190 



PHYSIOLOGICAL PHYSICS. [Chap. xix. 



in Relation to Ex- 

 tent of Surface. 



force, the force will act upon p' and p" to push them 

 out. The area of P' (p'q) being the same as that of p, 



the force exerted upon it will be 

 the same in amount as that acting 

 on P, but ab being twice the area 

 of P, the force acting on p" will 

 be doubled. Thus the amount of 

 the force exerted upon any surface 

 in a liquid under pressure is 

 proportional to its extent. 



The hydraulic press, made 

 by Bramah in 1796, embodies these 

 principles. Two cylinders of different diameter com- 

 municate by means of a transverse tube. In each 

 cylinder is fitted a piston. Suppose the large cylin- 

 der to be twenty times the diameter of the small, 

 then a weight of 1 pound on the small piston will 

 require a weight of 20 pounds on the large piston 

 to maintain equilibrium ; and if less than twenty 

 pounds be placed on the large piston it will be moved 

 upwards. Thus a small weight at one side of the 

 arrangement is capable of lifting a large weight at the 

 other. It is easily seen, however, that what is gained 

 by this arrangement in amount of force is lost in 

 extent of movement, for if the small piston be moved 

 downward for a distance of one foot the large piston 

 will be moved upward only the ^ of a foot. The 

 Bramah press consists of a small pump which forces 

 water through a pipe into a large cylinder in which is 

 fitted a large piston. The water forces up the piston, 

 which carries a cast-iron plate. Goods may be laid 

 on this plate, and by the upward movement of the 

 piston they are pressed against a second plate fixed 

 above. If the large piston be fifty or a hundred 

 times the cross section of that of the pump, then the 

 force of the small piston is increased fifty or a hundred 

 times, and is further multiplied by the pump being 



