383 MOTION AND RESISTANCR OF FLUIDS. 



affecting to form themselves into spheres. We must therefore ad- 

 mit in this case both powers, and that uhere one po\ver ends the 

 other begins, agreeably to Sir Isaac Newton's* idea of what take 5 ) 

 place, not only in respect to the constituent particles of bodies, but 

 to the bodies themselves, The inconipressibility of liquids (for I 

 know no decisive experiments which have proved them to 1)0 conu 

 pressible) seems most to favour the former supposition, unless we 

 admit, in the latter hypothesis, that the repulsive force is greater 

 than any human power which can be applied. The expansion of wa- 

 ter by heat, and the possibility of actually converting it into two 

 permanently elastic fluids, according to some late experiments, seem 

 to prove that a repulsive power exists between the particles ; for it is 

 bard to conceive that heat can actually create any such new powers, 

 or that it can of itself produce any such effects. All these uncer- 

 tainties respecting the constitution of fluids must render the con- 

 clusions deduced from any theory subject to considerable errors, 

 except that which is founded on such experiments as include in them 

 the consequences of all those principles which are liable to any de- 

 gree of uncertainty. 



A fluid being composed of an indefinite number of corpuscles, we 

 naust consider its action, either as the joint action of all the cor- 

 puscles, estimated as so many distinct bodies, or we must consider 

 the action of the whole as a mass, or as one body. In the former 

 case, the motion of the particles being subject to no regularity, or 

 at least to none that can be discovered by any experiments, it is 

 impossible from this consideration to compute the effects; for no 

 calculation of effects can be applied when produced by causes which 

 are subject to no law. And in the latter case, the effects of the 

 action of one body on another differ so much, in many respects, 

 from what would be its action as a solid body, that a computation of 

 its effects can by no means be deduced from the same principles. In 

 mechanics, no equilibrium can take place between two bodies of dif. 

 ferent weights, unless the lighter acts at some mechanical advantage ; 

 but in hydrostatics, a very small weight of fluid may, without ils 

 acting at any mechanical advantage whatever, be made to balance 

 a weight of any magnitude. In mechanics, bodies act only in t In- 

 direction of gravity ; but the property which fluids have of acting 



* See his Optics, Que. 31. Orig. 



