Ixii INTRODUCTION TO HYDROSTATICS. 



in its progress, meets with other rivulets of a similar description, and they 

 pursue their course together in the interior of the earth, till they are 

 stopped by some substance which they cannot penetrate ; for though 

 we have said that water under strong compression penetrates the pores 

 of gold, when acted upon by no other force than gravity it cannot make 

 its way even through a stratum of clay. This species of earth, though not 

 remarkably dense, being of great tenacity, will not admit the passage of 

 water. When, therefore, it encounters any substance of this nature, its 

 progress is stopped, and the pressure of the accumulating waters forms a 

 bed, or reservoir. 



Fig. 9 represents a section of the interior of a hill or mountain. A is a 

 body of water such as I have described, which, when filled up as high as 

 B (by the continual accession of waters it receives from the ducts or 



Fig 9. 



rivulets , a, a, ), finds a passage out of the cavity ; and, impelled by 

 gravity, runs on, till it makes its way out of the ground at the side of the 

 hill, and there forms a spring, C. The spring, during its passage from 

 B to C, rises occasionally, upon the same principle that water rises in the 

 spout of a tea-pot, but it cannot mount above the level of the reservoir, 

 whence it issues ; it must therefore find a passage to some part of the 

 surface of the earth that is lower or nearer the centre than the reservoir. 

 Water may thus be conveyed to every part of a town, and even to the 

 upper stories of the houses, provided that it be originally brought from a 

 height superior to any to which it is conveyed. 



Reservoirs of water are seldom formed near the summit of a hill, for in 

 such elevated situations there can scarcely be a sufficient number of rills 

 to supply one ; and without a reservoir there can be no spring. In such 

 situations, therefore, it is necessary to dig deep wells, in order to meet 

 with a spring ; and then it can rise in the well only as high as the 

 reservoir whence it flows. 



When reservoirs of water are formed in very elevated situations, the 

 springs which feed it descend from higher hills in the vicinity. There is 

 a lake on the very summit of Mount Cenis, which is supplied by the springs 

 of the higher Alps surrounding it. 



A syphon is an instrument commonly used to draw off liquids from 

 large casks or other vessels which cannot be easily moved. It consists 

 simply of a bended tube. If its two legs are of equal length, and filled 

 with liquid, if held perfectly level, though turned downwards, the liquid 



