are refracted by the convex lens C in the
same manner as they would have been by the circumscribed double prism D E ; and
in the same manner the concave lens F resembles in its operation the prisms G, H.
P. 326.

Fig. 383. A, a pencil of parallel rays, made to converge, by a double convex lens
of crown glass, to the centre of curvature of one of its surfaces. B a double con-

xxx DESCRIPTION OF PLATES.

cave lens, causing the rays to diverge from the centre of curvature. C, D a plano-
convex lens, of which the principal focus is at the distance of a diameter. P. 326.

Fig. 384. The lenses represented by the shaded surfaces are equivalent in their
effects to those of which the sections are shown by the dotted lines ; the figures at A
and B being of equal thickness in the middle, and at C at the edges also. P. 326.

Fig. 385. At A, a radiant point and its image are both situated at the distance of
twice the focal length from the lens ; at B, the one is more remote, the other nearer ;
and C D is to D E as E F to F G ; D and F being the principal foci of the lens.
P. 327.

Fig. 386. The oblique pencils of rays A, B, and the direct pencil C, are sup-
posed to be brought to their respective foci in the same plane D E. P. 327.

Fig. 387. The square A intercepts the whole light, proceeding from the point B,
which would fall on the surface C D, four times as great, placed at a double
distance. P. 329.

Fig. 388. The box of Count Rumford's photometer. The lights, being placed
at proper distances on the graduated arms or tables A, B, throw equally dark
shadows of the cylinders C, D on a white surface at E F. The wings of the cylinders
serve to make the shadows of equal breadth. The shadows are viewed through the
aperture at G. P. 329.

Fig. 389. Dr. Wollaston's instrument for the measurement of refractive densi-
ties. A is a rectangular prism of flint glass, under which the substance to be exa-
mined is attached ; BC is a rod, or ruler, 10 inches long, CD and DE are each
15fgg. When the sights at B and C are so placed that the division between the light
and dark portion of the lower surface of the prism is seen through them, the rod F,
which carries a vernier, shows the index of the refractive density, which, in the
situation here represented, would be 1.43. P. 329.

Fig. 390. A is the actual image of the candle B, formed by the convex lens C.
P. 330.

Fig. 391. A is the actual image of the candle B, formed by the concave mirror
C. P. 330.

Fig. 392. A is the actual image of the candle B, formed by the convex lens C,
being as much larger than the object as it is more distant from the lens. P. 330.

Fig. 393. A is the virtual image of the candle B, placed within the focal dis-
tance of the concave mirror C, the image remaining erect. P. 330.

Fig. 394. A is the virtual image of the candle B, formed by the concave lens C,
and less than the object. P. 330.

Fig. 395. When the object A is placed in the principal focus of the convex lens
B, a virtual image is formed at an infinite distance, which subtends, when viewed
from C, or from any other point, the same angle as the object subtends at the centre
of the lens. P. 330.

Fig. 396. The object A being placed a little within the focus of the lens B, a
virtual image C is formed, at such a distance as is most convenient to the eye,
which subtends the same angle as the object, from the centre of the lens, and there-
fore appears somewhat more magnified than when the object is in the principal
focus. P. 330.

DESCRIPTION OF PLATES. xxxi

PLATE XXVIII.

Fig. 397. An imperfect image of an external object, painted in a dark room, in
an inverted poskion, by the light coming in right lines through a small aperture.
P. 332.

Fig. 398. A portable camera obscura. A is a lens, B a mirror placed obliquely,
and throwing the image on a plate of ground glass, CD. E is a moveable cover,
and F G a screen attached to it, for excluding foreign light. P. 332.

Fig. 399. A camera obscura, which throws down an image, by means of the
mirror A, and the lens B, on the surface C, where it may be seen through the aper-
ture D. The surface C has here the curvature best adapted to receive every where a
perfect image of a distant object. P. 332.

Fig. 400. An arrangement proposed for a solar microscope, adapted to a window
facing the south. The mirror A is moved by a hinge into the position required for
the day, and during the employment of the instrument is turned only round the axis
A B, which is parallel to that of the earth. The mirror C is fixed : it receives the
beam of light from A, and throws it on the object through the lenses D and E, of
which the joint focus is near the magnifying lens F ; this lens paints an image of the
object in an inverted position on a screen at G. If the focus of the condensing
lenses were behind the object, as at H, the light would be liable to be condensed into
a spot on the screen at I. P. 333.

Fig. 401. An arrangement proposed for a phantasmagoria. The light of the
lamp A is thrown by the mirror B and the lenses C and D on the painted slider at
E, and the magnifier F forms the image on the screen at G. This lens is fixed to a
slider, which may be drawn out of the general support or box H : and when the box
is drawn back on its wheels, the rod I K lowers the point K, and by means of the
rod K L adjusts the slider in such a manner, that the image is always distinctly
painted on the screen G. "When the box advances towards the screen, in order that
the images may be diminished and appear to vanish, the support of the lens F suffers
the screen M to fall and intercept a part of the light. The rod K N must be equal
to I K, and the point 1 must be twice the focal length of the lens F, before the object,
L being immediately under the focus of the lens. The screen M may have a trian-
gular opening, so as to uncover the middle of the lens only, or the light may be inter-
cepted in any other manner. P. 334.

Fig. 402. The construction of the astronomical telescope. ABC and DEC are
the central parts of the pencils of rays, coming, from the extremities of the visible
field, through the middle of the object glass. P. 334.

Fig. 403. The extreme pencils of rays in the double or compound microscope.
P. 334.

Fig. 404. The extreme pencils in the Galilean telescope, or opera glass. P. 334.

Fig. 405. A, the directions of the extreme pencils in the common day telescope of
Rheita. If only two eye glasses were employed, as at B, the field would obviously
be more contracted. P. 334.

