But these equal pressures are counteracted by the pressures of the two unequal columns of water, D E and D C. And since the atmospheric pressure is more than sufficient to balance both these columns of fluid, that which acts with the lesser force, that is, the column D E will be more pressed against D c than D C is against D E at the vertex D; consequently the column D E will yield to the greater pressure, and flow off through the orifice c.

Emma. Would the same thing happen if the outer leg D C were shorter than the other?

Father. If D C were broken off at B, even with the surface of the water, no water would run over: or if it were broken off any where lower than B, it would only run away till the surface of the fluid descended to a level with the length of the outer tube, because then the column D E will be no more pressed against D C, than D C is against D E, and consequently the syphon will empty itself; the water in the outer leg will run

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out at the lower orifice, and that in the inner will fall back into the jar.

Charles. In decanting a bottle of wine, are you obliged first to fill the syphon with liquor, and then invert it?

Father. No: a small pipe is fixed to the outer leg of the syphon, by which the air is drawn out of it by the mouth, and the short leg being immersed in the wine, the fluid will follow the air, and run out till the bottle is empty.

The syphon is sometimes disguised for the sake of amusing young people. Tantalus's cup (Plate 111. Fig. 27.) is of this kind. The longer leg of the syphon passes through, and is cemented into the bottom of the cup; if water be poured into the cup, so as not to stand as high as the bend of the tube, the water will remain as in any common vessel; but if it be raised over the bended part of the syphon, it will run over and continue to run till the vessel is emptied. Sometimes a little figure of a man, representing Tantalus, conceals the syphon, so

that Tantalus, as in the fable, stands up to his chin in water, but is never able to quench his thirst, for just as it comes to a level with his chin, it runs out through the concealed syphon.

Emma. To this fable the lines in Pope's Homer refer:

E'vn in the circling floods refreshment craves,
And pines with thirst amids a sea of waves;
And when the water to his lips applies.

Back from his lips the treacherous water flies.

POPE.

Father. It is alluded to also by our own Milton:

-and of itself the water flies

All taste of living wight, as once it fled

The lip of Tantalus.

Par. Lost, Book II.

This is another kind of Tantalus's (Plate iv. Fig. 28.) but the syphon is con

is

cealed in the handle, and when the water in the cup, which communicates with the shorter leg at I, is raised above the bend of the handle, it runs out through the longer leg at P, and so continues till the cup empty. This cup is often made to deceive the unwary, who, by taking it up to drink, cause the water, which was, while at rest, below the bend of the syphon, to run over, and then there is no means of stopping the stream till the vessel is empty.

Charles. I have frequently seen at the doors of public houses large hogsheads of spirits in carts or waggons, and persons drawing off the contents by means of an instrument like a syphon.

Father. That is called a distiller's crane or syphon. B (Plate 1v. Fig. 29.) represents one of these barrels with the crane at work from the bung-hole n. The longer leg mr is about three feet long, with a stop-cock near the middle, which must be shut, and then the shorter leg is immersed in the liquor.

Emma. Is the air in the short leg forced into the other by the upward pressure of

the fluid?

Father. It is, and the cock being shut it cannot escape, but will be very much condensed. If then the cock be suddenly opened, the condensed air will rush out, and the pressure of the air on the liquor in the ves sel will force it over the bend of the syphon, and cause it to flow off in a stream, as the figure represents. If, however, the barrel be not full, or nearly so, then it is necessary to draw the air out of the syphon by means of a small tube, a b, fixed to it.

By the principle of the syphon we are enabled to explain the nature of intermitting springs.

Emma. What are these, Papa? ·

Father. They are springs or rather streams, that flow periodically. A figure will give a clearer idea of the subject than many words without. G F C (Plate IV. Fig. 30.) represent a cavity in the bowels of a hill, or mountain, from the bottom of which