Origin of Springs* $25 



have heard of as rifing on the tops of hills, almoft all 

 have been found, on examination, to iflue many feet be- 

 lovv the real apex. Were the fact otherwife, it would not 

 be inconfiftent with the Theory.— In ftiff clayed grounds 

 water may work its way a very confiderable diftance, 

 before it finds an outlet. Springs, in wet lands, fhould 

 be expected often to ilTue feveral miles from the refer- 

 voir. " And,'' in the language of Hutton, " if there 

 happen to be a valley, between a mountain on whofe 

 top is a fpring and the mountain which is to furnifh 

 it with water ; the Spring muft be confidered as water 

 conduced from a refervoir of a certain height, through 

 a fubterranean channel, to make a jet of an almoft equal 

 height." 



It is alfo faid, that fome fprings are not at all affeded 

 by drought. This may be owing to the capacity of the 

 reservoir ; and to the number of duels which fupply it, 

 and to the fmallnefs of the drain. 



But the principal objeftion is the infufficiency of va- 

 pour to fupply the demands of fprings and rivers. 



Dr. Halley tried the following experiment, to de- 

 ^"termine the actual evaporation from the Mediterranean j 

 so far as it is occalioned by heat. He filled a bafon 

 with brine, as fait as that of the Ocean ; and heated it, 

 over a pan of coals, to the temperature of the air in fum- 

 mer. By a careful examination he found, that the 

 quantity loft by evaporation was a tenth of an inch in 

 12 hours. He fuppofes the Mediterranean to be 40'' 

 long, and 4*^ broad ; making a furface of 160 fquare de- 

 grees. According to the experiment, therefore, it will 

 lofe 5,280,000,000 tons of water in a day. The Medit- 

 erranean receives the waters of the following confidera- 

 ble rivers ; the Ebro, the Rhone, the Tyber, the Po, the 

 Danube, the Neifter, the Neiper, the Don, and the 

 Nile. Dpv. Halley fuppofed, that, on an average, each 

 of thefe yields ten times as much water as the Thames ; 

 whereby he allowed for fmaller rivers which fall into the 

 fame fea. From an ins^enious meni'uration, he conclu- 

 ded, that the Thames difcharges daily 20,300,000 tons. 



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