The current is thus suddenly stopped; this causes a reaction, which produces pressure sufficient to open another valve (inlet valve) between the current-pipe and an air chamber, and a portion of water enters by means of the force of the current, but by so doing the current has spent its force; the outlet valve at the end of the chamber falls by its own weight, and the pressure in the pipe ceasing, the inlet valve in the air-chamber falls and closes the opening. The condition of things is then restored; the water then acquires a momentum which closes the outlet valve and forces more water again into the chamber. A very slight descending column is capable of raising one ascending very high. In all cases the drive-pipe or inlet pipe must be sufficiently long to prevent water being forced back into the reservoir. The air-chamber serves to keep up a steady supply from the reservoir, preventing spasmodic action. To prevent admixture of air with the water in the air chamber, which is caused by pressure of water when raised to a great height, a small hole should be made on the upper side of the inlet pipe, immediately in front of the same. By the action of the ram at each stroke, a partial vacuum is formed below the air chamber, and the air rushing through the small hole in the inlet pipe, passes into the air chamber, making good that which the water absorbs. NOTE.—In 1797 Matthew Boulton (manufacturer and practical engineer, and in later life a partner of Jas. Watt, the Father of the steam engine) obtained a patent for a mode of raising water by impulse. The apparatus had excited much attention in France, under the name of _Montgolfier’s hydraulic ram_, and Boulton added to it a number of ingenious modifications, which were the basis of his patent. [Illustration: FIG. 150.] Fig. 150 shows in section the construction of the ram in its simplest form in which E is the reservoir, A the pipe in which the water falls, B the channel, _a_ and _b_ the valves, C the air-chamber, and D the discharge. Water first flows out in quantity through the valve _a_, and as soon as it has acquired a certain velocity it raises that valve, closing the aperture. The impact thus produced, acting on the sides of the pipe and the valve _b_, raises this valve, and a quantity of water passes into the air-chamber shutting off air and compressing it in the space above the mouth _d_ of the discharge D. This air by its electric force closes the valve _b_, and the water which has entered is raised in the discharge D. As soon as the impulsive action is over, and the water in the channel A comes to rest, the valve _a_ again falls by its own weight, the flow begins afresh, and when it has acquired sufficient velocity the valve _b_ again closes, and the whole process is repeated.