two cylinders of different sizes are used, just as in an ordinary
compound engine. To compensate for the varying temperature of the
cooling water the cut-off valve to the first or primary expansion is
made adjustable; and this can either be regulated as occasion requires
by hand, or else automatically. The temperature in the depositors being
kept constant under all variations in cooling water, there is the same
abstraction of moisture in the tropics as in colder climates, and the
cold air finally discharged from the machine is also kept at a uniform
temperature.

[Illustration: Expansion Cylinder. Scale 1/60.92 deg. F. temperature of
entering air. Cooling water entering in at 86 deg. F.]

[Illustration: Expansion Cylinder. Scale 1/60. 68 deg. F. temperature of
entering air. Cooling water entering in at 65 deg. F. 125 revs. per minute,
or 312 ft. per minute per piston speed.]

The diagrams are reduced from the originals, taken from the compression
cylinder when running at the speed of 125 revolutions per minute, and
also from the expansion cylinder, the first when the cooling water
was entering the coolers at 86 deg. Fah., and the latter when this
temperature was reduced to 65 deg. Fah. In all cases the compressed
air is cooled down to within from 3 deg. to 5 deg. of the initial
temperature of the cooling water, thus showing the great efficiency
of the cooling apparatus. The machine has been run experimentally at
Dartford, under conditions perhaps more trying than can possibly occur,
even in the tropics, the air entering the compression cylinder being
artificially heated up to 85 deg. and being supersaturated at that
temperature by a jet of steam laid on for the purpose. In this case no
more snow was formed than when dealing with aircontaining a very much