Fig. 406. Dr. Herschel's forty feet telescope. ABC the path of a ray of light,
reflected by the mirror at B to the eye glass C. Da chair in which the observer
sits. E a moveable gallery, on which several persons may stand. F G a smooth
surface, on which the bottom of the telescope is made to roll along, while its opening
is raised or depressed by the pullies at H and I. K one of two rooms or huts for
the accommodation of the observer's assistants. The wheels, under the frame, serve
to turn the whole instrument round its centre. P. 335.

Fig. 407. The Newtonian telescope, with the direction of the central rays. These
are not the rays by which the object is actually seen, because they are intercepted by
the small speculum, but they afford the simplest determination of the magnitude of
the field of view. P. 335.

Fig. 408. The supposed path of the central rays in the Gregorian telescope.
P. 335.

Fig. 409. The supposed path of the central rays in Cassegrain's telescope. Here
the rays actually represented would not only be intercepted by the small mirror, but

xxviii DESCRIPTION OF PLATES.

which are higher than the parts represented, G the place of the cochlea, H the round
aperture. P. 302.

Fig. 352. A, B, C, a representation of the joint effect of two equal vibrations
variously combined, the middle line being always half way between the two outer
ones, and showing the compound vibration reduced to half its real extent : D shows
the mode of finding the joint effect of vibrations, by cutting a surface into sliders,
which are retained in their places by a screw. P. 305.

Fig. 353. The uppermost and lowermost curves represent a series of vibrations,
of which 12 occupy any given period of time : the third and sixth lines two series of
which 15 and 16 occupy respectively the same time : the joint effect of each pair is
shown by the dotted curves which are interposed between them, the middle one
representing the effect denominated a beat. P. 305, 306.

Fig. 354. The proportional lengths of a chord or pipe, constituting the different
notes of the simple diatonic scale, with their mutual relations, shown by their divi-
sions into aliquot parts. P. 307.

Fig. 355. A good practical mode of temperament ; making all the fifths and the
third in the first division perfect concords ; the three remaining fifths equally imper-
fect. P. 309.

Fig. 356. The trumpet of Marigni, with its bridge, which is supported by the
string A B nearly in contact with the sounding board ; this string being either
stretched by a pin at B, or by a cross string B C. The places at which the string is
to be touched, may be marked by frets fixed under them, as they are here shown by
points. At D, the scale of this instrument is exhibited, resembling that of the
trumpet and the French horn. P. 312.

PLATE XXVI.

Fig. 357. The right half of the human larynx. A B C is the outline of the cricoid
cartilage, D E F G H of the thyreoid, and I K L of the arytaenoid cartilage ; M is
the epiglottis, N K the upper ligament of the glottis, O P the lower ligament, and
Q the trachea. P. 313.

Fig. 358. A view of the ligaments of the glottis, seen from above, the larynx
being divided by a horizontal section a little above them. P. 313.

Fig. 359. Sections of the pipes employed by Kratzenstein for producing the
sounds of the different vowels ; in general by means of a larynx resembling the
mouth piece of a reed organ pipe, but in the case of the vowel I by simple inflation
through the tube B. The pipe for U produces the sound O, except when it is very
nearly shut up. P. 313.

Fig. 360. The vox humana organ pipe, with the mouth piece common to reed
pipes in general ; the lower part in contact with the tongue being nearly semicylin-
drical ; the tongue being adjusted to the proper pitch by means of a sliding wire,
which regulates the length of the part that is at liberty to vibrate. P. 314.

Fig. 361. The mouth piece proposed by Kratzenstein, for imitating the human
voice, the tongue A passing freely in and out of the tube, which is more than half of
a cylinder, as is seen at B. P. 314.

Fig. 362. The form of the regal organ pipe. P. 314.

Fig. 363. A front view and section of the open diapason organ pipe of metal. It
is tuned by opening or contracting the upper orifice. P. 314.

Fig. 364. A a front view of the flute organ pipe, of wood, which is tuned by a
plug. B a section of the pipe. P. 314.

Fig. 365. A stopped diapason organ pipe, of metal. It is tuned by altering the
position of the pieces on each side of the mouth. P. 314.

Fig. 366. A chimney pipe. P. 314.

Fig. 367. A spindle shaped organ pipe, contracted above. P. 314.

Fig. 368. A the form of a cromorn pipe, B, of a trumpet pipe, both having reed
mouth pieces. P. 315.

Fig. 369. A ray or pencil of light A B, C B, falling on the surface D E, a por-
tion of the light is reflected, and another portion is transmitted, in the direction
B F, B G, so that B G is equal to B C, and B H to B I, C I K and G H L being

DESCRIPTION OF PLATES. xxix

lines perpendicular to D E at any such distances, that B K may be to B L in a cer-
tain proportion, which is that of the sines of the angles of incidence ABM, C B M,
to those of the angles of refraction FBN, GBN. BO and B P are the reflected
portions of the rays. P. 322.

Fig. 370. A mode of determining the position of a refracted ray, which is parti-
cularly convenient in the case of refractions at spherical surfaces. ABC being any
circle, either touching the refractive surface at A, or being itself a section of the
refracting substance, if another circle D E F be drawn on the same centre, having its
diameter to that of the first as the sine of the angle of incidence to that of refrac-
tion, and a third circle G H I, which is less than the first in the same proportion as
the second is greater ; and if the direction of the incident ray K A be continued to D,
and L D be drawn from the centre cutting G H I in G, A G will be the direction of
the refracted ray ; and if this ray pass again out of the denser medium at B, its
direction B M may be found by drawing L I F, and F B M will be thus truly deter-
mined. P. 322.

Fig. 371. A ray or pencil AB, refracted at B to C, and there reflected by a
perpendicular surface into an opposite direction C B, will return also in the direction
B A, a portion of it being reflected in the first place to D, and hi the second to E.
P. 323.

