

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 



