The hydraulic piston compressor has one solitary advantage, and that is, it has no dead spaces.  It was conceived at a time when dead spaces were very serious conditions—­were positive specters!  Valves and other mechanism connected with the cylinder of an air compressor were once of such crude construction that it was impossible to reduce the clearance spaces to a reasonable point, and, furthermore, the valves were heavy and so complicated that anything like a high speed would either break them or wear them out rapidly, or derange them so that leakages would occur.  But we have now reduced inlet and discharge valves and all other moving parts connected with an air cylinder to a point of extreme simplicity.  Clearance space is in 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.

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[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.