92 Atmosphere. 



and from experiments it appears, that the 

 quantity of aqueous vapour contained 

 in the atmosphere is immense. Dr. 

 Halley, from an experiment on the eva- 

 poration from a fluid surface heated to 

 the same degree with that given by our 

 meridian sun, has calculated, that the 

 evaporation from the Mediterranean Sea 

 in a summer's day is 5280 millions of 

 tons of water, which is more than it re- 

 ceives from all the nine large rivers that 

 empty themselves into it. Dr. Watson, 

 in his Chemical Essays, has given an ac- 

 count of some experiments made with a 

 view to determine the quantity of the wa- 

 ter raised from the "earth itself alone in 

 time of drought. He informs us, that 

 when there had been no rain for above a 

 month, and the grass was become quite 

 brown and parched, the evaporation from 

 an acre was not less than 1600 gallons in 

 twenty-four hours. Making afterwards 

 two experiments, when the ground had 

 been wetted by a thunder-shower the day 

 before, the one gave 1973, the other 1905 

 gallons, in twelve hours. From this the 

 air is every moment purified by the 

 ascent of the vapour, which, flying off in- 