Fig. 372. A pencil AB passing through a substance CD contained between
parallel surfaces, continues its course in the direction EF parallel to AB.
P. 324.

Fig. 373. The ray A B, entering the medium C D through the transparent sub-
stance E F, contained between parallel surfaces, acquires the direction GH, parallel
to IK, into which LI is at once refracted. P. 324.

Fig. 374. The appearance of a prism, of which the lower surface is divided into a
bright and a dark portion, separated by a coloured arch ABC. P. 324.

PLATE XXVII.

Fig. 375. A is an actual focus of diverging rays, B an actual focus both of con-
verging and of diverging rays, C a virtual focus of converging rays, and D a virtual
focus of diverging rays; A and B, B and C, and C and D are foci conjugate to each
other, with respect to the refractions of the three lenses. P. 325.

Fig. 376. The image of the point N, formed by the plane mirror AB, is at an
equal distance behind the mirror ; and in this manner the whole image of the word is
formed in an inverted position. P. 325.

Fig. 377. A BCD represents a pencil of parallel rays falling on the concave
mirror C D, and collected into the principal focus at E, which is half way between
the surface and its centre. F is the principal focus of the convex mirror G ; and H
that of the refracting surface I. P. 326.

Fig. 378. A being the centre of the concave mirror B, the image of an object at
C will be found at D, and the reverse. P. 326.

Fig. 379. A pencil of light, deflected from its path by a prism of a denser sub-
stance, in different positions. P. 326.

Fig. 380. A pencil of light scattered into various directions by a multiplying
glass. P. 326.

Fig. 381. A is a section of a double convex lens, B of a double concave. C is a
planoconvex, D a planoconcave ; and E and F meniscus lenses ; but a meniscus of
the form represented by F is sometimes called a concavoconvex lens. P. 326.

Fig. 382. The pencils of light A, B are refracted by the convex lens C in the
same manner as they would have been by the circumscribed double prism D E ; and
in the same manner the concave lens F resembles in its operation the prisms G, H.
P. 326.

Fig. 383. A, a pencil of parallel rays, made to converge, by a double convex lens
of crown glass, to the centre of curvature of one of its surfaces. B a double con-

xxxiv DESCRIPTION OF PLATES.

PLATE XXX.

Fig. 436. A section of the human eye. A is the cornea ; B the aqueous humour,
in which the uvea hangs ; C the crystalline lens ; the ciliary processes being between
it and the uvea ; D the vitreous humour ; E F G is the choroid coat, lined by the
retina ; H I K the sclerotica, and L the optic nerve. P. 350.

Fig. 437. A picture painted on the retina in an inverted position, seen by dissect-
ing off the sclerotica and choroid behind it. P. 351.

Fig. 438. The apparent figure of the heavens being nearly like the curve ABC,
the sun or moon at A or C appears to be much larger than at B. P. 356.

Fig. 439. The red square A, inclosing a green square, produces, if viewed atten-
tively, in a strong light, a spectrum resembling B, which is red within and green
without, and which appears when we look soon after on any white object. P. 357.

Fig. 440. The spot, which is tinted with black lines only, appears, upon the
yellow ground, of a purple hue. P. 357.

Fig. 441. A grey spot on a purple ground appears of a greenish yellow or olive
hue. P. 357.

Fig. 442. The manner in which two portions of coloured light, admitted through
two small apertures, produce light and dark stripes or fringes by their interference,
proceeding in the form of hyperbolas ; the middle ones are however usually a little
dilated, as at A. P. 365,

Fig. 443. A series of stripes of all colours, of their appropriate breadths, placed
side by side in the manner in which they would be separated by refraction, and com-
bined together so as to form the fringes of colours below them, beginning from white.
P. 365.

Fig. 444. A series of coronae, seen round the sun or moon. P. 366.

Fig. 445. The internal hyperbolic fringes of a rectangular shadow. P. 367.

1Fig. 446. The external fringes seen on each side of the shadow of a hair or wire,
which is also divided by its internal fringes. The dotted lines show the natural mag-
nitude of the shadow, independently of diffraction. P. 367.

Fig. 447. Analysis of the colours of thin plates seen by reflection, beginning from
black. A line drawn across the curved fringes would show the portions into which
the light of any part is divided when viewed through a prism. P. 368.

Fig. 448. The coloured stripes of a film of soapy water, covering a wine glass.
P. 368.

Fig. 449. The colours of a thin plate of air or water contained between a convex
and a plane glass, as seen by reflection. P. 368.

Fig. 450. The colours of a mixed plate ; as seen by partially greasing a lens a
little convex, and a flat glass, and holding them together between the eye and the
edge of a dark object. One half of the series begins from white, the other from
black, and each colour is the contrast to that of the opposite half of the ring.
P. 369.

Fig. 451. The composition of the colours of the primary rainbow, when attended
by supernumerary bows. P. 369.

Fig. 452. The colours of concave mirrors. The small circles in the middle white
ring represent the aperture by which the light is admitted, and its image ; the
coloured rings are formed by the light irregularly dissipated before and after reflec-
tion. P. 370.

DESCRIPTION OF PLATES. xxxv

PLATE XXXI.

Fig. 453, 454. The appearance of the star Lyra, viewed with telescopes magnify-
ing 460 and 6450 times respectively. From Dr. Herschel. P. 390.

Fig. 455. The appearance of the nebula in Orion, about half a degree in length.
From Messier.* P. 391.

Fig. 456 . . 463. The appearances of different nebulae. From Dr. Herschel.
P. 391.

Fig. 464. A section of the nebula to which the sun is supposed to belong, its
projection forming the milky way ; taken in a plane perpendicular to its longest
diameter. From Dr. Herschel. The large star in the middle represents the sun,
and the circle drawn round it is at forty times the distance of the nearest fixed stars,
comprehending probably all the stars which are visible to the naked eye. P. 362.

