64 Mr Black adder on the Formation of Dew . 
probably but seldom, if at all, affected by that of the surface ; 
and at that height, it is found, on ordinary occasions, to be about 
one degree lower than at the level of the sea. But, we cannot 
infer that the temperature of the air at that height, or at a height 
beyond the cooling influence of the ground, would be the same 
on a calm evening, if no evaporation existed. Though the va- 
porific process may not reduce the temperature of the earth’s 
surface during the day, it is the means of preventing an im- 
mense accumulation of heat. Thus, in the deserts of Arabia, 
where evaporation is comparatively null, the temperature of the 
air in the shade has been observed as high as 120°; and even 
in this country, and at sea, the deck of a ship often acquires a 
very intense degree of heat. What, then, might we expect the 
temperature of the earth and atmosphere to be, if evaporation 
nowhere and at no time existed ! 
Had the preceding remarks been less extended, it might 
now be shewn, that all the objections hitherto made to the 
influence of evaporation in producing that decrement of heat so 
commonly observed in connection with the formation of dew, 
and on like occasions, can be readily and completely obviated. 
The peculiar phenomena that have been observed upon glass, 
metals, and other substances, when similarly exposed ; and the 
different results that are derived from a difference in the posi- 
tion, &c. of the same substance, may all be satisfactorily explain- 
ed on the same principle : that is, that the absorption of heat 
which takes place in the conversion of moisture into the form of 
vapour, is the means by which the depressed temperature in 
question is brought about. 
The explanation of these, however, and other interesting 
points, will be given at a more convenient opportunity, when 
the whole subject may be brought under more particular com. 
sideration. — I am, &c. 
To Professor Jameson, 
S51. Royal Circus. 
