46 HYDROSTATICS AND HYDRAULICS. 



the tube the pressure will hecome so astonishingly great 

 as to rend the cask asunder, as if with gunpowder. The 

 pressure, notwithstanding the smallness of the diameter 

 of the tube, being equal to a body of water of the height 

 of the pipe, whose base is equal to the base of the cask. 

 Bramah's press is made on this principle ; and a column 

 of water of half an inch in diameter is made to produce 

 a pressure of several hundreds of tons. Various amus- 

 ing experiments have been invented to exemplify this 

 peculiar property of fluids, which are detailed in most 

 works expressly on this science. 



If a body swims in a fluid, it is known to displace as 

 much of that fluid as is equal to its weight ; if it sink or 

 be immersed in a fluid, it will displace as much as is 

 equal to its bulk ; if it be suspended in a fluid, it will 

 lose as much of what it weighed in air as is equal in 

 weight to its bulk of the fluid. On this latter axiom 

 depends that which is termed the Specific Gravity of 

 bodies, or the relative weight of equal bulks of different 

 bodies. Bodies are generally compared with water, and 

 their specific gravity is usually found by weighing them 

 in water. If a cubic inch, or cubic foot, for instance, of 

 any body, be twice the weight of a cubic inch or cubic 

 foot of water, its specific gravity is said to be two, or 

 twice that of water ; and this is found by weighing the 

 body first in air, in the common way, and then in water,* 

 and dividing the weight in air, by the loss of weight in 

 water. Thus, if a guinea be found to weigh 129 grains 

 in air, and on its being suspended in water it weighs 7i 

 grains lighter, it shows that a quantity of water of equal 

 bulk with the guinea weighs 7-^ grains : 129 being divided 

 by 7^-, the quotient will be 18 nearly, which is its Specific 

 Gravity. If a body be lighter than water, as wood, cork, 

 &c., it is first weighed in air, and then it is attached to 

 some heavier body that will cause it to sink in the water, 

 the weight of which body has been counterbalanced in 

 the opposite scale ; these being immersed together will 



* This is done by the Hydrostatic Balance, an instrument like a 

 common balance, only one of the scales has a hook underneath to 

 suspend a body so as to let it dip into a glass of water. 



