CHAP, iv.] APPLICATIONS OF COMPRESSED AIR. 



81 



There is one of these railways carried under the Thames near 

 the Tower. 



It is only in very extensive and populous towns that an 

 underground network of pneumatic tubes can be established with 

 great advantage for the quick 

 distribution of parcels and tele- cr 



graphic or postal despatches. 



The pressure of water, used 

 to compress air, produces a 

 fountain on the surface of a 

 reservoir in the ingenious ap- 

 paratus known as Nero's foun- 

 tain, so called from the name 

 of a mathematician of the 

 Alexandrian school, to whom 

 the invention is attributed. 



A reservoir of water A com- 

 municates by a tube which 

 leads from the bottom with 

 the outer air ; it also com- 

 municates by a tube full of air 

 with 'a reservoir c partly filled 

 with water and surmounted by 

 a column of water a, b. Upon 

 the height of this column de-. 

 pends the pressure of the air 

 inclosed and compressed be- 

 tween A and c. This pressure 

 exerting itself at A on the 

 surface of the liquid of the 

 first reservoir, causes the water 

 to rise in the tube, and if the 

 height of this latter above the 

 level of the surface of the 

 water is less than the length a, b, the liquid will form a jet which 

 theoretically would be exactly equal to their difference. It would 

 rise to a' t if the line a, b', is taken equal to the height a, b ; but 

 the resistance to which the water is subjected in its movement in 



G 



FIG. 52 Nero's Fountain. 



