64 TECHNOLOGY, 
the axle w to turn through a certain arc, which, by means of the rod g and 
the lever 2, turns the cock / so as to shut off the main water from the 
cylinder v. The pressure upon m exceeding that upon o, will cause mu to 
descend, and admit the water from the main into T, when the main piston 
will commence its upward or working stroke. As it reaches the top, the 
projection B will, by acting on the arm a, turn the cock A, so as te admit 
the main water through x into v, and to close L’; when the piston-valve m 
will ascend and close the communication between the main pipe and cylin- 
der, leaving the piston to descend as before. The couplings of the pitman- 
rods are shown in jig. 14. 
3. Frre-EnGInes. 
A further application of the suction and forcing-pumps are jire-engmnes, 
which serve to throw water or other fire-extinguishing fluids to a consider- 
able distance or elevation. The chief requisites of a fire-engine are, that it 
should be as compact and portable as is consistent with the power of fur- 
nishing a large quantity of water, and that the stream of water thrown by 
it should not be intermittent but continuous, and that any desired direction 
may be given to it. All fire-engines consist of a single or double forcing- 
pump, provided with an air-chamber, the effect of which is to make the 
discharge of water continuous. The essential features of the machine are 
the following: one or two cylinders, at the bottom of which is the bottom 
or suction-valve, opening into the cylinder ; each cylinder is connected with 
the air-chamber by a pipe, at the junction of which with the air-chamber is a 
valve which admits the water into the latter, but prevents its return, In 
the lower part of the air-chamber is the pipe through which the water is 
expelled, and which consists of several portions so joined together as to 
allow the mouth or branch-pipe to be turned in every direction, or else a 
leather pipe or Aose ending in a brass nozzle is screwed to the first piece. 
The plungers in the cylinders are moved by levers on which the firemen 
operate. The cylinders and air-chamber generally are mounted in a water- 
box, which is supplied with water from a reservoir by means of a hose; 
when this reservoir is below the level of the fire-engine, the water is drawn 
from it by suction, and the suction-hose is stiffened out by spiral coils of 
wire (pl. 14, jig. 206) to prevent its being compressed by the atmospheric 
pressure. 
The operation of fire-engines differs from that of ordinary pumps only by 
the action of the air-chamber. At the beginning of the play of the engine 
the chamber contains a quantity of air corresponding to its volume; as 
water is forced into it while the nozzle remains closed, the air will be com- 
pressed in the chamber to a great degree, and will, on the nozzle being 
opened, expel the water with great velocity in a copious stream, which 
retains its force without much variation while the pump continues to be 
worked. The suction-valves are either conical, spherical, or plane-valves. 
The spherical or ball-valves (pl. 14, jig. 19) are the most usual, and deserve 
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