402 ON SIPHONS AND JETS OF WATf.R. 



the orifice, will unite in producing a vacuity in the centre ; and 

 when this happens, the discharge is considerably diminished. 



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 exhausted, as far as the level of its upper orifice. And 

 from this circumstance, the phenomena of lome intermitting springs 

 have been explained, which only begin to run, when the reservoirs 

 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 reservoirs, a great 

 number of alternations may sometimes be produced. 



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

 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 confirmed by experiment. 



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 demon- 

 strated, 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 simicircle, 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 simple projectiles : but the 

 experiment is most conveniently exhibited in the motion of a jet. 



\Ve have hitherto considered the motions of fluids as continued 

 principally in the same direction ; but they are frequently subjected 

 to alternations of motion, which bear a considerable analogy to the 



