792 



PLATE XXXV. 



Fig. 50'.!. AB btin s; the eRrth'3 axis, ll>c circle 

 A I'lj is tlie moridian o I tlu- pWe C,.aiid C E repre- 

 siiitii the [ilanc of its hori/oii. P. 537. 



rig. 60:5. Tlie (tVtet ol tlu- obliqiiity of the ecliptic 

 ii.i llie equation of time i.s lovvii by tlie tlitTeniicc of 

 tilt unalch ABC and D BE, ^ubiended at tiie. jiole B 

 bj equal portions of the oblique circle A 1'.. P. 5;5i>. 

 riif. .504. A Hbciiiii paiaUfl to llie earlii's axis, the 

 12 pUiues passiiijitbrnugli it, at cqir.il angular distances, 

 mark, on the circle CD perpendicular to it, the liour . 

 lines of an equatoiiai dial, and on the liorizoiitid .sur- 

 face P. V those of a tiori/outal dial. P. o38. 



Pig. 605. A method of coiistnictiug a dial on any 

 given plane. .'V li C is the eleviaion of the pole, or 

 more generally, the angle which the surface makes 

 with tlie i^nomon A B. J'he circles are divided into 

 equal parts, and 1,2,0,1,6,0 are the hour lines, B 

 tieiiig the place of the t;uonion. The reason of this 

 construction will appear by comparing the circle 111 

 the last figure with the ellipsis which is formed on the 

 horizontal surface. P. ^iliii. 



Pig. 50(). A dial for a pointed gnomon, or obcliic, 

 drawn on a liorizontal surface. P. 538. 



Fig. 507. A mural quadrant, with its telescope; 

 A P> is the plumb line, for adjusting the instrument, 

 and C the counterpoise for the telescope. P. 542. 



I'ig. 503. 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, w hich may 

 occasionally he 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. 542. 



Fig. 509. A transit circle, resembling Mr. Wollas- 

 ton's, with a horizontal circle, by means of which 

 both altitudes and azimuths may be measured. A is 

 a. microscope for viewing the plumb line, B anotlier 

 for reading ojT the divisions of tl>e horizonlid circle; 

 C and D are spirit levels. P. 642. 



Fig. 510. A zenith sector, with its telescope, which 

 has usually a reflecting prism, like that of the Nev^- 

 tonian telescope, for its eyeglass. P. 542. 



Pig. 511. The marine octant, introduced by lladley. 

 The mode of taking tlie common or front observation, 

 is shown by the lines drawn to the sun and moon : the 

 fcack observation by the two stars. A is a dark glass 



to be used in observations of the sun, and.wluch may 

 be fixed at B, when required. P. 5 12. 



Fig. 512. A B being the situation of the earth's ax- 

 is, if the angle C B U, or the altitude of the body D, be 

 measured, and wt subtract from it tlie elevation of the 

 equinoctial CHE, the remainder will be the decliua- 

 tion EBl). I'.aU, 543. 



Fig. 513. The aiigle ABC is the moon's horizon- 

 tal parallax, and DBC the parallax when she is ele- 

 vated above the horizon D E in the angle BDE. P» 

 61,"). 



Fi:;. 514. The situation of the earth at the transit of 

 Venus in June 17d9. A spectator at the North Cape 

 was carried during the trimsit from A to B,and the tran- 

 sit apjieared to liim to last while Venus moved from 

 C to 1) : tlie island of Otaheite, on the contrary, 

 wliidi 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 il. Hence tiie rotatory motion of the earth was 

 compared with the excess of the motion of Venus in 

 its orbit above that of the earth. P. 514.. 



I'ig. 515. A planisphere ncaijly resembling tliat of 

 Professor Bode. The outer circle i.s fixed to the chart, 

 and is divided either according to tiie degrees of the 

 ecliptic, or the dajs of the month; the graduated cir- 

 cle immediately within it is divided into 2 li hours, and 

 is fixed to a circle of pasteboard, out of which the 

 circle NF^SW, representing the horizon, is cut, the 

 place being filled by thin varnished paper, with circles 

 of azimuth and altitude engraved on it, which is car- 

 ried round with the hour circle. P. "jtiT. 



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 

 yoar being found in the graduated circle representing 

 portions of the ecliptic, and tlie latitude, on tjip mid- 

 dle line, by following the concentric circles of decli- 

 nation till they meet tiie horizon passing through the 

 given latitude, the line drawn from the pole ttiniugh 

 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 liours andJ, whence tiie sun 

 must rise about 37 minutes after six ; but in latitude 

 So'the sun never se ts on that day. P. 567. ,^ | 



PLATES XXXVr, XXXVIL 



Plate XXXVI. Fig. 517. Projection of the con- 

 stellations of the nortliern hemisphere ou the plane of 

 'fie equator. P. 498, 567. 



Plate XXXVn. Fi^-. 518. Projection of the south 

 ern hemisphere. P. 498, 567. 



