154 
THEORY OF THE DEW. 
In noticing these experiments, and the deductions at which Dr. Wells arrived, 
as we can perceive no reason materially to change the opinions we formed, and 
publicly announced several years ago, we now venture to present the same to the 
reader in a modified form. In reference to paragraph No. 1, the nature of the 
materials employed by Dr. Wells must he considered ; they were dry wool and 
swausdown — both of them electrics, both non-conductors of electricity ; they were 
placed upon a fir table, rendered insulating and a non-conducting medium, naturally 
by the resinous matter which sound fir contains, and still more so by the process of 
drying by exposure to the sun. Those electric substances, wool and swansdown, 
were thus insulated at an elevation of three feet above the surface of that grass which 
became 14° and 15° colder than a stratum of air four feet above the ground, and 
one foot above the table. Under those circumstances the swansdown remained dry, 
though cooled down to within half a degree of the grass, which, in all probability, 
was dewed, though we do not find it stated so to have been. 
From the facts thus announced it should appear that such non-conductors do not 
receive the dew, however low their temperature may be ; and therefore, that the 
precipitation of dew can neither depend upon the degree of cold, nor upon existing 
atmospheric moisture, independent of the agency of vegetable organisation, or pointed 
conductors of some kind. 
No. 2. The effects produced by the slight covering of a cambric handkerchief, 
lead to inferences of great practical utility : glass coverings may be much improved 
by screens, raised an inch or two above them, so as to interpose a stratum of dry air, 
which always acts as a very imperfect conductor; and plants in the open air can be 
considerably defended by simple raised coverings ; but while admitting the facts, we 
may be allowed to question the validity of the hypothesis, which refers the phenomena 
to the influence of radiated heat, active upwards, in the first place, and then 
downwards, as if by reflection. Again to cite Dr. Wells : — 
“ Dense clouds, near the earth, reflect back the heat they receive from it by 
radiation. But similarly dense clouds, if very high, though they equally intercept 
the communication of the earth with the sky, yet being, from their elevated situation, 
colder than the earth, will radiate to it less heat than they receive from it, and 
may consequently admit of bodies on its surface becoming several degrees colder than 
the air.” 
“ The best radiators,” said a public lecturer on heat, to his audience, “are soonest 
dewed ; hence grass and vegetables are more quickly covered with dew than gravel 
stones and metals ; and as the earth dissipates its heat by radiation, it will be seen 
that any light awning spread over the ground will prevent radiation, and keep the 
earth warm. 
“ This doctrine of radiation is the stumbling-block — it is, in fact, a mere name 
of an effect, for which no cause is assigned.” 
Let any of our observant readers call to memory the phenomena which usually-^ 
prevail during a very hot and dry summer, wherein the state of the atmosphere was?*, 
