Hydrostatics.

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Force of an equal Bulk of the Fluid, whichtherefore will destroy just so much of theGravity of the Solid; and consequently,the Residue or Excess of Gravity in the So-lid is that alone by which it must descend(LX).

From

(LX) 1. As those Bodies swim which are specifi-cally lighter than Water, as above explained ; so otherssink or descend in a Fluid by being specifically heavier,that is, by their relative Gravity. Thus, if the speci-fic Gravity of the Solid be to that of the Fluid as 3 to1, then 3 — 1 ~ 2 is the relative Gravity by which itdescends. If the Specific Gravities are as 7 to x, then7—x—6 is the relative Gravity. Whence we observe,

2. That the Descent of Solids in a Fluid Medium isthe very same with that of Bodies descending on an in-clined Plane, because in both Cases the absolute Gra-vity is only diminished, by the Resistance of the Medi-um in one Case, and by that of the Plane in the other ;and therefore all the Properties of the Motion of a Bo-dy falling freely, belong to this Motion thro’ a Mediumlikewise.

3. This relative Gravity of Solids, by which they pi. XIV.sink or swim, is usually illustrated by the Descent and jrjAscent of Glass Images and Bubbles included in a Jar

of Water cover’d over with a Bladder, so as to includea small Quantity of Air between the Bladder and Water.

The Images, &c. have small Holes in the Bottoms oftheir Feet, thro’ which some Water is put into theirBodies, and that in such Quantities as will renderthem but very little specifically lighter than Water,but some more so than others, that they may not beginto move all together.

4. The Images being thus put to float in Water, andthe Bladder tied down, if the Hand be laid on theBladder, and gently compresses the Air beneath, theAir by its Spring will act upon the Water, andcause it to compress the Air in the Bodies of the Images,by which Means more Water will be driven into their

T 3 Bodies;