and it was
found that not only was there a benefit from the operation of expansion,
but that there was a still greater benefit from the superior facility of
escape given to the steam, inasmuch as the application of lap involved the
necessity of turning the eccentric round upon the shaft, which caused the
eduction to take place before the end of the stroke. In 1840, one of the
engines of the Liverpool and Manchester Railway was altered so as to have 1
inch lap on the valve, and 1 inch opening on the eduction side at the end
of the stroke, the valve having a total travel of 4-1/4 inches. The
consumption of fuel per mile fell from 36.3 lbs. to 28.6 lbs, or about 25
per cent., and a softer blast sufficed. By using larger exhaust passages,
larger tubes, and closer fire bars, the consumption was subsequently
brought down to 15 lbs. per mile.

AIR PUMP, CONDENSER, AND HOT AND COLD WATER PUMPS.

324. _Q._--Will you state the proper dimensions of the air pump and
condenser in laud and marine engines?

_A_--Mr. Watt made the air pump of his engine half the diameter of the
cylinder and half the stroke, or one eighth of the capacity, and the
condenser was usually made about the same size as the air pump; but as the
pressure of the steam has been increased in all modern engines, it is
better to make the air pump a little larger than this proportion. 0.6 of
the diameter of the cylinder and half the stroke answers very well, and the
condenser may be made as large as it can be got with convenience, though
the same size as the air pump will suffice.

325. _Q._--Are air pumps now sometimes made double acting?

_A._--Most of the recent direct acting marine engines for driving the screw
are fitted with a double acting air pump, and when the air pump is double
acting, it need only be about half the size that is necessary when it is
single acting. It is single acting in nearly every case, except the case of
direct acting screw engines of recent construction.

326. _Q._--What is the difference between a single and a double acting air
pump?

_A._--The single acting air pump expels the air and water from the
condenser only in the upward stroke of the pump, whereas a double acting
air pump expels the air and water both in the upward and downward stroke.
It has, therefore, to be provided with inlet and outlet valves at both
ends, whereas the single acting pump has only to be provided with an inlet
or foot valve, as it is termed, at the