be small, but, with great caution,
he adds that "he was not acquainted with any means of determining the
proportion of this part to the whole."

A few observations of the temperature of the ground near the surface, and
of the air over it, first raised doubts as to the correctness of the now
generally received opinion that dew is formed of vapor existing at the
time in the air. These observations, made at night, showed the ground at
a short distance below the surface to be always hotter than the air over
it, and it was thought that so long as this excess is sufficient to keep
the temperature of the surface of the ground above the dew point of the
air, it will, if moist, give off vapor, and it will be this rising vapor
that will condense on the grass and form dew, and not the vapor that was
previously present in the air.

The first question to be determined was whether vapor does, or does not,
rise from the ground on dewy nights. One method tried of testing this
point was by placing over the grass, in an inverted position, shallow
trays made of thin metal and painted. These trays were put over the
ground to be tested after sunset and examined at night, and also next
morning. It was expected that, if vapor was rising from the ground during
dewy nights, it would be trapped inside the trays. The result in all the
experiments was that the inside was dewed every night, and the grass
inside was wetter than that outside. On some nights there was no dew
outside the trays, and on all nights the inside deposit was heavier than
the outside one.

An analysis of the action of these trays is given, and it is concluded
that they act very much the same as if the air was quite still. Under
these conditions vapor will rise from the ground so long as the
vapor-tension on the surface of the ground is higher than that at the top
of the grass, and much of this rising vapor is, under ordinary
conditions, carried away by the passing air, and mixed with a large
amount of drier air, whereas the vapor rising under the trays is not so
diluted; and hence, though only cooled to the same amount as the air
outside, it yields a heavier deposit of dew.

Another method of testing this point was employed, which consisted in
weighing a small area of the exposed surface of the ground, as it was
evident that if the soil gave off vapor during a dewy night, it must lose
weight. A small turf about 6 inches (152 mm.) square was cut out of the
lawn, and placed