'S AIR-PUMP. 



277 



pressure forces water from the reservoir through the valve 

 D into the hollow cylinder K, as in the common pump. 

 The cylinder C being pressed down, the valve D closes, and 

 the water is forced through the valve B into L, and, acting 

 on the cylinder A, makes it ascend, thus producing pressure 

 upon any substance included between M and N. A con- 

 tinued repetition of this process will produce any required 

 compression of the substance. 



Let R and r be the radii of the cylinders A and C, p the 

 power applied at the handle H, and P the pressure of the 

 water on A ; then we have, for the downward force p' on C, 



HF 



But (Art. 9) P:p' = 



By increasing the ratio of R to r, any amount of pressure 

 may be produced. Presses of this kind were employed in 

 lifting into its place the Britannia Bridge over the Menai 

 Straits, and for launching the Great Eastern. 



150. Hawksbee's Air-Pump.* B and B' are two cylin- 

 ders, in which pistons P and P', with valves V and V opening 

 upward, are worked by means of 

 a toothed wheel, the one ascend- 

 ing as the other descends. At the 

 lower extremity of the cylinders 

 there are valves v and v' opening 

 upwards, and communicating by 

 means of the pipe AC with the 

 receiver R, from which the air is 

 to be exhausted. Fi fl . 79 



* The air-pump was invented in 1660 by Otto von Guericke, Burgomaster of 

 Magdeburg. 



