Mechanics.

2o6

In WATF.R-MILLS, the Momentum ofthe falling Water is the Power ; the Forceto be overcome is the great Attrition ofthe two Stones in grinding the Corn, &c.which is effected wholly by a Complica-tion of Wheels and Axles. A Query mayhere be put, Why, since the Power con-stantly acts upon the Wheel, the Motionof the Wheel should be equable, and notaccelerated : The Answer is, The Incre-ments of Velocity keep rising, till theirMomentum is equall’d by the Resistance of

the

suppose the Sun to be, and is accordingly representedby a silvered Plate at Top.

5. The Place of the Comet at P is called the Peri-helion^ as being there nearest the Sun ; as I is its Aphe-lion or Point (>}' great ejl Dijlance. Since S P— S K, theVelocity of the Comet will be in the Point P equal tothat of the Point K ; and were the Comet’s Aphelionat r, its Velocity there would be equal to that of thePoint V when under s, viz. six Times less than before;but since the Cornet’s Aphelion is at I, and since thegreater Arch described in the fame Time must havea greater Velocity, the Velocity of the Comet at I willte about -J- of that at P.

6. The silvered Ellipsis on the Lid of the Box is di-vided into ioo Parts, shewing the Anomaly of the Co-met; and about the Axis of the Wheel G is placed aSilver Circle, divided into.75s equal Parts, represent-ing the Years or Period of the Comet, with a properIndex, pointing to those Divisions, by which Meansit is easy to shew the several Particulars relating to theTheory of elliptic Motions, whether of a Planet orComet,