CATARACTS AND INUNDATIONS. 3 



the mean of which two experiments is 0.084 inches, amounting for 

 the whole month of June to 2 62 inches. If we suppose this to 

 bear the same proportion to the whole year, that the evaporation in 

 Dr. Dobson's experiments for June do to the annual evaporation, 

 we shall obtain an annual evaporation amounting to about 22 

 inches; which is much smaller than the average obtained by Mr. 

 Williams. 



Mr. Dalton and Mr. Hoyle have offered us experiments still more 

 correctly conducted. They took place in the vicinity of Manchester 

 during 1796, au d the two succeeding years: and according to these 

 experiments the quantity of vapour raised in that quarter annually is 

 about 25 inches ; and if to this we add five inches for the dew, it will 

 make the average evaporation for the year 30 inches. Now if we 

 consider the situation of England, and the greater quantity of va- 

 pour usually admitted to be raised from water, it will not surely be 

 considered as too great an allowance if we estimate the mean annual 

 evaporation over the whole surface of the globe -at 115 inches. But 

 35 inches from every square inch on the superficies of the earth 

 make 94,4.50 cubic miles, equal to the water annually evaporated 

 over the whole globe. 



This may be a quantity altogether sufficient for the formation 

 and supply. of those immense masses of water which {^institute the 

 largest of those rivers which we shall presently nc.tice in their order. 

 But by what means is this prodigious expanse of vapour converted 

 into rain, in which form alone it can generate rivers, if it generate 

 them at all ? 



Rain never begins to fall while the air is transparent : the invi- 

 sible vapours first pass their maximum, and are changed into vesi. 

 cular vapours; clouds are formed, and these clouds are gradually 

 dissolved in rain. But clouds are not formed in all parts of the 

 horizon at once; the formation begins at one particular spot, while 

 the rest of the air remains clear as before: the first cloud rapidly 

 increases till it overspreads the whole horizon, and the 'rain then 

 commences. Now it is remarkable, that though the greatest quan % 

 tity of vapour exists in the lower strata of the atmosphere, clouds 

 never begin to form there, but always at some considerable height, 

 It is remarkable too, that, the part of atmosphere at which they 

 form has not arrived at the point of extreme moisture, ner near 



