ion of vapor;
and from this time on it had been known that heat is taken up when water
evaporates, and given out again when it condenses. Dr. Darwin had shown
in 1788, in a paper before the Royal Society, that air gives off heat
on contracting and takes it up on expanding; and Dalton, in his essay
of 1793, had explained this phenomenon as due to the condensation and
vaporization of the water contained in the air.

But some curious and puzzling observations which Professor Patrick
Wilson, professor of astronomy in the University of Glasgow, had
communicated to the Royal Society of Edinburgh in 1784, and some similar
ones made by Mr. Six, of Canterbury, a few years later, had remained
unexplained. Both these gentlemen observed that the air is cooler where
dew is forming than the air a few feet higher, and they inferred
that the dew in forming had taken up heat, in apparent violation of
established physical principles.

It remained for Wells, in his memorable paper of 1816, to show that
these observers had simply placed the cart before the horse. He made it
clear that the air is not cooler because the dew is formed, but that
the dew is formed because the air is cooler--having become so through
radiation of heat from the solids on which the dew forms. The dew
itself, in forming, gives out its latent heat, and so tends to equalize
the temperature.

Wells's paper is so admirable an illustration of the lucid presentation
of clearly conceived experiments and logical conclusions that we should
do it injustice not to present it entire. The author's mention of
the observations of Six and Wilson gives added value to his own
presentation.

Dr. Wells's Essay on Dew

"I was led in the autumn of 1784, by the event of a rude experiment,
to think it probable that the formation of dew is attended with the
production of cold. In 1788, a paper on hoar-frost, by Mr. Patrick
Wilson, of Glasgow, was published in the first volume of the
Transactions of the Royal Society of Edinburgh, by which it appeared
that this opinion bad been entertained by that gentleman before it
had occurred to myself. In the course of the same year, Mr. Six, of
Canterbury, mentioned in a paper communicated to the Royal Society
that on clear and dewy nights he always found the mercury lower in a
thermometer laid upon the ground in a meadow in his neighborhood than
it was in a similar thermometer suspended in the air six feet above the
former; and that upon one