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Chap. 4. PHILOSOPHY. 149

that is, the refiftance will be in a two-fold or duplicate propor-tion of the velocity, wherewith the body moves through thefluid.

10. Farther it is moft manifeft, that this latter kindof refiftance increafing with the increafe of velocity, even >in a greater degree than the velocity it felf increales, thefwifter the body moves, the lefs proportion the other fpeciesof refiftance will bear to this: nay that this part of the refift-ance may be fo much augmented by a due increafe of velo-city, till the former refiftances fhall bear a lefs proportion to*this, than any that might be afligned. And indeed expe-rience fhews, that no other refiftance, than what arifes from-the power of inactivity in the parts of the fluid, is of mo-ment, when the body moves with any confiderable fwiftnefs.

it. There is befides thefe yet another fpecies of refift-ance, found only in fuch fluids, as, like our air, are elaftic..Elafticity belongs to .no fluid known to us befide the air. Bythis property any quantity of air may be contracted into alefs fpace by a forcible preflure, and as foon as the com-prefling power is removed, it will fpring out again to itsformer dimenfions. The air we breath is held to its prefentdenfity by the weight of the air above us. And as this in-cumbent weight, by the motion of the winds, or other cauf-es, is frequently varied (which appears by the barometer;)fo when this weight is greateft, we breath a more denfe airthan at other times. To what degree the air would expandit felf by its fpring, if all preflure were removed, is not

known,