o2 Mr Blackadder on the Formation of Dew . 
even had the limits to which I have restricted myself otherwise 
admitted of it. 
It has been ascertained, that, on a serene evening, after sun* 
set, when dew is about to form, the air is never in a state of 
tension or saturation ; and that, even when a visible condensation 
of moisture has taken place on the blades of grass, the air is 
still capable of receiving an accession of moisture. As dry air, 
therefore, or air not saturated, cannot remain in contact with a 
moist surface without the existence of evaporation, whatever 
may be the respective temperatures of these bodies, it follows as 
a necessary consequence, that evaporation must be going for- 
ward, from a grassy surface, during the formation of dew ; and 
it might be inferred, not without some degree of probability, 
that these phenomena were not altogether disconnected. During 
the process of evaporation a quantity of heat is always absorbed, 
or becomes insensible ; and if there is not a constant influx of 
heat equal to that which is thus absorbed, the temperature of 
the body from whose surface the vapour is generated, is always 
very sensibly lowered. We find, accordingly, that on a hot 
summer-day, when the evaporation from a meadow is very co- 
pious, the temperature of the grass is not reduced below that of 
the air, as the heat absorbed in the vaporific process is constant- 
ly supplied by the sun’s rays. But as the sun declines, the in- 
flux of heat diminishes, and at sunset may be considered as al- 
together suspended. Still, however, the air not being saturated, 
the process of evaporation goes forward, and, as the supply of 
heat is now cut off, we would be led to expect that the tempera- 
ture of bodies capable of supporting evaporation would suffer a 
sensible depression. This is also agreeable to observation ; for 
even before sunset, the temperature of the grass is often sensibly 
diminished, and, in the course of the succeeding hour, generally 
indicates a very remarkable decrement of heat. When, there- 
fore, we find that the degree of cold that takes place on a grassy 
surface during a calm state of the air, always bears some rela- 
tion to the dryness of the latter body, the most natural conclu- 
sion would certainly be, that evaporation is the means by which 
the diminished temperature is produced. 
It has been observed, that different bodies, equally fitted to 
support evaporation, and similarly exposed to the influence of 
