When the piston descends, the valve S closes by its own weight, and prevents the return of the air from the cylinder into the tube A. The air compressed by the piston opens the valve O, and escapes into the atmosphere by the pipe C. With a second stroke, the same series of phenomena is produced, until after a few strokes the water reaches the cylinder. The effect is now somewhat modified; during the descent of the piston the valve S closes, and the water raises the valve O, and passes above the piston by which it is lifted into the upper reservoir D. There is now no more air in the pump, and the water forced by the atmospheric pressure rises with the piston, provided that when it is at the summit of its course it is not more than 34 feet above the level of the water into which the tube A dips. [Illustration: FIG. 154.] _In practice the height of the tube A does not exceed 26 to 28 feet_; for although the atmospheric pressure can support a higher column, the vacuum produced in the barrel is not perfect, owing to the fact that the piston does not fit exactly on the bottom of the barrel. But when the water has passed the piston, it is the ascending force of the latter which raises it, and the height to which it can be brought depends on the power which works the piston. The action of this pump, a model of which is represented in Fig. 154, _depends both on exhaustion and on pressure_. At the base of the barrel, where it is connected with the tube A, there is a valve, S, which opens upwards. Another valve, O, opening in the same direction, closes the aperture of a conduit, which discharges from a hole, _o_, near the valve S, into a vessel, M, which is called the _air-chamber_. From this chamber there is another tube, D, up which the water is forced. At each ascent of the piston B, which is solid, the water rises through the tube A into the barrel. When the piston sinks the valve S closes, the water is forced through the valve O into the reservoir M, and thence into the tube D. The height to which it can be elevated in this tube depends solely on the motive power which works the pump. If the tube D were a prolongation of the tube J_ao_, the flow would be intermittent; it would take place when the piston descended, and would cease as soon as it ascended. But between these motions there is an interval, which, by means of the air in the reservoir M, ensures a continuous flow. The water forced into the reservoir M separates into two parts, one of which, rising in D, presses on the water in the reservoir by its weight; while the other, by virtue of this pressure, rises in the reservoir above the lower orifice of the tube D, compressing the air above. Consequently, when the piston ascends, it no longer forces the water into M, the air of the reservoir, by the pressure it has received, reacts on the liquid, and raises it in the tube D, until the piston again descends, so that the jet is continuous.