204 Proceedings of the Boyal Society 
aether, from the upper and finer parts of living plants, in so great 
amount every clear night of summer, that destruction by frost 
could not be delayed for many hours after sunset without a com- 
pensating supply of heat from some extraneous source. This source, 
on windy nights, is the thermal capacity of the air whirled about, 
up and down, and among the stems and leaves of the plants. On 
still nights it is the latent heat of the vapour condensed into dew. 
This vapour is taken chiefly from the air engaged among the stems 
and leaves, which, in the case at least of fine grass, is all nearly 
at the same temperature as the leaves ; the temperature of the 
surface of these being of course rigorously the same as that of the 
air in contact. Thus the temperature of the leaves can never go 
helow the dew-jpoint of the air touching them, and any cooling 
which they experience after dew begins to deposit upon them is 
only equal to the lowering of the dew-point^ occasioned by the 
amount of drying experienced by the air in consequence of the 
condensation of vapour out of it. 
Clouds, as remarked first by Prevost, being practically opaque, 
prevent the surface of the earth from tending by radiation to a 
lower temperature than their own, which, unless they are very 
high, is generally not much colder than the dew-point of the lower 
air, but is at all events in general sufficiently warm to prevent the 
finest blades of grass from acquiring any very sensible dew, or to 
allow the general temperature of grass and the air engaged among 
it, even on the stillest night, to sink as low as the dew-point. 
Thus, either clouds, by their counter-radiation, or wind, by mixing 
a comparatively thick stratum of air with that next the earth, keep 
the grass and delicate parts of other plants from sinking to the 
dew-point ; or, when there is not enough of clouds and wind to 
afford this degree of protection, dew begins to form, and by 
preventing the temperature of any leaf or flower from sinking 
below the dew-point, saves them all from destruction, unless, as 
when hoar-frost appears, the dew-point itself is below the freezing 
point. 