Fig. 465. A large spot, traced through different forms in its path across the sun.
From Dr. Wilson. A is its place 23 Nov. 1769 ; B, 24 Nov. ; C, 11 Dec. ; D,
12 Dec. ; and E, 17 Dec. P. 399.

Fig. 466. A, a large spot on the sun ; B, the arrangement of the luminous and
opaque strata of clouds by which Dr. Herschel explains the appearance of the spot.
P. 399.

Fig. 467. A, a spot with a lighter portion in the middle ; B, the arrangement of
the strata corresponding to it. P. 399.

Fig. 468. The position assumed by the strata which had formed the spot shown in
the last figure, viewed about an hour afterwards. P. 399.

Fig. 469. A and B are the forms of a solar spot, at about two hours' distance of
time ; C, D, and E, are the successive forms of another spot. P. 399.

Fig. 470. The appearance of the zodiacal light, or solar atmosphere, as it is seen
in these climates, in the evening, about the beginning of March ; A B being the
horizon, and C the supposed place of the sun. P. 400.

PLATE XXXII.

Fig. 471. A representing the sun, B the earth, and C the planet Mars ; suppos-
ing Mars and the earth to set out together from D and E, the angle D A C was
determined by Kepler from calculation, and the angles BAD and A B C by observa-
tion ; whence it was easy to construct the triangle ABC, and to find the proportion
of A B to AC. P. 402.

Fig. 472. The solar system, representing the form and proportions of the orbits
of all the primary planets, and of three of the comets. The parts of the orbits
represented by entire lines are on the north of the ecliptic, the dotted parts on the
south : the letters A and P denote the aphelion and perihelion. The point in the
centre, which ought to be only ^ of an inch in diameter, represents the sun. The
figures of the respective planets show their comparative magnitude, that of the sun
being represented by the innermost of the graduated circles which inclose the whole :
they are placed according to their actual situations on the 14th June, 1806. The
letters M D show the mean distance of the comet of 1759, being placed at the
extremity of the lesser axis of the ellipsis in which it must be supposed to revolve.
P. 408.

Fig. 473. The periodical times of the different planets, represented by lines of
different lengths. P. 408.

Fig. 474. The comparative velocities of the different planets, represented bylines
which show the number of English miles described in a second, on the scale marked
on the lowest line. P. 408.

Fig. 475. The places of the ascending nodes of all the planets, marked on one
half of the ecliptic, supposed to be extended in a straight line ; together with the
inclinations of their orbits. The line marked F. F. E. E, shows the situation of the
fixed ecliptic. P. 408.

Hist, et Mem. 1771, p. 458.

XXXVI

DESCRIPTION OF PLATES.

PLATE XXXIII.

Fig. 476. A. The appearance of Venus, from Dr. Herschel: B, C, from
Mr. Schroeter. P. 408.

Fig. 477. A. .D, the appearance of Mars, from Dr. Herschel. The figures are
inverted, as they appear in the astronomical telescope. P. 408.

Fig. 478. A,B. The appearance of Jupiter, with his belts, from Dr. Herschel.
P. 408.

Fig. 479. The appearance of Saturn, with his ring, from Dr. Herschel. P. 408.

Fig. 480. The appearance of the moon, in an inverted position. The figure is
copied from Mr. Nicholson's plate, the references from Cassini and Lalande. Eq.
is the place of the moon's equator. P. 408.

Names of the spots, according to Riccioli, and Hevelius.

1 Grimaldus or

Palus Mareotis

26 Hermes Mons Bodinus

2 Galileus

Mons Audus

27 Dionysius

3 Aristarchus

Mons Porphyrites

(d) Albategnius

4 Keplerus

Loca paludosa

29 Plinius PromontoriumAche

5 Gassendas

Mons Cataractes

rusia

6 Schikardus

Mons Troicus

30 S. Theophilus Mons Moschi

7 Harpalus

Insula sinus hyper-

31 Fracastorius Lacus Thospitis

borei

32 Censorinus Promontorium acu

8 Heraclides

Caput mulieris

turn

(b) Vulcanus

33 Messala

9 Lansbergius

Insula Malta

34 Promontorium Som

10 Reinoldus

Mons Neptunus

nii

11 Copernicus

Mons Aetna

35 Proclus Mous Corax

12 Helicon

Insula erroris

36 Cleomedes Montes Riphaei

13 Capuanus

Regio Cassiotis

37 Snellius Mons Paropamisus

14 Bullialdus

Insula Greta

38 Petavius Petra Sogdiana

15 Eratosthenes

Insula Vulcania

39 Langrenus Insula major

16 Timocharis

Insula Corsica

40 Taruntius Sinus Phasianus,

17 Plato

Locus niger major

A Mare Humorum

18 Archimedes

B Mare Nubium

(a} Aratus

C Mare Imbrium

19 Insula sinus medii

D Mare Nectaris

20 Pitatus

Mare mortuum

E Mare Tranquili-

21 Tycho

Mons Sinai

tatis

22 Eudoxus

Mons Carpathes

F Mare Serenitatis

23 Aristoteles

Mons Serrorum

G Mare Foecundita-

24 Manilius

Insula Berbicus tis

25 Menelaus

Byzantium

H Mare Crisium

Fig. 481.. 483. The satellites of Jupiter, Saturn, and the Georgian planet, at
their proper distances, in proportion to the diameters of the planets, shown on the
same scale. P. 408.

Fig. 484. The figure of the tail of the comet of 1680, represented in the plane of
its orbit, from Newton. A B is the earth's orbit, C and D are the first and last
appearances of the tail, and E F is the line of the nodes. P. 408.

Fig. 485. A, B. Two successive appearances of the comet of 1723, from Lord
Paisley. P. 408.

DESCRIPTION OF PLATES. xxxvii

PLATE XXXIV.

