INTERMITTENT FOUNTAIN.

231

ment, without the intervention of the operator. It consists of a globe V (Fig. 174), which can be closed air-tight by means of a stopper, and is in communication with efflux tubes a, which discharge into a basin B, having a small hole o in its bottom for permitting the water to escape into a lower basin C.

A central tube t, open at both ends, extends nearly to the top of the globe, and nearly to the bottom of the basin B.

Suppose the globe to be filled with water, the basins being empty. Then the water will flow from the efflux tubes a, while air will pass up through the central tube. As the water issues from the efflux tubes much faster than it escapes through the opening o, the level rises in the basin B till the lower end of the tube t is covered. The pressure of the air in the upper part of the globe then rapidly diminishes, and the efflux from the tubes a is stopped. But as the water continues to escape from the

basin B through the opening o, the bottom of the tube t is again. uncovered, the liquid again issues from the efflux tubes, and the same changes are repeated.

271. Siphon. The siphon is an instrument in which a liquid, under the combined action of its own weight and atmospheric pressure, flows first up-hill and then down-hill, but always in such a way as to bring about a lowering of the centre of gravity of the whole liquid mass.

In its simplest form, it consists of a bent tube, one end of which is immersed in the liquid to be removed, while the other end either discharges into the air, at a lower level than the surface of the liquid in the vessel, as in Fig. 175, or dips into the liquid of a receiving vessel, the surface of this liquid being lower than that of the liquid in the discharging vessel.

We shall discuss the latter case, and shall denote the difference of levels of the two surfaces by h, while the height of a column of the liquid equivalent to atmospheric pressure will be denoted by H.

Let the siphon be full of liquid, and imagine a diaphragm to be drawn across it at any point, so as to prevent flow. Let this dia

IL

/BUNNAFOUX

Fig. 175.-Siphon.

C.LAPLANTE

phragm be at a height x above the higher of the two free surfaces, and at a height y above the lower, so that we have

y-x=h.

The pressure on the side of the diaphragm next the higher free surface will be H-x, (pressure being expressed in terms of the equivalent liquid column,) and the pressure on the other side of the diaphragm will be H-y, which is less than the former by yx, that is by h. The diaphragm therefore experiences a resultant force due to a depth h of the liquid, urging it from the higher to the lower free surface, and if the diaphragm be removed, the liquid will be propelled in this direction.

In practice, the two legs of the siphon are usually of unequal length, and the flow is from the shorter to the longer; but this is by no means essential, for by a sufficiently deep immersion of the long

leg, the direction of flow may be reversed. The direction of flow depends not on the lengths of the legs, but on the levels of the two free surfaces.

If the liquid in the discharging vessel falls below the end of the siphon, or if the siphon is lifted out of it, air enters, and the siphon is immediately emptied of liquid. If the liquid in the receiving vessel is removed, so that the discharging end of the siphon is surrounded by air, as in the figure, the flow will continue, unless air bubbles up the tube and breaks the liquid column. This interruption is especially liable to occur in large tubes. It can be prevented by bending the end of the siphon round, so as to discharge the liquid in an ascending direction. To adapt the foregoing investigation to the case of a siphon discharging into air, we have only to substitute the level of the discharging end for the level of the lower free surface, so that y will denote the depth of the discharging end below the diaphragm, and h its depth below the surface of the liquid which is to be drawn off.

As the ascent of the liquid in the siphon is due to atmospheric pressure on the upper free sur

face, it is necessary that the highest point of the siphon (if intended for water) should not be more than about 33 feet above this surface.

272. Starting the Siphon.-In order to make a siphon begin working, we must employ means to fill it with the liquid. This can sometimes be done by dipping it in the liquid, and then placing it in position while the ends are kept closed; or by inserting one end in the liquid which we wish to remove, and sucking at the other. It is usually more convenient to apply suction by means of a side tube, as in Fig. 176, this tube being sometimes provided with an enlargement to prevent the liquid from entering the mouth. One end of the siphon is inserted in the liquid which is to be removed, while the other end is stopped, and the operator applies suction at

Fig. 176.-Starting the Siphon.

the side tube till the liquid flows over. In siphons for commercial purposes, the suction is usually produced by a pump.

273. Siphon for Sulphuric Acid.-Fig. 177 represents a siphon used for transferring sulphuric acid from one vessel to another. The long branch is first filled with sulphuric acid. This is effected by means of two funnels (which can be plugged at pleasure) at the bend of the tube. One of these admits the liquid, and the other suffers the air to escape. The two funnels are then closed, and

Fig. 177.-Siphon for Sulphuric Acid.

the tap at the lower end of the tube is opened so as to allow the liquid to escape. The air in the short branch follows the acid, and becomes rarefied; the acid behind it rises, and if it passes the bend, the siphon will be started, for each portion of the liquid which issues from the tube will draw an equal portion from the short to the long branch. To insure the working of the sulphuric acid siphon, it is not sufficient to have the vertical height of the long branch greater than that of the short branch; it is farther necessary that it should exceed a certain limit, which depends upon the dimensions of the siphon in each particular case. In order to calculate this limit, we must remark that when the liquid begins to flow, its height diminishes in the long and increases in the short branch; if these two heights should become equal, there would be equilibrium. We see, then, that in order that the siphon may work, it is necessary that when the liquid rises to the bend of the tube, there should be in the long branch a column of liquid whose vertical height is at least equal to that of the short branch, which we shall denote by h, and the actual length of the short branch from the surface of the liquid in which it dips to the summit of the bend by . Then if a be the inclination of the long branch to the vertical, and L the length of the long branch, which we suppose barely sufficient, the length of the column of liquid remaining in the long branch will be h sec a. The air which at atmospheric pressure H occupied the length ', now under the pressure H-h occupies a length L-h sec a; hence by Boyle's law, we have

CUP OF TANTALUS.

235

=

Hh' (Hh) (L-h sec a), whence L= h sec a +

Hh'

H-h

In this formula H denotes the height of a column of sulphuric acid whose pressure equals that of the atmosphere.

n

274. Cup of Tantalus.-The siphon may be employed to produce the intermittent flow of a liquid. Suppose, for instance, that we have a cup (Fig. 178) in which is a bent tube rising to a height n, and with the short branch termi- n nating near the bottom of the cup, while the long branch passes through the bottom. If liquid be poured into the cup, the level will gradually rise in the short branch of the bent tube, till it reaches the summit of the bend, when the siphon will begin to discharge the liquid. If the liquid then escapes by the siphon faster than it is poured into the vessel, the level of the liquid in the cup will gradually fall below the termination of the shorter branch. The siphon will then empty itself, and will not recommence its action till the liquid has again risen to the level of the bend.

The siphon may be concealed in the interior of the figure of a man whose mouth is just above the top of the siphon. If water be poured in very slowly, it will continually rise nearly to his lips and then descend again. Hence the name. Instead of a bent tube we may employ, as in the first figure, a straight tube covered by a bellglass left open below; in this case the space between the tube and the bell takes the place of the shorter leg of the siphon.

It is to an action of this kind that natural intermittent springs are generally attributed. Suppose a reservoir (Fig. 179) to communicate with an outlet by a bent tube forming a siphon, and suppose it to be fed by a stream of water at a slower rate than the siphon is able to discharge it. When the water has reached the bend, the siphon will become charged, and the reservoir will be emptied; flow will then cease until it becomes charged again.

275. Mariotte's Bottle.-This is an apparatus often employed to obtain a uniform flow of water. Through the cork at the top of the bottle (Fig. 180) passes a straight vertical tube open at both ends, and