THEORY OF THE DEW. 
131 
" Dr. Wells, after verifying the facts observed, ascertained by numerous experi- 
ments and observations, that the cold at the surface, compared with that of the air 
above, precedes the formation of Dew, and often exists without Dew being formed ; 
and that bodies become colder than the neighbouring air before they are dewed. 
The cold, therefore, which Mr. Wilson and Mr. Six supposed to be the effect of dew, 
was found by Dr. Wells to be the cause of it." 
Dr. Wells then directed his attention to this depression of temperature, and he 
arrived at the conclusion that " it is produced by the radiation of heat, without an 
equivalent return." He reasoned thus — " The surface of the ground allows a portion 
of the heat which it receives from the solar rays, to escape by radiation when their 
action is withdrawn ; hence its temperature falls ; and if the air, holding watery 
vapour in solution, rest upon it without much agitation, a portion of that vapour will 
be condensed on the surface, and if the temperature is still lower, will be congealed ; 
thus it is that dew and hoar frost are formed only when the atmosphere is clear, for 
the clouds return an equivalent portion of radiant heat." 
A series of Letters on the Theory of Dew are in the course of publication. The 
Pharmaceutical Times contains a Letter by Melloni, abridged for the Comptes Rendus. 
It appears that its direct object is to confirm the theory of Wells, by experiments 
which make it clear that "Dew neither rises from the earth nor falls from the sky, 
but that it is produced by the elastic and invisible vapour which is everywhere present 
in the atmosphere," and, therefore, that "the precipitation of watery vapour is 
plainly owing to the cold caused by the radiation of bodies towards a clear sky." 
There is in this so much " begging of the question," or, in other and plainer 
terms, the admission of a certain principle that bears the stamp of authority, that we 
shall feel called upon to meet the question of radiation before assent be added to it 
as a fundamental principle. However, as this must be postponed, and referred to 
a second article on the Dew, we copy the following details of an experiment by 
Melloni, which the writer believes must fully establish Dr. Wells's principle. 
" On a tin disk, as large and as thin as possible, draw a concentric circle, whose 
radius shall be equal to one- third of that of the disk, and cover it with a thick layer 
of varnish. Then take another tin disk, less by ten millimetres (about of an 
inch) than the varnished circle ; and having soldered a pointed iron wire, two milli- 
metres broad, and two or three decimetres (better than 7 inches) long, at its centre, 
and perpendicular to it, pass the wire through a hole in the centre of the great disk, 
on the varnished side. The great disk is to be pushed along the wire until the two 
disks are about five millimetres from each other, at which distance they must be kept. 
The disks so joined are placed by means of the wire in the open ground of a field, or 
lawn, quite out of contact with any other body. If the night be calm and fine, the 
phenomena to be remarked will generally be the following, as a natural result of the 
arrangement : — The instrument, fixed as described above, has its small bright disk 
uppermost, hence an annular band of the varnished part of the lower disk will be 
exposed to the air. This band will (according to the theory) radiate heat, become 
