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 A, revolve once in a tropical year ; the wheels which turn Mercury are 60, upon 4, turning 14, that is 14 of a tropical year, or 85.223185 days ; this period, which is intended to be the tro pical revolution of Mercury, 'viz. 87d. 23h. 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 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 of ; 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. ao of a year, according to the origi nal intention, which time is only 219d.
311. 29m. 19.8s., instead of 224d. 161i. 41m. 30s., which is the true tropical period. The Moon-wheels, making 121 lunations, or synodic revolutions, give , one lunation at 29d. 14h. 44m. 29.8a., which is greatly too long ; the true pe riod being 29d. 12h. 44m. 3s. ; but , making 124 lunations, or one in 29d. 12h. 20m. 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 manlier.
For Mercury, instead of (or .4), put VI, in which case the wheel, 63, will produce 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 put and one revolution per annum will be pro- • , duced by the motion of the Earth's arm, .
and 0 of another by the wheels, mak- 2 ing, together, 1.625 in each year, or one tropical revolution in 224d. 18h. 21m. 27e.
The true 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 364d., and upwards.