82 
WINDS, AND THEIR CURRENT EFFECTS ON VEGETATION. 
air. Hence, as the latter frequently falls below the heat existent in animal and 
vegetable matters, it must necessarily be continually extracting their caloric at 
those seasons. When the atmosphere is calm, the radiation of heat from such 
bodies is comparatively slow, the involving and partially permeating stratum of air 
being tempered by that already given out, and consequently not abstracting so 
rapidly the portion retained ; whereas, if winds are travelling over, they bear away 
the volume transpired, and a rapid succession of cold strata demands a much more 
profuse effusion ; the extent of this loss being exactly adequate to the velocity of 
the breeze, or the quarter from whence it proceeds. 
But although the preceding account explains the manner in which winds 
dispose of the heat they abstract, and partially describes the mode of their minis- 
tration, we have left untouched that intricate element of their action to which all 
their effects must be ascribed in the season herein commemorated. We have said 
that currents of cold air naturally and directly disengage the temperature of bodies 
which they can penetrate, or around which they have full liberty to play. Let us 
now add, that their next highest office is the liberation of fluids from terrestrial 
objects, and that such a release can alone be accomplished by vaporization, in tlie 
transaction of which the emission of a considerable portion of caloric is requisite. 
Fluids and heat have a strong affinity to each other ; or rather, no substance 
can be liquified or retained in that state unless it comprises a certain quantity of 
heat. The temperature thus resident in a liquid, or a body containing moisture, 
induces a continual expansion of its watery parts, till at length, driven to the 
surface, it is there detached and concentrated into the distinct but imperceptible 
particles that constitute vapour, and then, by its own energy, diffuses itself through 
the atmosphere, carrying with it much of the inherent caloric, which is now com- 
paratively transmuted to a latent state. 
The escape of heat in the process of evaporation is far too little understood. 
As it intimately concerns the inquiry we have here undertaken, a few words may 
be devoted to its elucidation. It must first be stated that heat has a property of 
ascension, ascribable to its total want of specific gravity, by which it is always 
flying off into a more elevated medium. It thus plainly seeks the upper surface 
of any substance, and in its progress thither, forces with it the more subtile 
particles of fluid which are interposed between it and the air. By its constant 
accumulation in these, (which, being lightest, are always uppermost,) whether 
from additional extrinsic applications, or by its more tardy abstraction from 
interior and lower sources, it finally distends and etherealizes them to such a 
degree, that they are made capable of floating in the atmosphere without any other 
support than the heat they involve. 
Here, then, is the explication of this phenomenon. Water cannot be rarefied 
into vapour until it has collected into each individual atom of that vapour an | 
amount of heat sufficient to keep it dilated and poised in the atmosphere : when 
this takes place, it immediately quits the colder portions beneath it, and, both as an 
