802 FOUNTAINS. [Lesson xxx. 



I shall describe to you only three, they being the 

 only ones which in my opinion carry with them 

 any degree of probability. 



The first hypothesis is, that Springs are owing 

 to rain and melted snow. The water penetrates the 

 earth till it meets with a soil, or stratum of earth, 

 of a nature sufficiently solid to sustain it, and pre- 

 vent it from descending lower in such minute 

 quantities. It then glides gently along in that 

 way which the stratum declines, and in its passage 

 meets with fresh quantities which have been fil- 

 tered through in the same manner : these gradually 

 descend together till they arrive at an aperture in the 

 surface, through which they escape and form a 

 spring, and perhaps the source of a brook or rivulet. 



Another hypothesis, so nearly allied to that just 

 mentioned that it is almost unnecessary to dis- 

 tinguish them in a work of this kind, is that of the 

 ingenious Dr. Halley. When this gentleman made 

 his celestial observations upon the tops of the 

 mountains at St. Helena, he found that the quan- 

 tity of vapour which fell there (even when the sky 

 was clear) was so great, that his observations were 

 thereby much impeded: his glasses were so covered 

 with water through the condensation of the Vapours, 

 that he was obliged to wipe them every ten minutes. 

 In reflecting upon this, he was led to suppose that 

 the water raised by evaporation from the seas and 

 large rivers might afford a sufficient supply for the 

 water discharged by Fountains. In order to deter- 

 mine, with some degree of accuracy, how much 

 water would be raised in vapour in any space of 



time, 



