296 Hydrostatics. 



by the column NHGQ upon FL, should be equal to the down- 

 ward pressure exerted upon FL by the column EFLM. Now 

 the pressure of NHGO upon FL is equal to the weight of a 

 prism or cylinder of this fluid which has the surface FL for its 

 base and IK for its altitude; moreover this weight is equal 

 to the specific gravity multiplied by the bulk; accordingly, if 

 we call the specific gravity S, we shall have for the expression 

 of the weight S x IK X FL. For the same reason, if we call 

 S f , the specific gravity of the fluid EFLM, we shall have 



S' X EF X FL, 



as the expression for the absolute gravity of this fluid or the. 

 pressure which it exerts upon FL. Therefore 



S X IK X FL = S' X EF X FL, 

 or 



S X IK=& X EF-, 



whence 



S : S' :: EF : IK, 



that is, the altitudes are inversely as the specific gravities. Thus 

 if LFBCHN were mercury, and EFLM water ; since mercury 

 is 13,6 or nearly 14 times as heavy as water, the altitude IK 

 would be one fourteenth part of EF, whatever be the figure of 

 the vessel. 



412. From what has been said, it will be seen that the action 

 of fluids is very different from that of solids. Properly speaking, 

 it is only the part ECDG which exerts its action upon the surface 

 CD ; and in figure 1 95, the surface CD is pressed by ACDF, 

 as it would be by the weight of the fluid contained in the cylin- 

 der ECDG. If, on the contrary, the fluid ACDF were sudden- 

 ly to become a solid, by freezing, the bottom would support a 

 pressure equal to the weight of the entire mass ACDF in figure 

 194, and only equal to the weight of ACDF in figure 195. 



413. It is necessary here to distinguish between the force or 

 pressure exerted on the bottom CD, and that which would be 

 sustained by a person carrying the vessel. It is clear that if the 

 bottom CD were moveable, the only thing necessary to keep it 

 in its place, would be an effort equal to the weight of the cylin- 



