On Central-Station Power Supply. 475 
pressure or the velocity, or both, of the falling water. If 
coal is used, steam-engines and boilers give out the necessary 
mechanical work. In either case the energy is ready to 
be taken from a rotating shaft, which, if it is to be applied 
so as to compress air, may rot turn more than about eighty 
times a minute. This speed is for turbines a somewhat slow 
one. The slower the speed, the larger must be the turbine 
to develope a given power. This is a disadvantage, as it 
largely increases the first cost. 
Air-compressor.-—A machine having organs almost iden- 
tical with those of a steam-engine is used to transform the 
energy obtained from the above-mentioned rotating-shaft 
into that of a potential kind possessed by compressed air. 
Such an air-compressoz, if large, will consist of three cylin- 
ders and two intermediate vessels or receivers. These three 
cylinders are of different sizes. When the piston in the 
largest of the three moves in one direction it draws, by the 
ordinary principles of suction, a cylinder-full of air from the 
atmosphere. During the return stroke the air is compressed, 
by the closing of the inlet valve, into a smaller volume, and 
its pressure is correspondingly raised. When the pressure 
rises to that of the first of the two receivers, the outlet 
valve opens and the remainder of the stroke of the piston is 
- devoted to delivering this air into it. The second cylinder, 
whizh is smaller, acts in precisely the same way, except 
that it draws air from the first receiver instead of the atmos- 
phere, and delivers it into the second receiver at a still 
higher pressure. The third and smallest of the three cylin-. 
ders derives its supply of air at a high pressure from the 
second receiver, and finally delivers it in a still more com- 
pressed state into the storage reservoirs or transmission 
mains at the required high pressure suitable for conveyance 
to a distance. 
The two intermediate receivers have another and most 
important function to fulfil besides that of mere receptacles. 
In the act of compression, heat is very apt to be generated 
in the air; and the receivers are intended to act also as 
