_The efficiency of hydraulic rams_ has been much discussed; exhaustive practical tests have been made and the results have been reduced to formulas. Whittaker’s Mechanical Engineer’s Pocket Book gives the following: G × H E = ----- g × h where E = the efficiency; G = gallons of drive water used; g = gallons of water raised; H = height of fall, in feet; h = height to which the water is raised, in feet. _The Table_ given on page 174 is from the _American Engineer_. Its use is apparent, thus: when the height of fall in feet is, say 12 feet, and the elevation of discharge above the delivery valve of ram, in feet, is 30 feet, _the efficiency_ or per cent., is ·3282. (_Example_) of 100 gallons 32-82/100 gallons would be delivered. [Illustration: FIG. 151.] _The double hydraulic ram_ is shown in Fig. 151. A sectional view of the same device is shown in Fig. 152, the cuts represent the Rife hydraulic engine, or ram,—a so-called double acting or double supply type of the water ram. It is more clearly described by considering it, first, as a single machine by disregarding its double supply feature. First, suppose the opening at _H_, Fig. 152, to be closed, the valve _B_ being open, the water from the source of supply from more or less elevation above the machine flows down the drive pipe, _A_, and escapes through the opening at _B_ until the pressure due to the increasing velocity of the water is sufficient to close the valve, B. When the flow through this valve ceases, the inertia of the moving column of water produces a reaction, called the ramming stroke, which opens the valve at _C_, and compresses the air in the air chamber, _D_, until the pressure of the air plus the pressure due to the head of the water in the main, is sufficient to overcome the inertia of the moving column of water in the drive pipe. This motion may be likened to the oscillation of water in a ^U^ shaped tube. The instant the column of water in the drive pipe comes to rest, and the air pressure being greater than the static head alone, the motion of the moving column is reversed, and the valve, _C_, closes. The water in the drive pipe then moves backward, and with the closing of valve _C_ a partial vacuum is formed at the base of the drive pipe. This negative pressure causes the valve, _B_, to open again, and completes the cycle of operations. At the moment negative pressure appears the little snifting valve, _E_, admits a small quantity of air, and at the following stroke this air rises into the air chamber _D_, which would otherwise gradually fill with water, or the air is gradually absorbed by the water.