is a well-established
proposition that the pressure of the water vapour in the air does not
vary while the air is being cooled without change of its total external
pressure, the saturation pressure at the dew-point gives the pressure of
water vapour in the air when the cooling commenced. Thus the artificial
formation of dew and consequent determination of the dew-point is a
recognized method of measuring the pressure, and thence the amount of
water vapour in the atmosphere. The dew-point method is indeed in some
ways a fundamental method of hygrometry.

The dew-point is a matter of really vital consequence in the question of
the oppressiveness of the atmosphere or its reverse. So long as the
dew-point is low, high temperature does not matter, but when the
dew-point begins to approach the normal temperature of the human body
the atmosphere becomes insupportable.

The physical explanation of the formation of dew consists practically in
determining the process or processes by which leaves, blades of grass,
stones, and other objects in the open air upon which dew may be
observed, become cooled "below the dew-point."

Formerly, from the time of Aristotle at least, dew was supposed to
"fall." That view of the process was not extinct at the time of
Wordsworth and poets might even now use the figure without reproach. To
Dr Charles Wells of London belongs the credit of bringing to a focus the
ideas which originated with the study of radiation at the beginning of
the 19th century, and which are expressed by saying that the cooling
necessary to produce dew on exposed surfaces is to be attributed to the
radiation from the surfaces to a clear sky. He gave an account of the
theory of automatic cooling by radiation, which has found a place in all
text-books of physics, in his first _Essay on Dew_ published in 1818.
The theory is supported in that and in a second essay by a number of
well-planned observations, and the essays are indeed models of
scientific method. The process of the formation of dew as represented by
Wells is a simple one. It starts from the point of view that all bodies
are constantly radiating heat, and cool automatically unless they
receive a corresponding amount of heat from other bodies by radiation or
conduction. Good radiators, which are at the same time bad conductors of
heat, such as blades of grass, lose heat rapidly on a clear night by
radiation to the sky and become cooled below the dew-point of the
atm