99. Q.—­Where is the condenser situated?

A.—­The condenser K is immerged in a cistern of cold water.  At its side there is a tube I, for the admission of water to condense the steam, and which is governed by a cock, by opening which to any required extent, a jet of cold water may be made to play in the condenser.  From the bottom of the condenser a short pipe leads to the air pump J, and in this pipe there is a flap valve, called the foot valve, opening towards the air pump.  The air pump is a pump set in the same cistern of cold water that holds the condenser, and it is fitted with a piston or bucket worked by the rod L, attached to the great beam, and fitted with a valve opening upwards in the manner of a common sucking pump.  The upper part of the air pump communicates with a small cistern S, called the hot well, through a valve opening outwards and called the delivery valve.  A pump M, called the hot water pump, lifts hot water out of the hot well to feed the boiler, and another pump N lifts cold water from a well or other source of supply, to maintain the supply of water to the cold water cistern, in which the condenser and air pump are placed.

100.  Q.—­Will you explain now the manner in which the engine acts?

A.—­The piston being supposed to be at the top of the cylinder, the handle H will be raised by the lower pin or tappet on the air pump rod, and the valves V and E’ will be opened, and at the same time the other pair of valves V’ and E will be closed.  Steam will therefore be admitted above the piston and the steam or air which had previously filled the cylinder below the piston will be drawn off to the condenser.  It will there encounter the jet of cold water, which is kept constantly playing there by keeping the cock I sufficiently open.  It will thus be immediately condensed or reduced to water, and the cylinder below the piston will have a vacuum in it.  The steam therefore admitted from the steam pipe through the open valve V to the top of the cylinder, not being resisted by pressure below, will press the piston to the bottom of the cylinder.  As it approaches that position, the handle H will be struck down by the upper pin or tappet on the air pump rod, and the valves V and E’, previously open, will be closed, while the valves V’ and E, previously closed, will be opened.  The steam which has just pressed down the piston, and which now fills the cylinder above the piston, will then flow off, through the open valve E, to the condenser, where it will be immediately condensed by the jet of cold water; and steam from the boiler, admitted through the open valve V’, will fill the cylinder below the piston, and press the piston upwards.  When the piston has reached the top of the cylinder, the lower pin on the air pump rod will have struck the handle upwards, and will thereby have closed the valves V’ and E, and opened the valves V and E’.  The piston will then be in the same situation as in the commencement, and will again descend, and so will continue to be driven up and down by the steam.