THE EAllTII. 139 



But upon computing the quantity which passed through the 

 arches of one of its bridges in a yesu", it was found to amount 

 only to two hundred and eighty millions of cubic feet, which is 

 not above the sixth part of the former number. Kence it ap- 

 pears, that this river may receive a supply, brought to it by the 

 evaporated waters of the sea, by its current; and, therefore, 

 evaporation is more than sufficient for maintaining the greatest 

 rivers, and supplying the purposes also of vegetation.* 



• Tlie property which water has of evaporating spontaneously at all tem 

 peratures, is one of the most important in the whole economy of nature. Foi 

 upon it the growth of plants, aid the existence of living creatures upon the 

 oarth, depends. The vapours thus continually rising, not merely from the 

 surface of the sea, lakes, and rivers, but also from the dry land, are again 

 Condensed, and fall in the state of rain or dew. The rain penetrates into the 

 earth, and makes its way out again in springs. These Collecting together con. 

 stitute rivers, wliich making their way to the sea, afford the means of living 

 and enjoyment to numerous tribes and languages which occupy their banks. 

 Let us suppose for a moment that this spontaneous evaporation were to 

 rease, and let us contemplate the consequences. No more rain or dew could 

 fall, the springs would cease to flow, the rivers would be dried up ; the whole 

 water in the globe would be accumulated in the ocean ; the earth would be- 

 come dry and parched; vegetables being deprived of moisture, could no 

 longer continue to grow ; the cattle and beasts of every kind would ladt 

 their usual food ; man himself would perish ; the earth would become a dull, 

 inanimate, sterile mass, without any vegetables to embellish its surface, or 

 any living creature to wander through its frightful deserts. 



If the atmosphere contained no vapour whatever, the annual evaporation 

 from the surface of water could easily be determined, provided we were ac- 

 quainted with the mean temperature of the place. But as the atmosphere 

 is never free from vapour, we must either determine the mean quantity pre- 

 sent by trial, or determine the actual evaporation by experiment. Now as 

 far as evaporation is concerned, the surface of the globe presents three prin- 

 i-ipal varieties ; namely, water, ground covered with grass or other vegetables, 

 and bare soil. 



Dr Dobson made j. set of experiments during the years 1772, 1773, 177 1 

 and 1775, to determine the evaporation from the surface of water at IJver- 

 pool during these years. He took a cylindrical vessel of twelve inches dia- 

 meter, and having nearly filled it with water, exposed it beside a rain gau?e 

 r.f the same aperture, and by adding water, or removing it occasionally, he 

 Kept the surface at nearly the same height. By carefully registering the 

 •.uantities added or taken away, and comparing them with the rain that fell, 

 the amount of evaporation was ascertained. The mean annual evaporation 

 from the surface of water at Liverpool amounted to 36.37 inches. The mean 

 annual fall of rain at Liverpool, as ascertained by Dr Dobson, is (without 

 reckoning the dew) 37.48 inches. We see at once firom this that more raia 

 falls at Liverpool than can be accounted for by the evaporation. Consequent, 

 by there must be a supply of vapour from the sea, and jirobably from the 

 ivarmcr regions of the globe. 



