Below the piston of the upper cylinder is an annular space E (surrounding the common piston rod) with a capacity equal to the maximum displacement of the liftram, while the corresponding annular area C of the piston of the lower cylinder is just large enough when subjected to the working water pressure to enable the work of lifting the net load to be done and any friction to be overcome.
Distribution of steam is effected by a slide valve, sometimes fitted with a balancing device, and sometimes formed into a piston valve.
In this case the piston is solid, and the outlet pipe, G which is placed at the bottom of the cylinder, has a valve F opening outwards, the inlet pipe and valve are the same as before.
The cross-section of the cars was made to conform approximately to the section of the tunnel, the idea being that each train would act like a piston in a cylinder, expelling in front of it a column of air, to be forced up the station shaft next ahead of the train, and sucking down a similar column through the station shaft just behind.
The action is as follows: On raising the piston it cuts off communication with the inlet pipe and then compresses the air above, forcing it through the upper valve and oil into the atmosphere.
The total upward pressure on this piston is calculated to be equal to 150,000 lb; hence the shaft-bearings are practically relieved from pressure when the wheels are running.
Sand, driven between the wheel and the rail by a steam jet, used just at starting, increases the adhesion beyond the normal value and enables a larger pressure to be exerted on the piston than would otherwise be possible.
Stationary waves are formed in the air in the dust-tube if the length is rightly adjusted by the closely-fitting piston, and the lycopodium dust collects at the nodes in little heaps, the first being at the fixed end and the last just in front of the piston on the sounder.
P. Joule with the perforated piston and with the friction of water and mercury.
In practice it may be considerably less, owing to leakage at the valves and between the piston and cylinder.