CATARACTS AND INUNDATIONS. 5 



The experiment upon which he chiefly depended was the follow- 

 ing. He took a vessel of water, made of the same degree of salt- 

 ness as-the sea, which he ascertained by an hydrometer; and hav- 

 ing placed a thermometer in it, he brought it, by a chaffing-dish to 

 the heat of the air in the hottest summer. He then placed this 

 vessel, with the thermometer in it, in one scale, and nicely counter- 

 poised it with weights in the other. After two hours, he found that 

 about the sixtieth part of an inch had escaped in vapour, and con- 

 sequently, in ten hours, the length of a natural day, that one tenth 

 of an inch would have been evaporated. From this experiment it 

 should follow that every ten square inches of the surface of the 

 water yield a cubic inch of water hi vapour per day, every 

 square mile 6,914 tons, and every square degree (or 69 Eng- 

 lish miles) 33 millions of tons. Now if we suppose the Mediter- 

 ranean to be 40 degrees long, and 4 broad at a medium, which is 

 the least tlyat can be supposed, its surface will be 160 square de- 

 grees, whence there will evaporate 3280 millions of tons per day 

 in the summer time. The Mediterranean receives water from the 

 nine following great rivers, the Iberus, the Rhine, the Tiber, the 

 Po, the Danube, the Neister, the Boristhenes, the Tnais and the 

 Nile ; the other rivers that empty themselves into it being compara- 

 tively small, and their water inconsiderable. Now let us suppose 

 that each of these rivers conveys ten times as much water to the 

 sea as the Thames ; which is calculated to yield daily 76,032,000 

 cubic feet, equal to 320 millions of tons, which is little more than 

 one third of the quantity evaporated every day from the same sea : 

 the remainder being perhaps allotted to rains, which fall again into 

 different seas, after having served the purposes of vegetation. It is 

 highly probable, however, that by some means or other, a kind of 

 circulation is carried on through all nature; and that the sea re- 

 ceives back again, through the channel of the rivers, that water 

 which it parts with to the atmosphere. 



All rivers have their source either in mountains, or elevated lakes; 

 and it is in their descent from these, that they acquire that velocity 

 which maintains their future current. At first their course is gene- 

 rally rapid and headlong ; but it is retarded in its journey by the 

 continual friction against its banks, by the many obstacles it meets 

 to divert its stream, and by the plane's generally becoming more 

 level as it approaches towards the sea. 



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