6o 
THE TELEGRAPH. 
compressed air contained in the part beyond the working space 
expands, and the piston has to move a certain distance before 
that air is brought to the pressure corresponding to that of 
the vacuum reservoir. The work consumed in this expansion 
is in part lost, if compared with that which would be required 
with two cylinders, one for compression, and the other for 
rarefaction. 
A small 4 horse-power engine, driven by the town-water 
supply, is placed near the steam-engine, and is used in case 
the tubes have, from any unusual circumstance, to be worked 
during the night. 
It is formed of two horizontal cylinders acting on a hori¬ 
zontal shaft, provided with two fly-wheels, and transmitting 
motion to the piston of the compressing cylinder by means of 
a connecting rod. Each cylinder is fitted with a slide valve, 
and the pressure of the water is 36 lbs. on the square inch. 
The water supply of the city cannot be advantageously used 
in London, on account of the high price charged for it, 
amounting to Is. for 164 cubic feet. The bead being 84 feet, 
the theoretical worth of a cubic foot is 5,250 foot-lbs. To 
thus obtain the work of an engine of 40 horse-power, would 
require 15,000 cubic feet of water per hour, which at d. 
per cubic foot involves an expense of £4 11s. 5 d. per hour. 
Now, a steam-engine of 40 horse-power, of the construction 
least favourable for economy, that is to say, an engine neither 
working expansively nor condensing, would only consume per 
hour about 100 lbs. of coal, the value of which is Is. iod. 
The costs of maintenance and labour must be added to that of 
the coal, but these are insignificant compared with the expense 
incurred when water is used. 
At the Central Office in London, the boilers are placed in 
cellars close to the engines. Two of them have internal fire¬ 
places ; their diameter is 5 feet, and their total length, 16 
feet; the fire-bars being 5 feet long. The third boiler is 
tubular and vertical, 6 feet in diameter and 7 feet high. The 
maximum pressure of the steam is 40 lbs. per square inch. 
The Messrs. Siemens have successfully employed an apparatus 
for exhaustion, which is based on the same principle as the 
exhaust pipe by which the draught is maintained in loco¬ 
motives. A steam-jet, carried into a suitably-shaped pipe 
