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 



