some cases destroyed altogether by
what is, as it were, an elastic air head which is brought into direct
contact with the piston. All this reduces clearance to so small a point
that it has no influence of any consequence. The moving parts are made
extremely simple, even arriving at a point where inlet valves are opened
and closed by their natural inertia. Mr. Sturgeon, of England, has
applied a most ingenious and successful inlet valve, which is opened and
closed by the friction of the air piston rod through the gland. We have,
therefore, reached a point at which high speed is made possible.

Long-stroke air compressors are evidently objectionable on the basis of
greater expense of construction. All the parts must be larger and
heavier. The fly wheels are increased enormously in diameter and weight,
and the strength of bearings must be enlarged in proportion. It is
difficult to equalize power and resistance in air compressors with long
strokes. The speed will be jerky, and when slow, the fly wheel rather
retards than assists in the work of compression. This action tends to
derange the parts and makes large bearings a necessity. The piston in a
long-stroke compressor travels through considerable space before the
pressure reaches a point where the discharge valve opens, and after
reaching that point it has to go on still further against a prolonged
uniform resistance. This makes rotative speed difficult. During the
early part of the stroke, the energy of the steam piston must be stored
up in the moving parts, to be given out when the steam pressure has been
reduced through an early cut-off. With a short stroke and a large
diameter of steam cylinder we are able to get steam economy or early
cut-off and expansion without the complications of compounding.

* * * * *

[Continued from SUPPLEMENT, No. 793, page 12677.]

THE POWER OF WATER, OR HYDRAULICS SIMPLIFIED.

By G.D. Hiscox.

CURRENT WHEELS FOR POWER AND RAISING WATER.

The natural flow of water in a current is probably one of the oldest and
cheapest of the methods for obtaining power, or the lifting of water
within moderate elevations, for a supply for irrigation and domestic
purposes; and we propose, apart from the current wheel, to treat only of
self-water-raising devices in this chapter.

Water wheels of various forms for this purpose have been used from time
immemorial in Europe, Asia and Egypt, where the record gives e