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.

Q.Will you explain the construction and action of the single-acting engine, used for draining mines? A.Permit me then to begin with the boiler, which is common and necessary to all engines; and I will take the example of a wagon boiler, such as was employed by Boulton and Watt universally in their early engines, and which is still in extensive use.

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?

This differs from the one just described chiefly in that it is single-acting, while the other is double-acting.

By single-acting is meant that the air cylinders compress their respective volumes of air once every revolution.

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.

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.

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.