ON HTDROSTATICS. fS.6l 



them be horizontal; and this is in fact the state of the surface of common li- 

 quids, which is exposed to the pressure of the atmosphere. 



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

 lel 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 surface 

 of a fluid must always be perpendicular to the direction of the joint results 

 of all the forces which act on it; and since the earth turns round on its axis, 

 the centrifugal force resulting from its motion is combined with that of 

 gravity, in determining the position of the general surface of the ocean. 



A similar combination of a centrifugal force with gravitation may be ob- 

 served when a bucket is suspended by a rope, and caused to turn round on 

 its axis by twisting the rope: the direction of the joint forces is such that 

 the surface, iu order to be perpendicular to it, must assume a parabolic form. 

 When also any number of different fluids are made to revolve in the same 

 manner, or when they are inclosed in a glass globe and turned by means of 

 the whirling table, the surfaces which separate them acquire always the forms 

 of parabolic conoids, when the axis remains in a vertical position : but if the 

 axis be in any other position, the situation of the surface will be of more diffi- 

 cult determination. (Plate XX. Fig. 240.) 



In all these cases the equilibrium is stable ; for if any part of the fluid be 

 raised above the surface, it will immediately tend to return to its level. 

 But if a heavier fluid were contained in a bent tube or siphon, with its legs 

 or branches opening downwards, and immersed in a lighter fluid, the equili^ 

 brium would be tottering, since, if it were once disturbed, it would never be 

 restored. (Plate XIX. Fig 241.) 



From these principles, we may infer, that the pressure of a fluid on every 

 particle of the vessel containing it, or of any other surface, real or imaginaryf 

 in contact with it, is equal to the weight of a column of the fluid, of which 

 the base is equal to that particle, and the height to its depth below the sur- 

 foce of the fluid. Thus, if we have a vessel of water one foot deep, each 

 •quare foot of the bottom will sustain the pressure of a cubic foot of water, 



