In another form hydraulic motors have been adopted, in favorable locations, as first movers of machinery, and when thus used, _they exhibit a very striking resemblance to high pressure steam engines_. Indeed, the elemental features of steam and pressure engines are the same, and the modes of employing the motive agents in both are identical—it is the different properties of the agents that induces a slight variation in the machines—_one being an elastic fluid, the other a non-elastic liquid_. [Illustration: FIG. 131.] [Illustration: FIG. 132.] In steam engines a piston is alternately pushed forward and back in its cylinder by steam; and by means of the rod to which the piston is secured, motion is communicated to a crank and fly-wheel, and through these to the machinery to be driven: it is the same with pressure engines when used to move other machines, except that instead of the elastic vapor of water, a column of that liquid drives the pistons to and fro. NOTE.—“The hydraulic engine of Huelgoat, in Brittany, is used to drain a mine; is single-acting, and acts directly to lift the piston of the pump. It makes five and a half strokes per minute, the stroke being a little more than eight feet in length. The piston-rod is 767 feet long, and it weighs 16 tons. The power of the engine is derived from a source at a height 370 feet above its own level.”—KNIGHT. In default of a natural head of sufficient pressure, the head is sometimes established in _an accumulator_ of power; this is a body of water driven into a reservoir under heavy pressure, by forcing pumps worked by power. In cities where the water distribution is from elevated reservoirs, and in which the water supply is sufficiently abundant to justify the application of a portion of it to industrial uses, the water-engine or motor is recommended. The following description of a water engine of world-wide adaptation will, if attentively studied, show the working of an approved type of this machine: Ramsbottom’s hydraulic engine (Figs. 129 and 130), is oscillating, and employs two cylinders _b_, _l_, operating one crank-shaft, _a_, by means of two cranks at right angles to each other. In one of the accompanying figures the channels of induction are marked _j_ and are cast on the cylinders; the dotted circle _c_ shows the position of the supply and discharge pipes; in the other figure these pipes are indicated by arrows. The two views are vertical cross-sections at right angles to each other, one being through the axis of the cylinders and the other through the middle post in which the inner trunnions of the cylinders are journaled. The apertures of induction are seen at _h_ and those of eduction at _i_, and have the form of truncated circular sectors, whose center is the center of motion.