106 Daniell on Meteorological Observations. 



12°, and that of the vapour only about 9°. We accordingly 

 find that the dryness falls to 4°, which again diminishes, though 

 at a less rate, from autumn to winter. The exact correspondence 

 of the consequences, and their precise march, we cannot expect 

 to ascertain, but from the average of a series of years ; but 

 enough, I think, is apparent, to enable us to speculate a little 

 upon their causes. The sea, and other deep bodies of water, 

 are, no doubt, the grand reservoirs from whence is supplied the 

 greater part of the vapour of the atmosphere ; and the cooling 

 and heating of such bodies are regulated by particular laws. 

 It is a well known fact, that the heat of water cannot be raised 

 above 40°, till every particle of the mass, however deep, has 

 attained that temperature ; but that beyond this, the superficial 

 water may be heated, without affecting the lower stratum. So 

 in cooling, the whole volume must be cooled down to 40° before 

 the superficial water can fall below that point. Now the Spring 

 quarter, from March to May, includes, probably, the period 

 when the whole body of water, in this climate^of the globe, rises 

 to the point of 40°. A large portion of heat must, therefore, 

 be expended, in warming its entire bulk ; and, consequently, 

 its increase cannot keep pace with the surrounding air. In the 

 next quarter, however, having attained its maximum of density, 

 the whole heat is expended upon the superficial water alone ; 

 and the increase of heat in the upper stratum of water, and 

 consequently, in the vapour, is commensurate with that of the 

 air. The autumn again includes the point at which the whole 

 mass of heated water must fall to 40° ; which point being 

 passed, the cooling of the upper stratum keeps pace with the 

 cooling of the air. Daily observations upon the superficial 

 temperature of the sea would greatly tend to elucidate this 

 subject. 



The rate of evaporation is in compound proportion to the 

 heat of the water yielding the vapour, the degree of dryness, 

 and the velocity of the wind. In the calculation taken in the 

 tables, the temperature of the air has been always taken as the 

 temperature of the evaporating fluid ; whereas it is obvious that 

 this can seldom, if ever, be the true state of the case. Did we but 

 know the temperature of the sea at different hours of the day, 



