344 
XLVIUI. 
DESCRIPTION OF A COMETARIUM. 
BY JAMES DEAN, a.™. 4.4.5. 
Prof. Math. and Nat. Phil. in the University of Vermont. 
Communicated in a letter to Nathaniel Bowditch, A.M. As Ae 8. 
ean ai>—— : 
I HAVE been for some months excessively engaged i ina course 
of experimental lectures, during which I have constucted an instru- 
ment to represent the unequal motion of the planets round the sun, 
and the equation of the centre. As I conceive it equally useful with 
Ferguson’s cometarium, a description of which the compilers of the 
Encyclopzdia have thought proper to introduce, and much more con- 
"venient and easily constructed, I take the liberty to obtrude a short 
description of it on your patience. I shall refer to those expedients 
only by which the unequal motion is effected, as the rest may be sup- 
plied according to any one’s peculiar taste in mechanics. A and B 
(Pl. II. Fig. 6.) are two toothed wheels of the same diameter, turned 
uniformly, and in the same time, by two pinions on the same arbor C- 
The wheel A, turned by the upper pinion, carries a hollow arbor, 
which rising through the dial plate carries an arm with an arch of 60°, 
on which the equation of the centre may be reckoned. The wheel B 
under the other carries the flat iron bar, DE sliding through a slit in the 
upper end of its arbor. In the end E of this bar, which is bent up- 
wards, is a triangular notch, in which rests the tail or horizontal part 
EF, of a thick iron wire bent at a right angle, and turning in the hol- 
low arbor. This carries on its top, above the arbor, a slender rod re- 
presenting the vector radius, and moving with any desired degree of 
inequality. For when the wire FE is passing over the arbor of the 
