On Central- Station Power Supply. ATT 
which has a large pneumatic system of power distribution, 
the loss due to leakage was less than half a pound per 
square inch of the working pressure in one mile of main ; 
and the joints which produced these good results have been 
found satisfactory in every other respect. The loss of 
power due to friction in the pipes proved also unexpectedly 
small, as was shown in the experiments of Professors Riedler 
and Gutermuth. . 
Satisfactory results can certainly be obtained when the 
distance of transmission is as great as fifteen or even twenty 
miles. 
Distributing Pipes.—The same remarks regarding tight- 
ness and efficiency, when transmitting a supply of air under 
pressure, apply to the distribution pipes. And it should be 
further mentioned that the depth at which such pipes need 
be laid is much smaller than is required for water or gas 
mains. This is a very obvious advantage, especially in 
large cities. 
Motors and Pre-heaters—The pressure of the air in the 
distributing pipes is owing to friction and leakage, less than 
that at which it was delivered by the air-compressors. It 
ought, however, still to be higher than the greatest pressure 
to be used in any of the air motors of the system. The air 
has also lost ail superfluous heat above the temperature of 
the atmosphere it may have possessed on its entrance into 
the mains. 
The reader will perhaps remember that during the pro- 
cess of compression the air was unavoidably heated by the 
working pistons. He will then readily understand that 
during the process of expansion and abstraction of work 
from the compressed. air in the air motors a considerable 
fall of temperature will take place. So much is this the 
case that in many air motors working in mines the exhaust 
pipe becomes eventually choked up by the accumulations of 
frozen vapour in the escaping air. 
In the attempt to remedy this by heating the air before 
its entry into the cylinders of the motor, it has been found 
