Mr Blackadder on the Formation of Dew. 6 $ 
lure of the air a few inches from the surface was from 8 ° to 10 % 
and at the height of 24 feet about 14° higher than that of the 
snow. 44 This excessive cold,” we are told, “ was evidently not 
occasioned by evaporation, for, on blowing with bellows against 
the bulb when it lay on the snow, so far from sinking more, the 
mercury actually rose two degrees higher than its station in the 
air.” If the gentleman who starts this objection give the sub- 
ject a little farther consideration, he will find that the evidence 
is not so clear as he imagines. In the first place, the air which 
entered at the valve of the bellows was doubtless 8 ° or 10 °, more 
or less, warmer than the snowy surface. In the second place, 
if he will take the trouble of repeating the experiment on a fit- 
ting occasion, and take the precaution of having a thermometer 
inserted into the bellows, through a perforation made in the up- 
per board, near the muzzle, he will find no difficulty in discover- 
ing from whence the additional measure of heat was derived 5 
nor any thing remarkable in the circumstance that a thermome- 
ter should indicate an increase of temperature, at a time when a 
current of relatively warm air is directed against its bulb. 
Not only, however, is evaporation the immediate cause of the 
cold which determines the formation of dew and hoar-frost, and 
which reduces the temperature of the surface of snow, but is 
also frequently the chief agent in that absorption of heat which 
induces congelation ; whether that be exhibited in the freezing 
of the earthy mould, or in the solidification of water. In this 
country, congelation never goes forward at the surface of the 
earth when the air is in a state of saturation. 
It would be a curious, though not very useful investigation,, 
to endeavour to estimate how much the temperature of the air 
is influenced by evaporation, or, what the temperature of the 
earth and atmosphere would be, if that process had no existence 
in nature. Air having its density increased, by parting with a 
portion of its heat at the surface of the earth, cannot easily ac- 
quire a greater elevation. Currents of wind may have some in- 
fluence, and, in a calm state ot the air, the internal commotion 
excited by the vaporific process has an evident effect. On such 
nights, when the refrigerating influence of evaporation is most 
operative, the temperature of the air at the height of 800 feet is 
