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Art. IY.—O/i the Formation of Dew . By Henry Home 
Blackadder, Esq., Staff Assistant- Surgeon, 
Sir, 
Having for many years had my attention directed to sub- 
jects connected with meteorology, and having been led first to 
doubt the soundness of certain opinions, either generally received 
or confidently advanced, on that subject, and ultimately to form 
others of an opposite nature, I am induced to request your at- 
tention to a few general remarks on the spontaneous evaporation 
and condensation of moisture. It is impossible that, in the com- 
pass of a letter, such a subject can be noticed otherwise than in 
a very cursory manner. I have therefore to make the additional 
request, that you will consider what I may at present advance, 
merely as a few extracts from numerous notes on meteorological 
subjects, and not arranged, perhaps, with all the attention that 
might be wished. 
You are aware, that, at present, the general opinion is, that 
the decrement of heat, which is observed to precede the forma- 
tion of dew, and which takes place, on certain occasions, at the 
surface of snow, &c., cannot be accounted for on the principle 
of evaporation ; or, in other words, that the diminished tempera- 
ture observed on such occasions, cannot be the consequence of 
that absorption of heat which takes place during the conversion 
of moisture into the state of an elastic fluid or vapour. You are 
also aware, that the depression of temperature in question has 
by some been attributed to the radiation of heat into regions of 
empty space, — and, by others, to “ cold pulses showered down 
from the atmosphere. 1 "’ By the respective supporters of each of 
these hypotheses, the influence of evaporation seems to be alto- 
gether excluded : and is not this circumstance alone sufficient 
at least to place one on his guard ? 
In the course of the following remarks, reference has been 
made to facts, which, it is hoped, have been well ascertained ; 
and on these chiefly the reasoning is founded. Those who may 
be disposed to call these facts in question, have almost constant 
opportunities of either verifying or disproving them. A more 
minute detail, therefore, seems at present to be unnecessary, 
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