PLA 



PLA 



turned rou^d, while its plane continues 

 oblique to the axis, g, this ring represents 

 the plane of the Moon's orbit, and is en- 

 graved with the different phases of the 

 Moon. The Moon is not fixed to the 

 sirm which turns it, but its stem slides up 

 and down in a short tube fixed to the 

 arm, and rests upon the ring, so as to de- 

 scribe a parallel plane to it. On the end 

 of the frame, K E, a pillar is erected, to 

 support a small semi-circular piece of 

 bruss, m, inclosing the Earth, and show- 

 ing the line of light and darkness. N is a 

 tube screwed fast to the board, B B, by 

 a fianch at the lower end; it fits tbe out- 

 side of the tube of the wheel, 60, beneath 

 the board, and thus steadies the whole 

 frame as it turns round ; upon this tube 

 long arms are fitted, carrying Mars, Ju- 

 piter, and the other superior planets'; 

 but as there is no wheel-work to turn 

 these, tliey are omitted in the plate. This 

 instrument is defective in not having the 

 diurnal motion of the Earth upon its axis 

 shown, and the rotation of the Moon's 

 Nnodes; there have been instruments 

 made, which show all these motions, and 

 those of the superior planets with their 

 satellites ; but they are so complicated, 

 that it would far exceed the limits of our 

 plates to describe them. 



The numbers of the teeth of the wheels 

 of this planetarium are not correctly 

 calculated to produce true revolutions 

 -of the planets introduced in it, as the fix- 

 ed wheel, 64, and the wheel 64, on the 

 axis, h, are equal ; the latter, and all the 

 wheels on h h, revolve once in a tropical 

 year; the wheels which turn Mercury 

 are 60, upon b, turning 14, that is * 4 O f 

 a tropical year, or 85.223185 days; this 

 period, which is intended to be the tro- 

 pical revolution of Mercury, viz. 87 d. 

 23/j. 14m. 35s. is made the synodical re- 

 volution by the mechanism, by reason of 

 the wheel-work being carried round the 

 Sun again in a year, by the frame, E, re- 

 presenting the Earth's radius vector ; so 

 that the planet Mercury goes from con- 

 junction with the -Earth to conjunction 

 again, instead of going through the eclip- 

 tic only in this period, and the imperfec- 

 tion of the wheel-work is rendered still 

 more imperfect by its position, which 

 ought to have been on a stationary bar, 

 to have produced the true calculated ef- 

 fect ; this error causes it to make just one 

 revolution in a year more than intended. 

 The tropical period of Venus is also turn- 

 ed into a synodic one, by the same fault 

 in the position of the wheel-work ; be- 

 sides, the period itself being erroneous, 



viz. 2 4 of a year, according to the origi- 

 nal intention, which time is only 219J. 

 3h. 29m. 19.8s., instead of 224d. 16A. 41m. 

 30s., which is the true tropical period. 



The Moon-wheels, 6 , making 12i 

 lunations, or synodic revolutions, give 

 one lunation at 29d. I4h. 44m. 29.8s., 

 which is greatly too long ; the true pe- 

 riod being 29cl. 12h. 44m. 3s.; but ^, 

 making 12| lunations, or one in 29d. I2h. 

 20/?i. 54s., would be much more accurate, 

 and equally well made. Thus the instru- 

 ment before us is so very inaccurate, in 

 all respects, that it ought to have its 

 numbers rectified, which may be done in 

 this manner. 



For Mercury, instead of ?.4, (or .1-*)^ 

 put |_o, in which case the wheel, 63, will 

 produce 3^ revolutions; and the Earth's 

 arm will carry the 20 round oval in a 

 year, making together, 4.15 revolutions 

 of Mercury for one of the Earth's, 

 which is very near the truth, producing 

 one tropical revolution in 88d. Oh. 14m. 

 38s. 



For Venus, instead of 34, put .3 a , and 

 one revolution per annum will be pro- 

 duced by the motion of the Earth's arm, 

 aud |o of another by the wheels, mak- 

 ing, together, 1.625 in each year, or oue 

 tropical revolution in 224d. 18/i. 21w. 

 37>. 



The tfue synodic periods are : of 

 Mercury, 115.877d., and of Venus, 

 583.923d. ; therefore the said periods, by 

 the present wheel-work, are too short by 

 more than thirty days in Mercury, and in 

 Venus, by 3644., and upwards. 



PLANIMETRY, that part of geometry 

 which Considers lines and plain figures, 

 without considering their height orciepth. 

 See SuavKYiifG, &c. 



PLANISPHERE, signifies a projection 

 of the sphere, and its various circles on a 

 plane ; in which sense maps, wherein are 

 exhibited the meridians, and other circles 

 of the sphere, are planispheres. See MAP,. 

 SPHKRE, &,c. 



PLANISPHERE, is more particularly used: 

 for an astronomical instrument used in 

 observing the motions of the heavenly 

 bodies. It consists of a projection of the 

 celestial sphere upon a plane, represent- 

 ing the stars, constellations, &c. in their 

 proper order; some being projected on 

 the meridian, and others on the equator. 

 The use of the planisphere is to repre- 

 sent the face of the heavens for any day 

 and hour : find, on the lesser moveable 