Fig. 486. The gravitating body ABC, being supposed to revolve on the axis A C,
the fluid column B D must be longer than ED, in order to support its pressure.
P. 412.

Fig. 487. If A represent the place of the sun, B that of the earth, and C that
of the moon, taking A D to A C as the square of A C is to the square of A B, AD
will represent the sun's attraction acting on the earth, and C D the disturbing force,
which, together with A D, makes up A C, the force acting on the moon ; and it is
obvious that, when the nodes are in any oblique situation, as E F, the force being
directed to some point D, between B and A, while the moon moves from G to H,
the force C D will tend to lessen the inclination, while the moon is ascending from
E towards C, and to cause the node E to move back towards G, and, when it is
again descending towards F, the inclination will be increased, and the node F made
to recede towards H, until the moon arrives at H, and the force becomes directed to
a point on the other side of B ; the nodes only advancing while the moon is between
H and F, or between G and E. P. 413.

Fig. 488. A body attracted towards the centre A, and descending from B in the
ellipsis BCD, has the inclination of its orbit to the revolving radius A B, AC, AD,
perpetually changed, until at D it becomes perpendicular to it : but when the force
increases more rapidly, the radius does not become perpendicular to the orbit till it
arrives at E, and the line of the apsides A D moves forwards to E. P. 414.

Fig. 489. A represents the position of the limit of light and darkness on the
earth's surface at the vernal equinox, B at the summer solstice, and C at the winter
solstice ; E Q denotes the equator, N the north pole, and S the south. P. 417.

Fig. 490. N E S W being the horizon, and Z the zenith, E A W shows the sun's
apparent path in London at the time of the equinoxes, B C D at midsummer, and
F G H at midwinter, projected orthographically, as if the circles were described on
the surface of a globe, and viewed from a great distance. The circle I K L is the
boundary of twilight, supposing it 18° below the horizon, and its intersections with
the sun's path show the beginning and end of twilight, as at I and K. P. 418.

Fig. 491. The rays of light, coming in the direction AB, are bent by the at-
mosphere so as to arrive at C, and to illuminate a part of the atmosphere there,
which is visible, by means of a second refraction, to a spectator at D, and occasions
the first and last twilight. P. 527.

Fig. 492. Venus is at her greatest elongation or angular distance from the sun A,
when situated as at B, with respect to the earth at C ; and she is stationary at D,
when she is moving with the same velocity as the earth, with respect to the direction
of the earth's motion, the line E D being then more oblique, with respect to a fixed
line, than either before or after. P. 418.

Fig. 493. AB C D is the apparent path of Venus for the year 1806, supposing
the sun E to revolve round the earth F. The place of the sun and planet is marked
for every four weeks. P. 418.

Fig. 494. The apparent path of Saturn in the heavens for the year 1806, referred
to its proper place with respect to the ecliptic. The figures denote the places at the
beginning of each month. P. 418.

Fig. 495. The small figures represent the phases of the moon in different parts
of her orbit. The smaller detached figures show the appearance of the moon, as
seen from the earth; the larger ones, those of the earth at the same times, as seen
from the moon, which are always the reverse of the moon's appearance. At A the
moon is new ; B is the first quarter, C the full moon, and D the last quarter. A
and C are sometimes called the syzygies, and B and D the quadratures. P. 419.

Fig. 496. A, the moon passing through the earth's shadow ; which is dis-
tinguished into three parts, the perfect shadow, the true shadow, and the penumbra.
At B and C the moon is shown passing through the section of the shadow.
P. 420.

Fig. 497. The path of the moon's shadow passing over the earth, in the solar
eclipse of 1764, the earth being supposed at the same time to revolve on its axis.
The line A B is the part in which the eclipse appeared annular, C D being the
breadth of the whole shadow or penumbra. P. 420.

Fig. 498. The shadow of the moon falling on the earth. The true shadow not

xxxviii DESCRIPTION OF PLATES.

extending here to the earth, the cone formed by the continuation of its outlines
marks the extent of the parts in which the eclipse appears annular. P. 420.

Fig. 499. The termination of the moon's disc in a solar eclipse. From Dr.
Herschel. P. 420.

Fig. 500. The apparent magnitudes of the planets, that of the sun or moon
being supposed equal to a circle a foot in diameter : where there are two figures,
one of them shows the mean apparent magnitude, and the other the greatest.
P. 422.

Fig. 501. The apparent magnitude of the sun, as seen from the different
planets ; for Mercury, the magnitude is shown by that of the earth in fig. 497.
P. 424.

PLATE XXXV.

Fig. 502. A B being the earth's axis, the circle A C B is the meridian of the
place C, and C D represents the plane of its horizon. P. 426.

Fig. 503. The effect of the obliquity of the ecliptic in the equation of time is
shown by the difference of the angles ABC and D B E, subtended at the pole B by
equal portions of the oblique circle A E. P. 427.

Fig. 504. AB being parallel to the earth's axis, the 12 planes passing through
it, at equal angular distances, mark, on the circle C D perpendicular to it, the hour
lines of an equatorial dial, and on the horizontal surface E F those of a horizontal
dial. P. 427.

Fig. 505. A method of constructing a dial on any given plane. A B C is the
elevation of the pole, or more generally , the angle which the surface makes with the
gnomon AB. The circles are divided into equal parts, and 1, 2, 3, 4, 5, 6 are the
hour lines, B being the place of the gnomon. The reason of this construction will
appear by comparing the circle in the last figure with the ellipsis which is formed on
the horizontal surface. P. 427.

Fig. 506. A dial for a pointed gnomon, or obelise, drawn on a horizontal surface.
P. 427.

Fig. 507. A mural quadrant, with its telescope ; A B is the plumb line, for ad-
justing the instrument, and C the counterpoise for the telescope. P. 429.

