4. Siphon which is capable of discharging a greater or less quantity of Liquid with uniformity.
By the following arrangement we can produce a discharge at once uniform and variable; that is, a discharge in which, for a certain time at pleasure, the stream continues uniform from the beginning, and again, for any other period, is slower or quicker than before, but still uniform with itself. As before, let A B (fig. 4.) be a vessel of water, and C D a basin. Into the lid and bottom of the basin solder a tube L M wider than the inner leg of the siphon. On the lid place a wooden frame, C N X D, consisting of two upright pieces and a third lying across them on the top. In the inner sides of the upright pieces let grooves be cut down their whole length, along which another piece O P is to move freely. Let R S be a screw, working perpendicularly in the direction of the lid C D, and passing through a hole in O P: in O P let a pin be so fixed as to enter the spiral thread of the screw. The screw must project above N X, and a handle be fastened to its top by which to turn it, and by this means O P can be raised or lowered. Let the inner leg of the siphon be fixed in O P, and pass through the tube L M, so that its mouth may dip into the water in the vessel. Now if, as before, we draw off the liquid through the outer mouth, the siphon will flow with a uniform stream until the whole be exhausted. And when it is wished that a quicker stream should be produced through the siphon, but uniform with itself, let the screw be turned so as to lower the board O P; for then the excess of the outer leg is increased, and thus the stream is still of uniform velocity, but quicker than before. If a still greater velocity is desired, turn the screw again, so as to lower O P still further; and if a less velocity is sought, let O P be raised. Thus a discharge is produced through a siphon in one sense uniform, in another variable.