VOL. LXXXV.] PHILOSOPHICAL TRANSACTIONS. 46? 



bility of liquids (for I know no decisive experiments which have proved them to 

 be compressible) 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 water 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 hard to conceive that heat can actually create any such new powers, or 

 that it can of itself produce any such effects. All these uncertainties respecting 

 the constitution of fluids must render the conclusions 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 

 degree of uncertainty. 



A fluid being composed of an indefinite number of corpuscles, we must con- 

 sider its action, either as the joint action of all the corpuscles, 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 2 bodies of different 

 weights, unless the lighter acts at some mechanical advantage ; but in hydro- 

 statics, a very small weight of fluid may, without its acting at any mechanical 

 advantage whatever, be made to balance a weight of any magnitude. In me- 

 chanics, bodies act only in the direction of gravity ; but the property which fluids 

 have of acting equally in all directions, produces effects of such an extraordinary 

 nature as to surpass the power of investigation. The indefinitely small corpus- 

 cles of which a fluid is composed, probably possess the same powers, and would 

 be subject to the same laws of motion, as bodies of finite magnitude, could any 

 2 of them act on each other by contact ; but this is a circumstance which cer- 

 tainly never takes place in any of the aerial fluids, and probably not in any 

 liquids. Under the circumstances therefore, of an indefinite number of bodies 

 acting on each other by repulsive powers, or by absolute contact, under the un- 

 certainty of the friction which may take place, and of what variation of effects 

 may be produced under different degrees of compression, it is no wonder that 

 our theory and experiments should be so often found to disagree. 



Sir Isaac Newton seems to have been well aware of all these difficulties, and 

 therefore in his Principia he has deduced his laws of resistance, and the princi- 



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