4 SPRINGS, RIVERS, CANALS, LAKES, 



that point, even a moment before their formation. They are not 

 formed then, because a greater quantity of vapour had reached the 

 atmosphere than could remain there without passing its maximum. 

 It is still more remarkable, that when the clouds are formed, the 

 temperature of the spot in which they are formed is not always 

 lowered, though this is sometimes the case. On the contrary, the 

 heat of the clouds themselves is sometimes greater than that of the 

 surrounding air. Neither then is the formation of clouds owing to 

 the capacity of air for combining with moisture being lessened by 

 cold : so far from this, indeed, we often see clouds which had re- 

 mained in the atmosphere during the heat of the day disappear in 

 the night after the heat of the air has diminished. And hence the 

 formation of clouds and rain, from which rivers are so generally 

 supposed to proceed, are themselves not to be accounted for upon 

 any principles with which we are acquainted. 



It is a very remarkable fact, that evaporation often goes on for a 

 month together in hot weather without any rain. This occasionally 

 occurs in our own country ; and takes place every year in the torrid 

 zone. Thus at Calcutta, during January 1785, it never rained at 

 all : the mean of the thermometer for the whole month was 665 

 degrees: there was no high wind, and indeed during great part of 

 the month little wind at all. And this is also a fact that it is im- 

 possible for us to account for. The enquiry therefore is involved in 

 great difficulty. In the beginning of the late century, the phi- 

 losophical world was agitated by a variety of opinions upon the 

 subject. One party contended strongly for the existence of a large 

 mass of water within the bowels of the earth, which supplied not 

 only the rivers but the ocean itself; at the head of these we may 

 place the ingenious but fanciful Burnet. The French philosophers, 

 on the contrary, asserted, that the waters of the ocean were con- 

 veyed back by some subterraneous passages to the land, and being 

 filtrated in their passage, returned again to the sea in the course of 

 the rivers ; but this opinion appears contrary to all the known prin- 

 ciple of hydrostatics. 



It was in opposition to these hypotheses, that our illustrious 

 countryman Halley contended for the process of evaporation, and 

 maintained that the immense 'deposition of water in consequence of 

 ?t, is fuily adequate to the whole supply* 



