INTRODUCTION TO HYDROSTATICS. Ixiii 



will not flow out, but remain suspended in the tube (Jig. 10) ; for there 

 is no pressure of the atmosphere above the liquid, while there is a 

 V in pressure from below upwards upon the open ends of the 

 tube ; and so long as this pressure is equal on both ends, 

 the liquid cannot flow out; but if the smallest inclination be 

 given to the syphon, so as to destroy the equilibrium of the 

 water, it will immediately flow from the lowest leg. When 

 syphons are used to draw off liquids, the legs are made of 

 unequal lengths, in order to render the pressure of the liquid 

 unequal ; the shortest leg is immersed in the cask, and the 

 liquor flows out through the longest. To accomplish this, it 

 is however necessary to make the liquor rise in the shortest 

 leg, and pass over the bended part of the tube, which is 

 higher than the level of the liquor in the cask. There are two modes of 

 doing this : one is, after immersing the shortest leg in the liquor to be 

 drawn off, to suck out the air of the tube from the orifice of the longest 

 leg ; then the liquor in the cask, which is exposed to the pressure of the 

 atmosphere, will be forced by it into the tube which is relieved from 

 pressure. As long as the tube continues full, no air can gain admittance ; 

 the liquor will therefore flow on till the cask is emptied. The other mode 

 is to fill the syphon with the liquor, then stopping the two ends with the 

 fingers, immerse the shortest leg in the vessel, and the same effect will 

 follow. In either case, the water in the highest part of the syphon must 

 not be more than 32 feet above the reservoir ; for the pressure of the 

 atmosphere will not support a greater height of water. 



The phenomena of springs which flow occasionally, and occasionally 

 cease, may often be explained by the principle of the syphon. The reservoir 

 of water which supplies a spring may be considered as the vessel of 

 liquor to be drawn off, and the duct the syphon, having its shortest leg 

 opening in the reservoir, and its longest at the surface of the earth whence 

 the spring flows ; but as the water cannot be made to rise in the syphon 

 by either of the artificial modes which we have mentioned, the spring will 

 not begin to flow till the water in the reservoir has risen above the level 

 of the highest part of the syphon : it will then commence flowing upon 

 the principle of the equilibrium of fluids ; but it will continue upon the 

 principle of the syphon; for, instead of ceasing as soon as the equilibrium 

 is restored, it will continue flowing as long as the opening of the duct is 

 in contact with the water in the reservoir. Springs which do not con- 

 stantly flow are called intermitting, and are occasioned by the reservoir 

 being imperfectly supplied. 



Reservoirs of water which are formed in the bosom of mountains gene- 

 rally find a vent either on their declivity, or in the valley beneath ; while 

 subterraneous reservoirs formed in a plain can seldom find a passage 

 to the surface of the earth, but remain concealed, unless discovered by 

 digging a well. When a spring once issues at the surface of the earth, it 

 forms a rivulet, and continues its course externally, seeking always 

 a lower ground, for it can no longer rise : if therefore it flows into 

 a- situation which is surrounded by a higher ground, its course is stopped, 

 the water accumulates, and forms a pool, pond, or lake, according to the 

 dimensions of the body of water. Thus the Lake of Geneva is filled by 

 the Rhone, which passes through it. When the river enters the valley 

 which forms the bed of the Lake, it finds itself surrounded by higher 

 grounds : its waters, consequently, are pent up, and accumulate till they 

 rise to a level with that part of the valley where the Rhone continues its 



