SIPHOX. 



179 



flow. Therefore, if the liquid "be, water, the height, 

 ab, must be less than 34- feet ; if it be mercury, db 

 must be less than the mercury column of the barometer. 



3O1. Intermittent Springs. Occasionally a 

 spring is found which flows freely for a time, and then 

 Deases to flow for a time. Fig. 117 represents an under- 

 ground reservoir, fed with water through fissures in the 

 earth. The channel through which the water escapes 



FIG. 117. 



FIG. 118. 



from this reservoir forms a siphon. The water escaping at 

 the surface constitutes a spring. When the water in the 

 reservoir reaches the level of the highest point in the 

 channel, the siphon begins to act, and continues to do so 

 until the water level in the reservoir falls to the mouth of 

 the siphon. The spring then ceases to flow until the 

 water has regained the level of the highest point of the 

 siphon-like channel. This action is well illustrated by 

 Tantalus' Cup," represented in Fig. 118. 



EXERCISES. 



1. How high can water be raised by & perfect lifting-pump, when 

 the barometer stands at 30 inches ? (See 253, [2].) 



