J138 PHILOSOPHICAL TRANSACTIONS. [aNN0 1777. 



climate. There may be little or even no rain, and yet the air be constantly 

 damp and foggy ; or there may be heavy rains, with a comparatively dry state of 

 the atmosphere. The same depth of rain will likewise produce different effects 

 on the air, according as it falls on a flat or hilly country ; for large quantities 

 soon quit the hills or high grounds, while smaller quantities have more lasting 

 and powerful effects on a fiat country. Much also depends on the nature of the 

 soil, whether clay or sand, whether firm and compact, or loose and spongy. Is 

 not evaporation then a more accurate test of the moisture or dryness of the at- 

 mosphere, than the quantity of rain ? 



It is well known, that air is an active solvent of water, and that its powers of 

 solution are in proportion to its dryness. It is also well known, that in chemi- 

 cal solutions, the action of the menstruum is greatly promoted by heat and agi- 

 tation. If the temperature of the air then, and the state of the winds, be as- 

 certained, which in the present case denote the heat and agitation of the men- 

 struum, the evaporation will be the true index of the dryness of any particular 

 season, situation, or climate. 



To determine the annual evaporation in the neighbourhood of Liverpool, Dr. 

 D. procured 2 well varnished tin vessels ; one of which was to serve the purpose 

 of a rain-gage ; the other to be employed as an evaporating vessel. The evapo- 

 rating vessel was cylindrical, 12 inches in diameter, and 6 inches deep. The 

 rain-gage consisted of a funnel also 12 inches in diameter, the lower end of 

 which was received into the mouth of a large stone bottle ; and, to prevent any 

 evaporation from the bottle, the pipe of the funnel was stopped with a grooved 

 cork. These vessels were placed in the middle of a grass-plot, on a rising 

 ground adjoining, and immediately overlooking the town, about 75 feet above 

 the level of the sea, and with a free exposure to the sun, winds, and rain. 

 The cylindrical vessel was filled with water within 2 inches of the top ; and if, in 

 consequence of heavy rains, there was danger of its overflowing, a quantity of 

 water was taken out ; but if, in consequence of long drought, it sunk lower, a 

 quantity of water was then occasionally added ; and these additions or subtrac- 

 tions were carefully registered. At the end of every month, the depth of rain 

 was first calculated ; and, as each vessel received the same depth of rain, he had 

 only to examine the quantity of water which had been added to, or taken out of 

 the evaporating vessel, and the evaporation of the month was ascertained. 



The 1st column of the following tables points out the mean temperature of 

 the air at 2 in the afternoon. The 2d, the character of the month with respect 

 to the winds, the number of dots expressing their strength ; and, to make this 

 part tolerably accurate, daily observations on the winds were marked down, and 

 the character of the month formed from a general survey of these observations : 

 our winds are westerly for near two-thirds of the year. The 3d column points 



