512 PHILOSOPHICAL TRANSACTIONS. [aNNO 1741, 



the axes being fixed into the columns, so that the balls are held fast in a posi- 

 tion parallel to the horizon of the globe. 



The upper parts of the axes are round and polished, as well as the upper sur- 

 faces of the balls ; and they receive round plates of brass, efg, efg, resting on 

 the balls in such a manner, that, being turned on the axes, they always remain 

 parallel to the plane of the horizon. The plates are about 3 inches diameter, 

 and have each a notch in the circumference, to receive a thread, but the plate 

 efg a little less than the other plate efg ; and this latter has a circle inscribed 

 on it, divided into degrees, with an index h, to point to any part. 



There are 3 brazen rays, ik, il, im, connected at i, containmg equal angles, 

 kil, lim, mik ; the place i being perforated with a very small hole. The rays 

 are elastic, and very thin, and in length nearly equal to a 4th part of the dia- 

 meter of the globe. They have also small perforations at 1 and m, through 

 which a thread being drawn, is brought round the plates in the direction 

 mEFGgfel, the ends being fastened together between 1 and m : therefore the 

 skeleton of the penumbra is also rendered immoveable at the part of the thread 

 elmE. Hence the skeleton is turned either way, in a right line, by turning the 

 plate EFG or efg. 



The following circumstances must be changed according to each particular 

 eclipse. The chief of these is the disk of the penumbra With regard to this, 

 having found, by the tables of the eclipse to be represented, the semidiameter 

 of the lunar penumbra on the earth's disk, as also the moon's horizontal paral- 

 lax, it is, as the moon's horizontal parallax, is to the radius of the disk of the 

 penumbra, so is the semidiameter of the terrestrial globe used, to the quadrant, 

 which expressed the radius of the penumbra by the magnitude of the globe. 

 ' To set every thing in order for any moment of a given eclipse, we must pro- 

 ceed in the following manner. Having found by calculation the points of the 

 bounds of light and shade, by which the moon's centre first enters the earth's 

 disk, and departs from it again, they are to be marked on the horizon of the 

 globe, and the arms ab, ab, are to be placed so, that the plate efg being turned 

 round, the centre i of the disk of the penumbra klm, may pass over them ; 

 which when done, will be shown by the pendulum in. Then find the time 

 when the centre of the penumbra is in any remarkable place, as when it first 

 enters the earth's disk, and place the globe, by means of the meridian and 

 equator, in such a manner, that the part above the horizon may show the 

 earth's hemisphere, at that time illuminated by the sun. Then turn the plate 

 EFG till the centre of the penumbra, i, is over that particular place. This 

 done, move the index h of the plate to the beginning of the division. Thus 



