86 Mr Blackadder on Circumstances connected with the 
2° below that of the latter. When, however, from some tem- 
porary cause, the surrounding atmosphere became agitated, that 
is, when a progressive, undulatory, or convolving motion had 
been communicated to it, the air contiguous to the snow was 
thereby either mixed with, or altogether displaced by, the adja- 
cent air of a higher temperature. By this means the snow ac- 
quired an accession of heat, and the thermometer in contact with 
it indicated an increase of temperature. As long as the atmo- 
sphere continued agitated, fresh portions of air would every in- 
stant be brought into contact with the snow ; and in this way 
supplies of heat would be furnished equal to that which was car- 
ried off from the snow by the evaporating process. When still- 
ness again took place, though the air contiguous to the ground 
was not absolutely at rest, fresh parcels of the higher adjacent 
air were not now, as formerly, brought incessantly into contact 
with the snow ; and hence the latter did not receive a quantity 
of heat equal to the whole amount of that carried off by the va- 
pour, until its temperature was again reduced about £° below 
that of the air, a few feet from the ground. I shall here merely 
introduce an experiment of Mr Howard. On a night, when the 
minimum temperature was 19°, that gentleman exposed 1000 
grains of snow, on a dish 6 inches in diameter, and in the course 
of the night 60 grains were lost by evaporation. I have repeat- 
edly made observations and experiments similar to those above 
adverted to, but it seems quite unnecessary on the present occa- 
sion to multiply examples. I shall therefore conclude this 
part of the subject with noticing an observation to be met with 
in the writings of a well known meteorologist. He informs us, 
that 4 a ploughed field is more affected by the sun’s rays than 
a plot of grass ; because a loose spongy bod}^, by exposing nu- 
merous surfaces, dissipates more quickly the heat communicated 
to it and, in confirmation and illustration of this opinion, he 
adds, that 4 the inferiority of a grassy surface was not owing to 
the waste of heat by a more copious evaporation ; for that, on 
spreading a layer of hay, or even wool, over a part of the naked 
soil, the temperature of it was in a few minutes reduced to the 
same degree as that of the grass.’ 
Hay and wool, as has already been remarked, are hygrosco- 
pic bodies, and bad conductors of heat ; and they are rarely met 
