——

Book IV. Mechanic Powers. 85

wonted rectitude, or to hinder ſuch bending or crookedneſs;therefore when a weight is taken up in this manner, oft-timesthe endeavour is in the knees and the whole Body; or theweight to be raiſed is placed in an upper place, and then inlike manner we muſt have recourſe to the force of the Nerves,but when we uſe a Pully hung in an upper place, we may ap-ply the weight of our body, beſides this kind of force of theNerves, and oft- times without any endeavour of the Nerves,therefore tis not ſo troubleſome.

———————

PRO O. III.A Theorem.Large Pullies are moſt Aſeful.

Fig. 71. Lthough. as J have ſhewy in the firſt Propoſition,.

that one Pully whoſe Snatch-block is immova-ble, neither increaſes nor diminiſhes the force of the power;nevertheleſs we have ſaid alſo, it adds to the facility, becauſethat it ſaves the wearing, or rubbing of the Rope, but there isfound ſome, although but little reſiſtance, in the Axis ofthe Pully, which ought to be turn d about; I ſay, this diffcul-ty will be leſſencd, by how much the Orb of the Pully is grea-ter, for than the Scmidiameter of the Pully, or Rundle, is as.it were a Leaver, and the Semidiameter of the Axis as it werea Clinder in a wheel, or Peritrechie; and by how much greaterthe proportion of the handle is to the Clinder, ſo much thecaſier is che motion; as becauſe there is greater proportion ofthe line G D, to the radius of the Axis, than of the line G B,the reſiſtance is eaſier overcome, which is made in the circum-ference of the Axis, while the Pully is turned about it.

There is alſo another conveniency in large pullies, to wit,that the Rope is not ſo often folded, ard conſequently not ſo-much rubbed, or worn; but J ſuppoſe alwaies Pullys to be-moveable about their Axes, for if chey ſhould not, we muſt:reaſon other wiſe. ö

P R. O-