154 Sir Isaac Newton’s BookL

deles of the fluid are endued with perfect elasticity; so thatas the globe impinges upon the particles of it, they shallbound off and separate themselves from the globe, with thefame velocity, with which the globe strikes upon them; thenthe resistance, which the globe moving with any known ve-locity suffers, is to be thus determined. From the velocityof the globe, the time, wherein it would move over twothird parts of its own diameter with that velocity, will beknown. And such proportion as the density of the fluid bearsto the density of the globe, the fame the resistance given tothe globe will bear to the force, which acting, like the powerof gravity, on the globe without intermission during the spaceof time now mentioned, would generate in the globe thefame degre of motion, as that wherewith it moves in thefluid a . But if neither the globe nor the particles of thefluid be elastic, so that the particles, when the globestrikes against them, do not rebound from it, then theresistance will be but half so much b . Again, if the par-ticles of die fluid and the globe are imperfectly elastic, sothat the particles will spring from the globe with part onlyos that velocity wherewith the globe impinges upon them;then the resistance will be a mean between the two preced-ing cafes, approaching nearer to the first or second, accor-ding as the elasticity is more or less c .

17. The elasticity, which is here ascribed to the parti-cles of the fluid, is not that power of repelling one another,

1 Princ.philos. Lib.II. Prop. I c Id.

■> Ibid. |

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