at the other end. The instrument is then inverted, the end being submerged in the liquid, and the stop removed from A. The liquid will begin to flow through the tube, and the flow will continue till the level of the liquid in the reservoir reaches that of the mouth of the tube C.

To find the velocity with which water will issue from the siphon, let us consider an infinitely small layer at the orifice A. This layer will be pressed downwards, by the tension of the atmosphere exerted on the surface of the reservoir, diminished by the weight of the water in the branch BD, and increased by the weight of the water in the branch BA. It will be pressed upwards by the tension of the atmosphere acting directly upon the layer. The difference of these forces, is the weight of the water in the portion of the tube DA, and the velocity of the stratum will be due to that weight. Denoting the vertical height of DA, by h, we shall have, for the velocity (Art. 173),

v = √2gh.

This is the theoretical velocity, but it is never quite realized in practice, on account of resistances, which have been neglected in the preceding investigation.

B

The siphon may be filled by applying the mouth to the end A, and exhausting the air by suction. The tension of the atmosphere, on the upper surface of the reservoir, will press the water up the tube, and fill it, after which the flow will go on as before. Sometimes, a sucking-tube AD, is inserted near the opening A, and rising nearly to the bend of the siphon. In this case, the opening A, is closed, and the air exhausted through the sucking-tube AD, after which the flow goes on as before.

The Wurtemburg Siphon.

A

Fig. 180.

215. In the Wurtemburg siphon, the ends of the tube are

bent twice, at right-angles, as shown in the figure. The advantage of this arrangement is, that the tube, once filled, remains so, as long as the plane of its axis is kept vertical. The siphon may be lifted out and replaced at pleasure, thereby stopping the flow at will.

A AJ

Fig. 181.

It is to be observed that the siphon is only effectual when the distance from the highest point of the tube to the level of the water in the reservoir is less than the height at which the atmospheric pressure will sustain a column of water in a vacuum. This will, in general, be less than 34 feet.

The Intermitting Siphon.

The

E

D

C'

D'

216. The intermitting siphon is represented in the figure. AB is a curved tube issuing from the bottom of a reservoir. reservoir is supplied with water by a tube E, having a smaller bore than that of the siphon. To explain its action, suppose the reservoir at first to be empty, and the tube E to be opened; as soon as the reservoir is filled to the level of CD, the water will begin to flow from the opening B, and the flow once commenced, will continue till the level of the reservoir is again reduced to the level C'D', drawn through the opening A. The flow will then cease till the cistern is again filled to CD, and so on as before.

Intermitting Springs.

B

Fig. 182.

217. Let A represent a subterranean cavity, communicating with the surface of the earth by a channel ABC, bent like a siphon. Suppose the reservoir to be fed by percolation through the crevices, or by a small channel D. When the

Fig. 183.

water in the reservoir rises to the height of the horizontal plane BD, the flow will commence at C, and, if the channel is sufficiently large, the flow will continue till the water is reduced to the level plane drawn through C. An intermission of flow will occur till the reservoir is again filled, and so on, intermittingly. This phenomena has been observed at various places.

Siphon of Constant Flow.

218. We have seen that the velocity of efflux depends upon the height of the water in the reservoir above the external opening of the siphon. When the water is drawn off from the reservoir, the upper surface sinks, this height diminishes, and, consequently, the velocity continually diminishes.

If, however, the shorter branch CD, of the tube, be inserted through a piece of cork large enough to float the siphon, the instrument will sink as the upper surface is depressed, the height of DA will remain the same, and, consequently, the flow will be uniform till the bend of the siphon comes in contact with the upper edge of the reservoir. By suitably adjusting the siphon in the cork, the velocity of efflux can be increased or decreased within certain limits. In this manner, any desired quantity of the fluid can be drawn off in a given time.

The siphon is used in the arts, for decanting liquids, when it is desirable not to stir the sediment at the bottom of a vessel. It is also employed to draw a portion of a liquid. from the interior of a vessel when that liquid is overlaid by one of less specific gravity.

The Hydraulic Ram.

219. The hydraulic ram is a machine for raising water by means of shocks caused by the sudden stoppages of a stream of water.

The instrument consists of a reservoir B, which is supplied with water by an inclined pipe A; on the upper surface

of the reservoir, is an orifice which may be closed by a spherical valve D; this valve,

G

PFE

B

Fig. 184.

when not pressed against the opening, rests in a metallic framework immediately below the orifice; G is an air-vessel communicating with the reservoir by an orifice F, which is fitted with a spherical valve E; this valve closes the orifice F except when forced upwards, in which case its motion is restrained by a metallic framework or cage; H represents a delivery-pipe entering the air-vessel at its upper part, and terminating near the bottom. At P is a small valve, opening inwards, to supply the loss of air in the air-vessel, arising from absorption by the water in passing through the air vessel.

To explain the action of the instrument, suppose, at first, that it is empty, and all the parts in equilibrium. If a current of water be admitted to the reservoir, through the inclined pipe A, the reservoir will soon be filled, and commence rushing out at the orifice C. The impulse of the water will force the spherical valve D, upwards, closing the opening; the velocity of the water in the reservoir will be suddenly checked; the reaction will force open the valve E, and a portion of the water will enter the air-chamber G. The force of the shock having been expended, the spherical valves will both fall by their own weight; a second shock will take place, as before; an additional quantity of water will be forced into the air-vessel, and so on, indefinitely. As the water is forced up into the air-vessel, the air becomes compressed; and acting by its elastic force, it urges a stream of water up the pipe H. The shocks occur in rapid succession, and, at each shock, a quantity of water is forced into. the air-chamber, and thus a constant stream is kept up. To explain the use of the valve P, it may be remarked that water absorbs more air under a great pressure, than under

a smaller one. Hence, as it passes through the air-chamber, a portion of the air contained is taken up by the water and carried out through the pipe H. But each time that the valve D falls, there is a tendency to produce a vacuum in the upper part of the reservoir, in consequence of the rush of the fluid to escape through the opening. The pressure of the external air then forces the valve P open, a small portion of air enters, and is afterwards forced up with the water into the vessel G, to keep up the supply.

The hydraulic ram is only used where it is required to raise small quantities of water, such as for the supply of a house, or garden. Only a small fraction of the amount of fluid which enters the supply-pipe actually passes out through the delivery-pipe; but, if the head of water is pretty large, the column may be raised to a great height. Water is often raised, in this manner, to the highest points of lofty buildings.

Sometimes, an additional air-vessel is introduced over the valve E, for the purpose of deadening the shock of the valve in its play up and down.

Archimedes' Screw.

220. This machine is intended for raising water through small heights, and consists, in its simplest form, of a tube wound spirally around a cylinder. This cylinder is mounted so that its axis is oblique to the horizon, the lower end dipping into the reservoir. When the cylinder is turned on its axis, by a crank attached to its upper extremity, the lower end of the tube describes a circumference of a circle, whose plane is perpendicular to the axis. When the mouth of the tube comes to the level of the axis and begins to ascend, there will be a certain quantity of water in the tube, which will flow so as to occupy the lowest part of the spire; and, if the cylinder is properly inclined to the horizon, this flow will be towards the upper end of the tube. At each revolution, an additional quantity of water will enter the tube, and that already in the tube will be forced, or raised, higher and