may be a vacant space between the piston and the water; and at the
return stroke the momentum of the water in the pipe expends itself in
giving a reverse motion to the column of water approaching the pump.
Messrs. Kirchweger and Prusman, of Hanover, have investigated this subject
by applying a revolving cock at the end of a pipe leading from an elevated
cistern containing water, and the water escaped at every revolution of the
cock in the same manner as if a pump were drawing it. With a column of
water of 17 feet, they found that at 80 revolutions of the cock per minute,
the water delivered per minute by the cock was 9.45 gallons; but with 140
revolutions of the cock per minute, the water delivered per minute by the
cock was only 5.42 gallons. They subsequently applied an air vessel to the
pipe beside the cock, when the discharge rose to 12.9 gallons per minute
with 80 revolutions, and 18.28 gallons with 140 revolutions. Air vessels
should therefore be applied to the suction side of fast moving pumps, and
this is now done with good results.

339. _Q._--What are the usual dimensions of the cold water pump of land
engines?

_A._--If to condense a cubic inch of water raised into steam 28.9 cubic
inches of condensing water are required, then the cold water pump ought to
be 28.9 times larger than the feed pump, supposing that its losses were
equally great. The feed pump, however, is made sufficiently large to
compensate for leaks in the boiler and loss of steam through the safety
valve, so that it will be sufficient if the cold water pump be 24 times
larger than the feed pump. This ratio is preserved by the following rule:--
multiply the capacity of the cylinder in cubic inches by the total pressure
of the steam per square inch, or the pressure on the safety valve plus 15,
and divide the product by 200. The quotient is the proper capacity of the
cold water pump in cubic inches when the engine is double acting, and the
pump single acting.

FLY WHEEL.

340. _Q._--By what considerations do you determine the dimensions of the
fly wheel of an engine?

_A._--By a reference to the power generated, each half stroke of the
engine, and the number of half strokes that are necessary to give to the
fly wheel its standard velocity, supposing the whole power devoted to that
object. In practice the power resident in the fly varies from 2-1/2 to 6
times that generated each half stroke; and if the weight of the wheel be
equal to the