Fig. 508. A portable transit instrument. A and B are screws for adjusting the
axis by a vertical and a horizontal motion ; C D is a spirit level, which may occa-
sionally be hung on the telescope by the pins E and F. G is a small graduated arch,
to be viewed through the microscope H, for taking elevations of a few degrees.
P. 429.

Fig. 509. A transit circle, resembling Mr. Wollaston's, with a horizontal circle,
by means of which both altitudes and azimuths may be measured. A is a micro-
scope for viewing the plumb line, B another for reading off the divisions of the hori-
zontal circle ; C and D are spirit levels. P. 429.

Fig. 510. A zenith sector, with its telescope, which has usually a reflecting prism,
like that of the Newtonian telescope, for its eyeglass. P. 429.

Fig. 511. The marine octant, introduced by Hadley. The mode of taking the
common or front observation, is shown by the lines drawn to the sun and moon : the
back observation by the two stars. A is a dark glass to be used in observations of
the sun, and which may be fixed at B, when required. P. 430.

Fig. 512. A B being the situation of the earth's axis, if the angle C B D, or the
altitude of the body D, be measured, and we subtract from it the elevation of the
equinoctial C B E, the remainder will be the declination E B D. P. 430.

Fig. 513. The angle ABC is the moon's horizontal parallax, and DBC the
parallax when she is elevated above the horizon D E in the angle B D E. P. 430.

Fig. 514. The situation of the earth at the transit of Venus in June 1769. A
spectator at the North Cape was carried during the transit from A to B, and the
transit appeared to him to last while Venus moved from C to D : the island of
Otaheite, on the contrary, which is situated on the lower part of the illuminated
hemisphere, was carried from E to F, and the duration of the transit was there only
while Venus moved from G to H. Hence the rotatory motion of the earth was

DESCRIPTION OF PLATES. xxxix

compared with the excess of the motion of Venus in its orbit above that of the
earth. P. 431.

Fig. 515. A planisphere nearly resembling that of Professor Bode. The outer
circle is fixed to the chart, and is divided either according to the degrees of the
ecliptic, or the days of the month ; the graduated circle immediately within it is
divided into 24 hours, and is fixed to a circle of pasteboard, out of which the circle
NE S W, representing the horizon, is cut, the place being filled by thin varnished
paper, with circles of azimuth and altitude engraved on it, which it carried round
with the hour circle. P. 433.

Fig. 516. A diagram showing the length of the day, and the time of the sun's
rising and setting in any part of the globe, within a few minutes ; the time of the
year being found in the graduated circle representing portions of the ecliptic, and the
latitude on the middle line, by following the concentric circles of declination till
they meet the horizon passing through the given latitude, the line drawn from the
pole through this point will cut the equator in the point showing the length of the
day or night. Thus, on the first of March, in latitude 50° north, the length of the
day appears to be nearly 10 hours and f , whence the sun must rise about 37 minutes
after six ; but in latitude 85° the sun never sets on that day. P. 433.

PLATE XXXVI.

Fig. 517. Projection of the constellations of the northern hemisphere on the
plane of the equator. P. 433.

PLATE XXXVII.

Fig. 518. Projection of the southern hemisphere. P. 433.

PLATE XXXVIII.

Fig. 519. A scale of the height of different parts of the earth's surface above the
level of the sea, in English feet and miles, and in French toises. P. 439.

Fig. 520. A. The dotted ellipsis shows the section of a spheroid, which would
be the form of the earth and sea if it were always in a state of equilibrium with the
attraction of a distant body, and the shaded ellipsis the actual form assumed in con-
sequence of its rotation round its centre, the depth of the sea being less than 13 miles.
B. The surface of the sphere being supposed to be flattened, and the tides spread on
it, they would assume the form of the waves here shown. The dotted straight line
shows the mean height, which is a little above the surface in the principal sections of
the spheroid, although not universally. C. The nature of the tides of lakes, the sur-
face being regulated by that of the dotted line at B, nearly agreeing with it in direc-
tion, as at D, when the lake is narrow and deep, but differing from it, as at E, when
shallower. P. 444.

Fig. 521. The progress of the tides from the Atlantic through the channels sur-
rounding the British islands, the lunar tides happening in any part of the shaded
lines nearly at the hour after the moon's southing, which is indicated by the figure
annexed to it. P. 446.

Fig. 522. The lines AB and BC, representing the heights of the lunar and solar
tides, and the angle ABC twice their angular distance, or A D C being simply the

xl DESCRIPTION OF PLATES.

angular distance, the line AC shows the height of the compound tide, and the angles
B AC and A CB its distance from the lunar and solar tides respectively. P. 448.

Fig. 523. The two unequal tides represented by the elevation of the ellipsis above
the smaller circle may be considered as composed of two equal tides cut off by the
dotted circle, and the single tide between the two circles ; as the tides B and C make
the unequal tides at D. P. 449.

Fig. 524. The first and second curves represent two equal semidiurnal and one
diurnal tide, which would make together two unequal tides : the third and fourth
the same tides six hours more advanced : and when these are combined, the first and
third destroy each other, but the second and fourth together compose the fifth, or a
large diurnal tide. P. 449.

Fig. 525. A, the ancient system of the world, adopted by Ptolemy. B, the
arrangement supposed by some other astronomers. P. 452.

Fig. 526. The Egyptian system of the world. P. 452.

Fig. 527. The system of the Pythagoreans, and of Copernicus. P. 454.

Fig. 528. The mode of representing the inequalities of the celestial motions
employed by Ptolemy, the small circle being carried round the circumference of
the larger, while the luminary revolves in it, so as to describe the dotted curve.
P. 456.

Fig. 529. The Tychonic system of the world. P. 457.

PLATE XXXIX.

