153 
THEORY OF THE DEW. 
{Continued from page 132.) 
It should appear, from the third and latest letter of M. Melloni, that the 
editors of periodicals who inserted the series, had mistaken the writer's object. 
Certain it is that, judging from the style of his first epistle, we ourselves concluded 
that M. Melloni was perfectly satisfied with the soundness of Dr. Wells's theory, 
and therefore had adduced further experiments, in order to " make assurance doubly 
sure." It now, however, appears that such was not the case ; we shall therefore 
recur to Dr. Wells himself, in order to put the reader in possession of several 
interesting facts that were not alluded to in the last article. 
1. " After a long period of drought, when the air was still and the sky serene, 
at twenty-eight minutes before sunset, parcels of wool and swansdown, previously 
weighed, were exposed to the sky upon a smooth, unpainted, and perfectly dry fir 
table, five feet long, three broad, and nearly three feet high, which had been placed 
an hour before in the sunshine, in a large grass field. The wool, twelve minutes 
after sunset, was found to be 14° colder than the air, but it had acquired no weight," 
(hence, was not wetted with dew). " The swansdown, much greater in quantity than 
the wool, was at the same time 13° colder than the air, and was also without any 
additional weight. In twenty minutes more the swansdown was 14^° colder than 
the neighbouring air, and was still without additional weight. At the same time the 
grass was 15° colder than the air, at four feet above the ground." 
2. "A very slight covering will exclude much cold." Every gardener can bring 
this asserted fact to the test, and we urge it upon him so to do, in order to leave no 
doubt remaining upon a point of such practical utility. " Being desirous," says 
Dr. Wells, "of acquiring some precise information on this subject, I fixed, perpen- 
dicularly in the earth of a grass-plot, four small sticks, and over their upper extremities, 
which were six inches above the grass, and formed the corners of a square, the sides of 
which were two feet long, I drew tightly a very thin cambric handkerchief. The 
temperature of the grass, which was thus sheltered from the sky, was always found 
higher than that of the neighbouring grass, which was uncovered, if this was colder 
than the air. One night, when the fully-exposed grass was 11° colder than the air, 
the latter was 3° warmer than the sheltered grass ; and the same difference existed 
on another night, when the air was 14° warmer than the exposed grass. 
" A difference in temperature of some magnitude was always observed on still 
and serene nights, between bodies sheltered from the sky by substances touching 
them, and similar bodies which were sheltered by a substance a little above them. 
Dr. Wells found that grass sheltered by a cambric handkerchief raised a few inches 
in the air was, on one occasion, 3°, and on another 4°, warmer than a neighbouring 
piece of grass, with which the handkerchief was in contact." 
VOL. XV. NO. CLXXT. X 
