Mr Blackadder on the Formation of Few. 5% 
therefore, as the heat acquired from the condensation of mois- 
ture is not greater than the measure contemporaneously ab- 
sorbed in the process of evaporation, the temperature of the 
grass will not be sensibly increased. But it now becomes neces- 
sary to remark, that, as the grass becomes colder from the in- 
fluence of evaporation, the contiguous air is at the same time 
cooled by communicating its heat to the grass ; and were it not 
for this influx of heat from the air, the degree of cold produced 
by evaporation would be much greater, and would be conti- 
nually increasing as long as that process continued in action. 
But as things are constituted, the cold produced by evaporation 
only increases until the contiguous air can furnish a supply of 
heat equal to that which is absorbed in the formation of vapour; 
and hence, in the ordinary course of nature, the depression of 
temperature soon attains a certain limit, beyond which it can- 
not extend, unless we suppose the capacity of the air for mois- 
ture to be somehow increased without a contemporaneous eleva- 
tion of its temperature. 
As the air, by losing heat, becomes denser, and consequently 
has its specific gravity increased, that which has been most 
cooled by contact with the grass must retain the lowest position, 
and hence will remain contiguous to the grassy surface, pro- 
vided there be no lower level to which it can remove. We And, 
accordingly, that over a level meadow, whose surface has been 
cooled by evaporation after sunset, the air increases in tempera- 
ture as we ascend upwards ; and that the air in contact with the 
grass lias always the same, or very nearly the same, temperature 
with that body. If, then, the atmosphere be calm, and there 
be no lower level to which the cold air can withdraw, the grass 
remains immersed in a body of air as cold, or almost as cold, as 
itself ; for that which is higher and warmer cannot descend, so 
as to come into contact with the grass ; and the quantity of heat 
that can be conveyed by conduction through air is well known 
to be extremely small, and on the present occasion need not be 
estimated. If the temperature of the contiguous air, therefore, 
diminishes with that of the grass, and if the latter, after having 
been cooled by evaporation, should acquire an accession of heat, 
from a condensation of vapours rising through it from the sub- 
jacent earthy surface, this acquired heat would be partly ab- 
