night the difference amounted to five degrees
of Fahrenheit's scale. Mr. Six, however, did not suppose, agreeably to
the opinion of Mr. Wilson and myself, that the cold was occasioned by
the formation of dew, but imagined that it proceeded partly from the
low temperature of the air, through which the dew, already formed in the
atmosphere, had descended, and partly from the evaporation of moisture
from the ground, on which his thermometer had been placed. The
conjecture of Mr. Wilson and the observations of Mr. Six, together
with many facts which I afterwards learned in the course of reading,
strengthened my opinion; but I made no attempt, before the autumn of
1811, to ascertain by experiment if it were just, though it had in
the mean time almost daily occurred to my thoughts. Happening, in
that season, to be in that country in a clear and calm night, I laid a
thermometer upon grass wet with dew, and suspended a second in the air,
two feet above the other. An hour afterwards the thermometer on the
grass was found to be eight degrees lower, by Fahrenheit's division,
than the one in the air. Similar results having been obtained from
several similar experiments, made during the same autumn, I determined
in the next spring to prosecute the subject with some degree of
steadiness, and with that view went frequently to the house of one of my
friends who lives in Surrey.

"At the end of two months I fancied that I had collected information
worthy of being published; but, fortunately, while preparing an account
of it I met by accident with a small posthumous work by Mr. Six, printed
at Canterbury in 1794, in which are related differences observed on dewy
nights between thermometers placed upon grass and others in the air that
are much greater than those mentioned in the paper presented by him to
the Royal Society in 1788. In this work, too, the cold of the grass is
attributed, in agreement with the opinion of Mr. Wilson, altogether to
the dew deposited upon it. The value of my own observations appearing to
me now much diminished, though they embraced many points left untouched
by Mr. Six, I gave up my intentions of making them known. Shortly after,
however, upon considering the subject more closely, I began to suspect
that Mr. Wilson, Mr. Six, and myself had all committed an error
regarding the cold which accompanies dew as an effect of the formation
of that fluid. I therefore resumed my experiments, and having by means
of them,