ON HYDROSTATICS. 197 



no immediate tendency to expand when at liberty, is commonly considered 

 as a liquid : thus water, oil, and mercury, are liquids ; air and steam are 

 fluids, but not liquids. 



We shall for the present consider a liquid as without either compressi- 

 bility or expansibility : and we must neglect some other physical properties 

 essential to liquids, such as cohesion and capillary attraction ; although in 

 reality the partigles of liquids are found, by very nice experiments, to be 

 subject to the same laws of elasticity which we have already examined with 

 regard to solids, and are possessed also of cohesive powers, which essentially 

 distinguish them from elastic fluids, and which resist any force tending 

 directly to separate the particles from each other, while they admit any 

 lateral motion with perfect facility. In treating of hydrostatics, therefore, 

 we suppose the fluids concerned to be of uniform density throughout ; and 

 as far as elastic fluids agree with this description, they are subject to the 

 same laws with liquids ; on the other hand, all fluids, as far as they are 

 compressible, possess properties similar to those which will hereafter be ex- 

 amined, when we investigate the subject of pneumatic equilibrium. 



The first law of hydrostatics which arrests our attention, is this, that the 

 surface of every homogeneous gravitating fluid when at rest, is horizontal. 

 If any part of the surface were inclined to the horizon, the superficial par- 

 ticles would necessarily tend towards its lowest part, in the same manner 

 as if they moved without friction on the inclined surface of a solid. And 

 if any two portions of the surface of the fluid are separated, as in two 

 branches of a tube or pipe, however they may be situated, the fluid cannot 

 remain at rest unless the surfaces be in the same level plane : for if we 

 imagine such a tube, containing water, to be made of ice, and to be 

 immersed in a large reservoir of water, and then thawed, the water will 

 make a part of the general contents of the reservoir, and consequently will 

 remain at rest, if its surfaces are level with that of the reservoir : and it is 

 obvious that the tube has acquired no new power of supporting it from 

 being thawed ; consequently, the water would have remained in equili- 

 brium at the same height in the original state of the solid tube. The 

 experimental proof of this proposition is easy and obvious, and the property 

 affords one of the most usual modes of determining a horizontal surface. 

 But when we compare the heights of fluids occupying tubes of different 

 magnitudes, it is necessary, if the tubes are small, to apply a slight cor- 

 rection on account of the actions of the tubes on the fluids which they con- 

 tain, which are more apparent as their diameters are smaller. The same 

 cause produces also a curvature in each separate surface, which is always 

 visible at the point of contact with the tube or vessel. (Plate XIX. Fig. 

 239.) 



If several separate fluids of different kinds be contained in the same 

 vessel, they will never remain at rest unless all the surfaces intervening 

 between them be horizontal ; and this is in fact the state of the surface of 

 common liquids, which is exposed to the pressure of the atmosphere. 



The power of gravitation, strictly speaking, does not act precisely in 

 parallel lines, so that the surface of lakes, instead of being perfectly plane, 

 becomes, like that of the earth, a little convex. It is obvious that the sur- 



