62 
Mr Blackadder on the Formation of Dew. 
snow is different from rain. We learn from observation, that 
evaporation always accompanies the formation of hoar-frost ; 
and when dew is forming on low plains, hoar-frost is often form- 
ed at the same instant, and at a small distance, on parts of the 
earth’s surface having a general elevation of a few hundred 
feet. 
Next to the formation of hoar-frost, that reduction of tempe- 
rature which has been observed to take place at the surface of 
snow, may perhaps be considered as most closely allied to the 
cold that takes place previous to the formation of dew. On a 
clear night, the surface of snow has been observed to be even as 
much as 16° below that of the air, and this degree of cold ex- 
tended but a small way below the surface. So great a diffe- 
rence between the temperatures of the surface of snow and of 
the air is not of frequent occurrence ; but a difference of seve- 
ral degrees is far from being a rare phenomenon ; and whenever 
it occurred, the air was never in a state of saturation, — evapora- 
tion was always found to be going forward. If, therefore, it be 
well known that evaporation is capable of producing very in- 
tense degrees of cold, and if we find a solid body whose surface 
is capable of supporting evaporation, greatly reduced in tempe- 
rature, at a time when that process is going forward at its sur- 
face, are we not entitled to conclude that they are in the rela- 
tion of cause and effect ? And, would it not be highly unphi- 
losophical, in such a case, to admit the existence of other hypo- 
thetical causes, even should they merit the title of ingenious or 
plausible ?— though, by the way, a much more applicable term 
might readily be selected. 
A theoretical objection, indeed, has been made to the fact of 
evaporation being the cause of the reduction of temperature ob- 
served at the surface of snow *. Reference is made to the ob- 
servations and experiments made at Glasgow by Dr Wilson, in 
the year 1780, as detailed in the London Philosophical Trans- 
actions. That gentleman found the temperature of the surface 
of snow on a clear night to be 23° below zero. The tempera- 
See Suppl % to the Encydop. Brit . vol. iii. p. 555. 
