Professor Dean’s account of a cometarium. 345 
wheel B, it is carried slower than the wheel; but when the bar DE 
- is passing under the hollow arbor of the wheel A, the wire being 
shorter, the arm DE is carried faster than the wheel. The wheel B 
is set ina frame by itself, moveable on the centre C, so that it is not 
detached from the pinion, while the distance between the centres of 
the wheels, and of course the inequality of the totion may be chosen 
at pleasure. If a very great inéquality, like those of the comets, be 
desired, the claw FE, may be moved nearer its arbor, even so near as just 
to pass round the arbor in which the perpendicular part of the wire 
turns; but in that case the wire should pass through an eye which 
may turn in the bar DE, on account of the — nr of the two 
to each other, 
~ The avintaaies of this construction I conceive to be its being 
eapable of representing any degree of eccentricity, from that of Ve- 
nus to that of Mercury ; that toothed wheels are much more secure 
than banded ones ; and that circular ones are much more easily form- 
ed than elliptical ones. ae od Sores i hi : = of is, 
that it represents the € qui reatest, when the 
anomaly i is exactly 3or9 signs. But the principal use of all such in- 
struments, as orreries, globes, maps, &c. to me has been, to give a 
a general notion of the subject. A slight difference of degree cannot 
much affect our conceptions, and accurate measures of distance or quan- 
tity must be preserved in quite a different manner. If this subject 
require any illustration to youthful minds, (which I find clearly to be 
the case) this method of effecting it seems so simple and obvious that 
I can scarcely imagine it to have escaped those who have tortured 
vulgar fractions for ratios of planetary motions far more accurate than 
our conceptions can take cognizance of; and yet had it ever been pro- 
posed, I'sce not why it has not been as much noticed as “the more 
a though = more difficult one, introduced: tt fo the En, Brit. 
