130 
On the Influence of Water 
stances : hence, again, the advantage of drainage. These im- 
portant processes, viz., the absorption of moisture, and the radiation 
of heat, will be carried on with more or less energy in proportion 
to the inherent qualities of a soil, to its state of mechanical pre- 
paration, and to the proper adjustment of its supply of water. 
Cause and Physical Action of Dew. 
The quantity of moisture attracted from the atmosphere, in the 
form of dew, is unknown ; but the cause, and many of the laws of 
its formation, deposition, and physical action, have been disclosed 
to us by the talents and labours of Dr. Wells, whose experiments 
and Essay on this subject stand almost unrivalled in the records 
of science, as examples of skilful investigation and profound in- 
duction.* Previously to the conclusive experiments of 4his admir- 
able philosopher, the formation of dew was held to be the ca\isc 
of the cold observed with it, and he originally entertained the 
same opinion. 
" But," he observes, " I began to see reason, not long after my re- 
gular course of experiments commenced, to doubt its truth, as I found 
that bodies would sometimes become colder than the air, without being 
dewed ; and that, when dew was formed, if different times were com- 
pared, its quantity, and the degree of cold which appeared with it, were 
very far from being always in the same proportion to each other. The 
frequent recurrence of such observations at length converted the doubt 
of the justness of my ancient opinion into a conviction of its error, and 
at the same time occasioned me to conclude that dew is the production 
of a preceding cold in the substances on which it appears." 
Further — 
" that the cold which produces dew is itself produced by the radiation of 
heat from those bodies upon which dew is deposited." 
Thus it was discovered that an effect had heretofore been mis- 
taken for a cause ; and the explanation of the various phenomena 
connected with the subject, afforded by this theory, has since 
remained unchallenged, and is admitted to be incontrovertible. 
Besides the determination of the immediate cause of dew. Dr. 
* The original ' Essay on Dew,' which appeared in 1814, is very scarce, 
but is republished in the ' Works of Dr. Wells' (1818), containing a memoir 
of his life, written by himself. 
A distinguished living philosopher thus writes of this theory, after 
making a concise but searching analysis of it : — " We have ])urpose]y se- 
lected this theory of dew, first developed by the late Dr. Wells, as one of 
the most beautiful specimens of inductive experimental intjuiry lying 
within a moderate compass. It is not possible, in so brief a space, to do it 
justice ; but we earnestly recommend liis work (a short and entertaining 
one) for perusal to the student of natural philosophy, as a model with 
which he will do well to become familiar." — Discourse on the Study of 
Natural Plulosophij, by Sir J. F. Ilerschel, 1832, p. 1G3. 