Fig. 530. The repulsive force of two particles of matter, situated at the distance
A B or A C, is represented by the ordinates or perpendiculars B D, C E, drawn to
the curve D E, supposing the force to be inversely as the distance ; but the law of
the force appears to be more nearly represented by a curve like F E. The line D FG
shows the magnitude of the cohesive force, which overcomes the repulsion at the dis-
tance AG, and is balanced by it when the particles arrive at the distance A B or AH.
The dotted lines represent the nature of the changes made in the lines FE, DFG,
and FH, by an elevation of temperature. P. 474.

Fig. 531. The general direction of the cohesive force acting on a particle of a
liquid at A being represented by A B or AC, that of the repulsive force will be DA
or EA, and in order to maintain the equilibrium, the forces BF and CG, making
together H A, must be supplied by the pressure or reaction of the internal parts.
P. 475.

Fig. 532. A. The transverse section of a drop, supposed to be of considerable
length, and flat at the sides : the curvature of the outline being every where propor-
tional to its distance from the horizontal line A B. B, a round drop, the concavity
at the horizontal line being equal to the convexity which would be found by cutting
off the drop horizontally ; the sum or difference of the curvatures being every where
proportional to the distance from this line. P. 476.

Fig. 533. The solid AB possessing half the attractive power of the liquid CD,
the surface of the liquid will remain horizontal : for the attractions will be repre-
sented by DA, DE, and DC ; and of these DA and DE make DB, and DB and
DC make DF, which is in a vertical direction. If the solid be more attractive, the
forces will be combined nearly as at G, and if less attractive, as at H. P. 476.

Fig. 534. The form of the surface of a liquid in contact with a plane and ver-
tical side of a solid which is wetted by it. The height of the ascent of water is about
one fourth of that which is here represented. P. 477.

Fig. 535. The form of the surface of a liquid elevated between two plates which
meet at A, and are at a little distance from each other at B ; about one third of an
inch, supposing the liquid to be water. P. 477.

Fig. 536. The height at which water will stand in tubes of the form and magni-
tude which are here represented. P. 477.

Fig. 537. The depression of mercury, in contact with a large or flat glass vessel,
is one fourth as great as that which is here represented. P. 478.

Fig. 538. The depression of mercury within a small tube of glass. P. 478.

DESCRIPTION OF PLATES. xli

Fig. 539. The actual elevation of a portion of water in contact with a hori-
zontal surface which is wetted by it. P. 478.

Fig. 540. The elevation of mercury in contact with a horizontal surface of glass.
P. 478.

Fig. 541. A, a wide drop of water standing on a dry surface, not attracting it.
B, a wide drop of mercury, standing on glass. P. 478.

Fig. 542. A magnified representation of the manner in which the seeds of lico-
podium prevent a drop of water from wetting the substance on which it stands.
P. 478.

Fig. 543. The bodies A and B, and the bodies C and D, appear to attract, and E
and F to repel each other. P. 479.

Fig. 544. The apparent cohesion of two plates, between which a fluid is inter-
posed. P. 479.

Fig. 545. The apparent attraction of a drop between two plates, tending to draw
it towards the line of their junction, causes the drop to rest in an inclined position
of the plates. P. 479.

Fig. 546. Dr. Herschel's figure, representing by the distance of the curve ABC
from the line AC the heat thrown on different parts of AC by a prism, while DC is
the illuminated part, divided according to Newton's experiments, the quantity of
light being expressed by the distance of the line DEC. P. 490.

Fig. 547. Dr. Herschel's figure of the distribution of heat and light corrected
according to the division of the coloured spectrum, as ascertained by Dr. Wollaston.
P. 490.

Fig. 548. Bernoulli's air thermometer. P. 499.

Fig. 549. A differential air thermometer, or thermoscope, from which the pres-
sure of the atmosphere is excluded. From Kunze. P. 499.

Fig. 550. A differential thermometer on Mr. Leslie's construction. P. 499.

Fig. 551. The distribution of the electric fluid in spheres of different sizes, and at
different distances, and in a conical point. The density is represented by the dis-
tance of the dotted line from the surface. P. 51 1.

PLATE XL.

Fig. 552. A. A spark passing between a negative and a neutral ball; B, between
a neutral and a positive ball ; C, between a negative and a positive ball. D, two
sparks between a negative and a positive cylinder, each of the same form as if it
were passing singly from the end of a charged to the side of a neutral cylinder.
From Mr. Nicholson. P. 518.

Fig. 553. A compound galvanic circuit, formed by portions of an acid, pieces of
zinc, and wires of silver ; the arrows show the directions of the electric current.
P. 522.

Fig. 554. A compound galvanic circuit, formed by an acid, charcoal and water,
the water and acid communicating by a small siphon. P. 522.

Fig. 555. A compound galvanic circuit, formed by portions of an alkaline sul-
furet, and water, and pieces of copper : the liquids being connected by a siphon.
Fig. 522.

Fig. 556. A simple galvanic circuit, formed by wires of zinc and silver, or platina,
the lower ends being immersed in an acid, and the upper being brought into contact
at pleasure. P. 522.

Fig. 557. A galvanic battery, in the form of a trough, composed of plates of
zinc, silvered on one side, with vacant spaces for the reception of an acid : the letters
show the order of the elements, and the arrows the direction of the current, from the
positive wire + to the negative wire — . P. 523.

Fig. 558. An electrical machine, on Nairne's construction. A, the cylinder of
glass ; B, the cushion, or rubber ; C, the silk flap ; D, the negative conductor ; E,
the positive conductor ; F, a ball connected with the internal coating of a glass jar,
contained in the conductor. The conductors are insulated by varnished rods of
glass. P. 525.

e

xlii DESCRIPTION OF PLATES.

Fig. 559. A plate machine. A and B, the rubbers, which are usually double ;
C D, double flaps of oiled silk, for confining the electricity ; E, the conductor.
P. 525.

Fig. 560. An electrophorus. A, the cake of resin ; B, the plate of metal ; C,
the ball for taking the spark ; D, the handle of glass. P. 526.

