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In 1876 the Otto-Crossley silent engine was introduced. As you are aware, it is a single-acting engine, in which the gas and air are drawn in by the first outward, and compressed by the first inward stroke. The compressed mixture is then ignited; and, being expanded by heat, drives the piston outward by the second outward stroke.

Q. in the double-acting engine the piston is pushed by the steam both ways, whereas in the single-acting engine it is only pushed one way? A. The structure and action of a double-acting land engine of the kind introduced by Mr. A is the cylinder in which a movable piston, T, is forced alternately up and down by the alternate admission, to each side, of the steam from the boiler.

Q. What are the chief varieties of the steam engine in actual practical use? A. There is first the single-acting engine, which is used for pumping water; the rotative land engine, which is employed to drive mills and manufactories; the rotative marine engine, which is used to propel steam vessels; and the locomotive engine, which is employed on railways.

It was composed of two partsthe fore part consisting of a small steam-engine, formed of a round copper boiler, with a furnace inside, provided with two small chimneys and two single-acting brass steam cylinders, whose pistons acted alternately upon the single driving-wheel.

Thus, as the oil is gradually used the pressure of the water in the other leg of the hydrostatic balance raises the level of the remaining portion. When a fresh supply of oil is poured into the box, it forces out some of the water and descends very nearly to the level of the hole, B. The engine is made with either one or two cylinders, and is, of course, single-acting.

The air is admitted to the large cylinder through the piston, is compressed to about 30 pounds, and on the return stroke the pressure is raised to almost any point required, and in proportion to the diameter of the smaller cylinder. Though single-acting, the capacity of one of these compressors is about equal to that of the double-acting machine of the same cost of construction.

We shall end by quoting the greatest living authority, Lord Kelvin, now Lord Chancellor of Glasgow University, which Watt and he have done so much to render famous: Precisely that single-acting, high-pressure, syringe-engine, made and experimented on by James Watt one hundred and forty years ago in his Glasgow College workshop, now in 1901, with the addition of a surface-condenser cooled by air to receive the waste steam, and a pump to return the water thence to the boiler, constitutes the common-road motor, which, in the opinion of many good judges, is the most successful of all the different motors which have been made and tried within the last few years.

The maximum strains are reduced considerably below those which exist in compressors that do not compound the air. The advantage of the single-acting air cylinder over the double is that it compresses a volume of free air only once every revolution, hence there is a better chance to cool the air during compression.

The last is always a high-pressure engine; the others are, for the most part, condensing engines. Q. Will you explain the construction and action of the single-acting engine, used for draining mines?

An open-ended "trunk" piston, similar in principle to that used in gas engines, is employed; and the valve is of the piston type, which is less complicated than the box form of valve, though less easily made steam-tight in small sizes. The engine is single-acting, making only one power stroke per revolution.