IN SPRINGS AND ARTESIAN WEILS. 441 



water; for the pipe let down into the lower stratum will not allow 

 the impure water from above to mix with the pure ascending from 

 below. Water from two different strata may thus be brought to the 

 surface by one borehole of a sufficient size to contain a double pipe, 

 viz. a smaller pipe included within a larger one, with an interval 

 between them for the passage of the water. The smaller pipe may 

 thus discharge the water of the lower, and the larger pipe that of the 

 upper stratum ; for in either case the fluid is but endeavouring to 

 regain the level at its feeding source on the surface of the earth. 

 Fountains of this sort Artesian wells are very well known on the 

 eastern coast of Lincolnshire by the name of blow wells. This district 

 is low, covered by clay between the wolds of chalk near Louth and 

 the sea shore ; and by boring through the clay to the subjacent chalk, 

 a spring is found that yields a perpetual jet, rising several feet above 

 the surface. But wells of this kind are common in many parts of the 

 world ; in the neighbourhood of London ; in Artois, Perpignan, Tours, 

 Roussillon, and Alsace, in France ; in some parts of Germany ; in the 

 duchy of Modena ; in Holland, China, and North America. 



565. But whence come those vast issues of fresh water that sometimes 

 rise up in the sea, as in the Mediterranean near Genoa, and in the 

 Persian Gulf, where the ascending volume is so vast as to allow 

 mariners in the one case, and divers in the other, to water ships ? 

 Springs such as these are the issues of subterranean rivers, all of 

 which consist of meteoric water, or that which the atmosphere had 

 transferred to itself from the ocean, distilled and discharged upon the 

 undulating surface of the earth. 



566. The annual fall of rain between the tropics is about ten feet 

 in depth ; and estimating this in other countries as nearly propor- 

 tional to the cosine of the latitude, the quantity of moisture ex- 

 haled in a year, over the surface of the globe of our earth, would 

 form a sheet of water five feet deep ; therefore the number of cubic 

 feet of water turned into vapour, and dispersed through the mass of 

 the atmosphere every minute, would be 5x10,424,000,000, or fifty- 

 two thousand one hundred and twenty millions. But this enormous 

 mass Leslie further multiplies by 18,000, the mean height* of the 

 atmosphere in feet, and again by 62 J, the weight in pounds avoir- 



* In taking 18,000 feet as the mean height of the atmosphere, we have followed 

 Leslie ; but the mean height is 27,800 feet in round numbers, for air is to water as 

 1 to 1000; therefore we have 1 : 1000 : : 34 : 27,818 feet for the height of the 

 cloud sustaining atmosphere j that is to say, there are no clouds carried higher than 

 five miles. 



