also a considerable portion of the moisture, which, when fresh air
is being used, must of necessity enter the compression cylinder, is
condensed and deposited as water.

[Illustration: COMPRESSION CYLINDER. SCALE 1/60]

After being cooled, the compressed air is then admitted to the expansion
cylinder, but as it still contains a large quantity of water in
solution, which, if expansion was carried immediately to atmospheric
pressure, would, from the extreme cold, be converted into snow and ice,
with a positive certainty of causing great trouble in the valves and
passages. It is got rid of by a process invented by Mr. Lightfoot,
which is at the same time extremely simple and beautiful in action, and
efficient. Instead of reducing the compressed air at once to atmospheric
pressure, it is at first only partially expanded to such an extent that
the temperature is lowered to about 35 deg. to 40 deg. Fah., with the
result that very nearly the whole of the contained aqueous vapor is
condensed into water. The partially expanded air which now contains the
water as a thick mist is then admitted into a vessel containing a number
of grids, through which it passes, parting all the while with its
moisture, which gradually collects at the bottom and is blown off. The
surface area of the grids is so arranged that by the time the air has
passed through them it is quite free from moisture, with the exception
of the very trifling amount which it can hold in solution at about 35
deg. Fah., and 30 lb. pressure. The expansion is then continued to
atmospheric pressure and the cooled air containing only a trace of snow
is then discharged ready for use into a meat chamber or elsewhere. In
small machines the double expansion is carried out in one cylinder
containing a piston with a trunk, the annulus forming the first
expansion and the whole piston area the second, but in larger machines