of vapor sufficient, true dew makes its appearance, and
now the plants get equally wet all over, in the same manner as dead
matter. The difference between true dew on grass and these exuded drops
can be detected at a glance. The drops are always exuded at a point near
the tip of the blade, and form a drop of some size, while true dew is
distributed all over the blade. The exuded liquid forms a large
diamond-like drop, while the dew coats the blade with a pearly luster.

Toward the end of the paper the radiating powers of different surfaces at
night is considered, and after a reference to some early experiments on
this subject, the paper proceeds to describe some experiments made with
the radiation thermometer described by the author in a previous paper.
When working with this instrument, it is placed in a situation having a
clear view of the sky all round, and is fixed at the same height as the
ordinary thermometer screen, which is worked along with it, the
difference between the thermometer in the screen and the radiation
thermometer being observed. This difference in clear nights amounts to
from 7 deg. to 10 deg.. By means of the radiation thermometer the radiating
powers of different surfaces were observed. Black and white cloths were
found to radiate equally well; soil and grass were also almost exactly
equal to each other. Lampblack was equal to whitening. Sulphur was about
two-thirds of black paint, and polished tin about one-seventh of black
paint. Snow in the shade on a bright day was at midday 7 deg. colder than the
air, while a black surface at the same time was only 4 deg. colder. This
difference diminished as the sun got lower, and at night both radiated
almost equally well. In the concluding pages of the paper some less
important subjects are considered.

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