791 



PLATE XXXIV. 



Fig. 486. Tlic gravitating body ABC, being sup- 

 posed to revolve on the axis A C, the fluid column 

 B D must be longer than ED, in order to support its 

 pressure. P. 510. 



Fig. 487. If A represent the place of the sun, B 

 that of the earth, and C that of the moon, taking A 1) 

 to A C as tlie square of A C is to the square of A B, 

 AD will represent the sunV attraction acting on the 

 earth, and CD the disturbing force, wliich, together 

 with AD, makes up AC, the force actin;; 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, bclwcen B and A, while t!ie moon 

 moves from G to H, the force CD will tend to lesson 

 the inclination, while the moon is ascending from E 

 towards C, and to cause the node E to move back to- 

 wards G, and, when it is again de-cending towards 

 F, the inclination will he increased, and the node F 

 made to recede towards 11, until the nioon arrives at 

 II, and the force becomes directed to a point on the 

 other side of B; the nodes only advancing while the 

 moon is between II and F, or between G and E. P. 

 520. 



Fip:. 488. A body attracted towards the centre A, 

 and descending from B in the ellipsis BCD, has the 

 inclination of its orbit to tlie revolving radius A B, A C, 

 AD, perpetually changed, until at D it becomes per- 

 pendicular 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 

 AD moves forwards to E. P. 521. 



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 win- 

 ter solstice: EQ denotes the equator, N the north 

 pole, and S the south. P. 525. 



Fig. 400. NESW being the horizon, and Z the 

 7cnith, F' A W shows the sun's apparent path in Lon- 

 don at the time of the equinoxes, BCD at midsummer, 

 and F G II 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° be- 

 low the horizon, and its intersections with the sun's 

 path show the beginning and end of twihght, af at I 

 andK. P. 627. 



Fig. 491. The rays of light, coming in the direction 

 AB, arc bent V)y the atmosphere so as to arrive at C^ 

 and to illuminate a part of the atmosphere there, 

 which is visible, by mean.s of a second retraction, to a 

 upectator at D, and occasions the first and last twi- 

 gbt. • P. 527. 



Fig. 492. Venus is at her greatest elongation or 

 angular distance from the sun A, when situated as at 

 B, with respei t 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 dhection of the earth's mo- 

 tion, the line E D being then more oblique, with respect 

 to a fixed line, than either before or after. P. 627. 



J'ig. 493. A BCD is the apparent path of Venusfor 

 the year 1806, supposing the sun E to revolve round 

 the earth F. "rhe place of the sun and planet is mark- ' 

 cd for every four weeks. P. 527. 



Fig. 494. The apparent path of Saturn in the hea- 

 vens for the year 1806, referred to its proper place 

 with respect to tbe eclijitic. The figures denote the 

 places at the beginning of eacii month. P. 527. 



Fig. 495. I he small figures represent the phases of 

 the moon in different parts of her orbit. The smaller 

 detached fii'inf s 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 some- 

 times called the syzygies, and B and D the quadra- 

 tures. P. 528. 



Fig. 496. A, the moon passing through the earth's 

 sliadow ; which is distinguished 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. 529. 



F'ig. 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 ihe eclipse ap- 

 peared annular, CD being the breadth of the whole 

 shadow or penumbra. P. 529. 



Fig. 498. The shadow of the moon falling on the 

 earth. The true shadow not extending here ta the 

 earth, the cone formed by tUe continuation of its out- 

 lines marks the extent of the parts in which the eclipse 

 appears annular. P. 529. 



Fig. 499. The termination of the moon's disc in a 

 solar eclipse. From Dr. Ilerschel. P. .')29. 



Fig. 500. The apparent mag^iiludes of the planets, 

 that of the sun or moon being supposed equal to a 

 circle a foot in diameter; whtrr there are two figures, 

 one of them shows the mean apparent niagnilude, and' 

 the other the greatest. P. 531. 



F'ig. 501. "Vhe apparent n ngnitude of tlie sun, as 

 seen from the different planets ; for Mercury, the mag- 

 nitude is shown by that of the tarth in fig. ')07. P. 

 535. 



