54 Mr Blackadder on the Formation of' 'Dew. 
disposition for moisture, in some degree modified perhaps by the 
relative temperature of the ground, and the facility with which 
it can furnish a supply of moisture. This also is consonant with 
observation ; for, when about sunset the air is still capable of 
receiving a considerable accession of vapour, the moisture 
transpired by the grass is rapidly carried off, and likewise that 
at the surface of the subjacent soil. Hence in such circum- 
stances the temperature of the grass is considerably reduced, 
without any manifest deposition of moisture on its surface. 
When, however, the temperature of the grass has suffered a 
certain reduction below that of the vapour issuing from beneath 
it, and when the air, from its decreasing temperature and the 
effects of evaporation, has its disposition for moisture diminished, 
the vapour generated at the surface of the subjacent soil is partly 
condensed by coming into contact with the piles of grass, and, 
sooner or later, according to circumstances, acquires the form of 
distinct drops of dew. 
It perhaps cannot be easily shewn, that a solid body has its 
temperature always as much increased from the condensation of 
a certain quantity of vapour on its surface, as it would in simi- 
lar circumstances have its temperature diminished by the evapo- 
ration of a like quantity of moisture. Yet as heat is always ex- 
tricated on the condensation of vapour, it cannot be questioned 
that grass must have its temperature in some degree affected 
when dew is deposited on its surface ; and it has been found, 
that though, on certain occasions, when the condensation is sud- 
den and copious, there is an evident accession of heat, it fre- 
quently happens that the previous degree of cold is not sensibly 
diminished. If, therefore, it were found that the air is com- 
pletely saturated whenever dew begins to form, and if it were 
the case that grass readily acquired heat by conduction from the 
ground, this circumstance of grass maintaining its depressed 
temperature after moisture has been condensed on it, could not 
be satisfactorily explained on the principle of evaporation. But 
we learn by observation, that the state of things is exactly the 
reverse of what we have supposed. For, dew may often be ob- 
served on the grass long before evaporation has altogether 
ceased ; and, as has already been observed, grass acquires but 
little heat by conduction from the subjacent soil. As long, 
