140 PHILOSOPHICAL TRANSACTIONS. [aNNO 1/77. 



surface of water in 1774. Consequently the dryness of the atmosphere, or its 

 power of solution, during the year 1775} exceeded that of 1774. 



2. If we take the medium of 4 years observations, it appears, that the 

 annual evaporation at Liverpool amounts to 36.78 inches. Dr. Halley observed 

 at London, that water placed in a close room, where neither the winds nor sun 

 could act on it, exhaled only 8 inches during the whole year. He makes no 

 doubt but that the free access of the \\ inds would have trebled the quantity carried 

 away ; and that this again would have been doubled by the assistance of the sun. 

 Dr. Halley therefore fixes the annual evaporation of London at 4 8 inches. If 

 this calculation be admitted, it follows, that the annual evaporation of London 

 exceeds the annual evaporation of Liverpool 1 1 inches ; but were the experi- 

 ments to be made in London, in the same circumstances with those made at 

 Liverpool, it is probable that this would be found to be more than the real 

 difference. ' ' , 



The learned Cruquius observed at Delft in Holland, that there exhaled from 

 water set in the open air, but in a calm and shady place, about 30 inches ; and 

 it is not to be doubted, says Dr. Brownrigg, in the Art of making common 

 Salt, but that double this quantity, or 6o inches, would have exhaled, had it 

 been placed where the sun and winds could have had their due effects. In 

 another part of this publication. Dr. Brownrigg fixes the evaporation of some 

 parts of England at 73.8 inches during the 4 summer months, May, June, 

 July, and August ; and the evaporation of the whole year at upwards of 140 

 inches. These are calculations however which do not appear to correspond with 

 experience; for the whole evaporation at Liverpool, instead of 140 inches, was 

 only 36.78 inches. And the evaporation of the 4 summer months, on a medium 

 of 4 years, instead of 73 inches, was only 18.88 inches. 



3. Dr. Hales calculates the greatest annual evaporation from the surface of the 

 earth in England, even that from a surface of hop-ground, at 6.G6 inches. If 

 we compare this with the annual evaporation from a surface of water as deter- 

 mined by experiment, we find, that the latter exceeds the former about 30 

 inches ; and that the annual evaporation from a surface of water, is to the 

 annual evaporation from the surface of the earth in this part of England, 

 nearly as 36 to 6, or as 6 to 1. 



4. On comparing the depth of rain with the annual evaporation of this part of 

 Lancashire, we find that more falls in rain than is raised in vapour, even though 

 the whole were a surface of water; for the rain is to the evaporation as 37-43 

 inches to 36.78 inches: and we further find, that tlie quantity exhaled from the 

 surface of the earth, is little more than a 6th part of what descends in rain ; we 

 must therefore have very large supplies from other regions, from the surrounding 

 sea, and from the ocean of warmer climates. Hence we see, why our south 



