Meteorology of Whitehaven. 59 
this process takes place to a certain extent by day as well as by night, 
in situations shaded from the direct rays of the sun, as] have frequently 
found by direct experiment. And, with a clear sky, dew may often be 
observed on vegetation in the shade, when the sun is a considerable 
height above the horizon ; in such cases, the temperature of the surface 
must not only be considerably under the temperature of the air, but also 
below that of the dew-point, as from several experiments which I made 
during the past winter, it appears that dew is rarely deposited on grass, 
until its temperature has fallen several degrees below the point of satura- 
tion, at the ordinary height of a thermometer in the air. 
The radiating powers of the various substances in nature are probably 
as various as the bodies themselves, in all their numerous combinations 
and modifications of condition ; and this property appears to obtain in the 
direct ratio of their absorbing, and in the inverse ratio of their conduct- 
ing and reflecting powers. The leaves of plants are excellent radiators, 
and from their sharp angular form, are well calculated to throw off su- 
perabundant heat. Hence, the natural grassy surface of the globe plays 
an important part in modifying the temperature of the air, and in etfect- 
ing the formation of dew,—a material highly conducive to the healthy 
growth and sustentation of the vegetable kingdom. As there is a con- 
stant tendency towards an equilibrium of temperature in all bodies, re- 
latively warm substances will radiate latterly a portion of their heat, 
which will be absorbed by neighbouring bodies at a lower temperature. 
Moreover, the freely radiating surface of grass will receive by conduc- 
tion, an accession of heat from the subjacent soil; and the quantity of 
caloric which it would naturally part with under any given circumstances, 
will be apparently diminished by that amount. As it is necessary to keep 
the radiation thermometers level with the points of the grass, for several 
years I made use of a flat piece of cork (about 1} inch in thickness), 
which is a very slow conductor, for this purpose; but latterly I have 
employed raw white wool, which is perhaps the most perfect non-con- 
ducting material hitherto discovered; and it is my intention to reduce 
all past and future results to this standard. The mean monthly differ- 
ence in the results between the temperature on cork and on wool, varies 
from 0°-95 to 2°-46, the average difference being about 14°. 
I am now (1849) making daily observations of the temperature on the 
surface of grass, with a naked thermometer suspended on Ys, and so far 
as the experiments have proceeded, the mean difference of the readings 
from those on wool is about 3°.* When this series is complete, the ob- 
servations taken on cork and on wool, can at any time be reduced to the 
standard of short grass. 
From the above table, it will be seen that the temperature has been 
below 32° in every month of 1848, except in July and August, when the 
minima were 35° and 34°°5 respectively ; but vegetation is liable to be 
* Vor the results of experiments (made at Whitehaven) on the radiating 
powers of various substances, also on the amount of radiation as indicated by a 
thermometer placed in the focus of the parabolic mirror, vide Mr Lowe’s Treatise 
on Atmospheric Phenomena, 
