61. Water flowing from a Siphon ceases on surrounding the End of its longer Side with Water.
Let there be an air-tight vessel provided with an open spout, and by its side a thyrsus under which is a cup full of water: if the cup is removed, as long as it is withdrawn, a small stream shall flow from the mouth; but when the cup is pushed back, the spout shall run no longer. Let A B (fig. 61), be the vessel described, having its neck closed by the partition C D; from C D, and fitted air-tight in it, a tube, E F, extends, about which lies another tube, K L, forming an inclosed diabetes. With K L another tube, M N, communicates, of which the mouth M is open, while the outer leg is placed in a cup, O X, into which water has been poured until it is full; it is clear that so much of the leg of the siphon as is in the cup will be filled at the same time. Into the neck of the vessel A B a little water must be poured, just enough to close all entrance for the air; and, when A B is full, the spout P, though open, will not run, since the air has no means of entrance, because of the water poured into the neck. But if the cup is drawn slightly downwards, some portion of the leg of the siphon which is in the cup must be emptied, and into the part emptied the contiguous air will be drawn: this air will attract some of the water which was poured into the neck, so that the water shall rise above the mouth F; and hence, the air having found an entrance, the spout P will run until the cup O X is pushed up again, causing the water to return to its old position and to close the passage for the air so that the spout will cease to flow. This will happen as often as the cup is withdrawn and applied: it is necessary however that the cup be not wholly drawn away, that the siphon leg may not be wholly emptied. Let the tube M N be fashioned like a thyrsus, R N being its shaft: thus the spectacle will be properly arranged.