178 PRESSURE UPON THE BASE OF VESSELS. 



The pressure upon the bottom of vessels, as the pre- 

 ceding experiments have demonstrated, is independent 

 of the form of the vessels : it is solely dependent on 

 the area of the base, on the perpendicular height of 

 the surface of the liquid above the base, and also, as is 

 evident, on the nature of the liquid. The pressure is 

 always equal to the weight of a liquid column with 

 vertical sides standing upon a horizontal base, and it is 

 quite immaterial whether the actual weight of the 

 liquid is greater or less than such a column, as is 

 respectively the case in vessels wider or narrower at the 

 top than at the base. The pressure upon the base of 

 a vessel can consequently be easily calculated. The 

 volume of a straight column is found by multiplying the 

 area of the base into the vertical height. If the liquid 

 is water, the number of cubic centimetres in the volume 

 equals the number of grammes in the weight with which 

 the liquid presses upon the base, because l cc of water 

 weighs I* 1 . If the liquid is not water, its specific gravity 

 must be determined in order to know how much heavier 

 or lighter a unit-volume of it is than an equal volume of 

 wafer, and the pressure of the liquid is then found by 

 multiplying its specific gravity into the pressure which an 

 equal column of water would exert upon the same base. 



For example, let it be required to find the pressure 

 exerted by a solution of common salt, which has a specific 

 gravity of 1*2, and fills a rectangular vessel 10 cm long, 

 8 cm wide, and 6 cm high upon the base. The area of the 

 base is 10 x 8 = 80 square centimetres ; hence the 

 volume of the whole liquid 80 x 6 = 480 CC . If the 

 liquid were water/ the pressure would be 480^; but, -as 





