S8(5 tzcTvnx xxiii. 



which must necessarily revolve round tile orifice, will unite in producing a 

 vacuity in the centre; and when this happens, the discharge is considerably di- 

 minished. 



In order that a siphon may run, it is obvious that it must first be filled; 

 and when it is once filled, it will continue to run till the reservoir is ex- 

 hausted, as far as the level of its upper orifice. And from this circumstance, 

 the phenomena of some intermitting springs have been explained, which 

 only begin to run, when the resei-voirs from which they originate have been 

 filled by continued rains, and then go on to exhaust them, even though the 

 weather may be dry. From a combination of several such siphons and re- 

 servoirs, a great number of alternations may sometimes be produced. (Plate 

 XX. Fig. 260.) 



Since the velocity of a stream or jet issuing in any direction, out of a simple 

 orifice, or a converging one, is nearly equal to that of a heavy body falling 

 from the height of the reservoir, it will rise, if directed upwards, very nearly 

 to the same height, excepting a slight difference occasioned by the resistance of 

 the air, and by the force which is lost, in producing the velocity with which the 

 particles must escape laterally, before they begin to descend. The truth of 

 this conclusion is easily confinned by experiment. (Plate XX. Fig. 261.) 



If a jet issue in an oblique or in a horizontal direction, its form will be 

 parabolic, since every particle tends, as a separate projectile, to describe the 

 same parabola in its range: and it may be demonstrated, that if it be emitted 

 horizontally from any part of the side of a vessel, standing on a horizontal 

 plane, and a circle be described, having the whole height of the fluid for its 

 diameter, the jet will reach the plane, at a distance from the vessel twice as 

 great as the distance of that point of the circle, through which it would have 

 passed, if it had continued to move horizontally. And if the jet rise in any 

 angle from the bottom of the vessel, the utmost height of its ascent will be 

 equal to that of the point in which it would meet the same semicircle, if it 

 continued to move in a right line, and the horizontal range will be equal to four 

 times the distance, intercepted between the same point and the side of the vessel. 

 This law is equally true with regard to pimple projectiles : but the experiment 

 is most conveniently exhibited in the motion of a jet. (Plate XX. Fig. 262.) 



