abundantly on roads, and that the reason it has
not been observed is that it has not been sought for at the correct
place. We are not entitled to expect to find dew on the surface of roads
as on the surface of grass. because stones are good conductors of heat,
and, the vapor-tension being higher underneath than above the stones, the
result is, the rising vapor gets condensed on the under sides of the
stones. If a road is examined on a dewy night, and the gravel turned up,
the under sides of the stones are found to be dripping wet.

Another reason why no dew forms on the surface of roads is that the
stones, being fair conductors, and in heat communication with the ground,
the temperature of the surface of the road is, from observations taken on
several occasions, higher than that of the surface of the grass
alongside. The air in contact with the stones is, therefore, not cooled
so much as that in contact with the grass.

For studying the formation of dew on roads, slates were found to be
useful. One slate was placed over a gravelly part of the road, and
another over a hard dry part. Examined on dewy nights, the under sides of
these slates were always found to be dripping wet, while their upper
surfaces, and the ground all round, were quite dry.

The importance of the heat communicated from the ground is illustrated by
a simple experiment with two slates or two iron weights, one of them
being placed on the ground, either on grass or on bare soil, and the
other elevated a few inches above the surface. The one resting on the
ground, and in heat-communication with it, is found always to keep dry on
dewy nights, whereas the elevated one gets dewed all over.

The effect of wind in preventing the formation of dew is referred to. It
is shown that, in addition to the other ways already known, wind hinders
the formation of dew by preventing an accumulation of moist air near the
surface of the ground.

An examination of the different forms of vegetation was made on dewy
nights. It was soon evident that something else than radiation and
condensation was at work to produce the varied appearances then seen on
plants. Some kinds of plants were found to be wet, while others of a
different kind, and growing close to them, were dry, and even on the same
plant some branches were wet, while others were dry. The examination of
the leaf of a broccoli plant showed better than any other that the
wetting was not what we might expect if it w