Fig. 561 . A condenser, as arranged by Mr. Cavallo, under the name of a collector :
the middle plate is insulated : the two outward plates communicate with the earth ;
they stand near the first plate when the electricity is imparted to it, and are after-
wards removed by means of their hinges. P. 526.

Fig. 562. Mr. Cavallo's multiplier. ' The electricity being first communicated to
the insulated plate A, the moveable plate B is brought near it, while the wire C
touches the pin D so as to form a communication with the earth ; the plate B is
then made to communicate with E, which is insulated, and stands near the plate F,
which enables it to receive almost the whole of the electricity brought at each alter-
nation by B ; and when the plate F is removed from the neighbourhood of E, this
plate becomes strongly charged. P. 527.

Fig. 563. A revolving doubler, on the principle of Mr. Bennet's instrument.
The fixed and insulated plate A first receives the electricity, and when the moveable
plate B stands opposite to it, it receives by a wire from the stand of the instrument C
the opposite electricity ; when it is brought opposite to D, this plate is made to com-
municate with the stand by the wire E, and acquires a charge similar and nearly
equal to that of A. When B comes again to A, the wire F forming a communica-
tion between A and D, nearly the whole charge of both these plates is brought into
A, and B receives a charge almost twice as great as at first. P. 527.

Fig. 564. Mr. Coulomb's electrical balance. The needle A is made of silk,
covered with sealing wax ; it supports, at the end B, a ball of the pith of elder,
another similar ball being fixed at C ; the force of attraction or repulsion is ascer-
tained by the torsion of the wire A D, which is measured by a graduated circle E.
P. 528.

Fig. 565. Mr. Henry's quadrant electrometer; it is made of box wood, sup-
ported by metal : the ball is of cork, the graduated arc of ivory. P. 528.

Fig. 566. A, Mr. Bennet's gold leaf electrometer ; B, a piece of excited sealing
wax held over it, for distinguishing the electricity. Instead of the pieces of gold leaf
C, we may substitute Mr. Cavallo's pith balls D, or the straws E, employed by Volta.
P. 528.

Fig. 567. Mr. Lane's discharging electrometer. The distance of the balls A, B
is measured by the turns of the screw on the scale C ; and the parts of a turn are
ascertained by the graduated circle D. P. 528.

Fig. 568. A discharger for a battery. When the repulsion of the balls A, B,
becomes greater than the weight of a wire which passes through a perforation in the
balls, they separate, and the ball C, descending to D, forms a communication, which
completes the circuit, so that the shock passes through any substance placed at E.
P. 528.

PLATE XLI.

Fig. 569. The form of the curves which show the direction of the magnetic
needle, in consequence of the attraction and repulsion of two poles, situated at A and
B. They are found by drawing the lines A CD, BED, so that the sum or dif-
ference of the parts A C, B E, shall be always equal, A C E B being a semicircle ;
and the direction D F may be found by making A F to B F as the cube of A D to
that of B D. P. 534.

Fig. 570. The arrangement of iron filings in the neighbourhood of a magnet.
P. 534.

Fig. 571. The particle of iron A B, lying on a card nearly over the magnet C,
assumes, when the card is shaken, first the position D, then, falling to E and F,fe
left a little further from the magnet than at first. P. 534.

Fig. 572. An azimuth compass. The box is turned round, until the shadow of
the thread A B or A C falls on the line CD: the position of the needle is theft

DESCRIPTION OF PLATES. xliii

ascertained by that of the card E, which is fixed on it. The compass is kept always
in a horizontal position, by means of a double suspension on the gimbals E G.
Instead of this suspension, Mr. M'Culloch makes the bottom of the box in the form
of a hollow cone, resting on a point, and loaded with a weight, which brings the
centre of gravity below the point of support, as at H. P. 535.

Fig. 573. A dipping needle. The piece AB is brought into such a situation,
that the line drawn on it coincides with the middle of the vibrations of the needle.
The position of the needle may be changed, either by turning the stand half round,
or by turning the needle within the stand. P. 535.

Fig. 574 . . 576. The situations of the lines of equal declination in 1700, 1744,
and 1794, in the hemisphere, which is bisected by the meridian of London. The
first two from Mountaine's Tables,* the last from Churchman's Chart.f P. 536.

Fig. 577. The actual situations of the lines of equal dip. From Churchman's
Chart. P. 537.

Fig. 578. The lines of equal dip, calculated from the supposition of a small
magnet, situated at the centre of the earth, directed to a point in latitude 75° N. and
longitude 70° W. P. 537.

Fig. 579. A, Six's thermometer ; B, the wire with a fine spring, which serves as
an index. P. 545.

Fig. 580. Rutherford's double thermometer. P. 545.

Fig. 581. Deluc's whalebone hygrometer. A, the slip of whalebone ; B, a spiral
spring, serving to keep it stretched ; C, the index. P. 554.

PLATES XLII. XLIII.

Fig. 582. A chart of the world, on Mercator's projection, from Arrowsmith ;
with the dip and variation of the compass, principally from Churchman, for the year
794 ; and with the trade winds and monsoons. P. *437, 536.

* Ph. Tr. 1757, p. 329. Account of Methods, 4to, 1758.
t Churchman's Magnetic Atlas, 1794.

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RENEWALS: CALL (415) 642-3405

DUE AS STAMPED BELOW

JAN 3 01988

. NOV 2 4 2UU/

mm NOV D g

987

Air 13 1999

WJu *fl

APR o 8 2001

MAU A * MQl

HUT V9 *W*

•

MAR 2 2 2004

fAUG 1 5 2m

SEP 2 9 2005

UNIVERSITY OF CALIFORNIA, BERKELEY
FORM NO. DD6, 60m; 1 /83 BERKELEY, CA 94720

®s

"(H241SJ. v-

22701

U.C. BERKELEY LIBRARIES