129 
THEORY OF THE DEW. 
Every one knows, or believes that he knows, what the Dew is ; but we believe, 
if the phenomena which attend it were strictly and impartially investigated, candour 
would require the admission that real knowledge is very superficial indeed ! Persons 
are not prone to inquire : — to do so requires trouble, and it is more facile to admit, 
than to investigate ; hence it has happened that Dr. Wells, a native of Charlestown, 
in South Carolina, (but who came to England in 1775, and studied as a physician,) 
is regarded as an authority — one from whose " Theory of the Dew " it is almost 
heretical to appeal. It is far from our desire to disparage the doctrine, or its ob- 
servant writer. Dr. Wells has advanced many unquestionable truths, which it is 
our duty to notice, in order to induce a spirit of research and enquiry among our 
horticultural amateurs. Loudon has done the same thing above twenty years ago, in 
the " Encyclopedia of Gardening," and at this time the subject has been revived by 
another author, whose opinion shall, in the course of this enquiry, be duly noticed. 
In .1788, Mr. Wilson, of Glasgow, addressed a paper to the Eoyal Society of 
Edinburgh, in which he suggested that " cold is occasioned by the formation of 
dew." Mr. Six, the inventor of the self-registering thermometer, which bears his 
name, observed "that the production of dew is accompanied with cold on the 
surface of the ground greater than in the atmosphere a few feet above it; the 
differences being frequently five, ten, or more degrees of Fahrenheit." This is true ; 
and to that difference we must ascribe some of the irregularities of our meteorological 
reports, because — let us select the aspect how one may, even if three thermometers 
are suspended above the ground, — and admitting that they agree to a fraction, 
other three instruments will all differ ; if one lie on the grass, another be 
suspended against a pole in the shade, and the third be attached to a wall, where 
a reflected sun heat shall in any degree strike it. It is never fair to let the in- 
strument lie on the grass, unless it be with the express object to determine the 
lowest degree upon the herbage during the night or early morning. Suspended 
thermometers always present returns the most equable : those which are otherwise 
situated exhibit the strongest contrasts. 
The Dew, whatever be the prime, actuating cause of its deposition, must depend 
upon the presence of watery vapour in the atmosphere ; and this being understood, 
we may now approach the theory of Dr. Wells. 
" During nights which are equally clear and calm, Dew often appears in unequal 
quantities, even after due allowance is made for their different lengths : thus it is 
more abundant shortly after rain than during a long season of dry weather. In 
particular localities it is generally more abundant duriug southerly and westerly 
winds than when they blow from the north and east. To the greater or less 
quantity of moisture in the atmosphere at the time of dewing — subject to the 
immediately exciting cause — is its copiousness or deficiency to be referred. 
VOL. XV. NO. CLXXIV. S 
