124 
On the Injluence of Water 
the two vessels in time ; but the difference in the quantity of 
heat emitted from each of them is immense, as appears from 
what is stated above with reference to the constituent heat of 
vapour. 
Fourthly ; as the temperature of water diminishes during the 
night, or in the day time, according to the varying conditions of 
the atmosphere, by radiating its heat to the heavens, its specific 
gravity increases ; and the superficial stratum, which is first cooled, 
immediately descends by reason of its augmented density. This 
film of cooled and heavier water is as quickly replaced by rela- 
tively warmer and lighter portions which become cooled in turn, 
and successively sink. Water, therefore, though a non-conductor 
of heat downwards, when warmed on the surface, becomes a ready 
vehicle of cold in that direction when cooled on its surface ; and 
this cooling process may even continue, under fitting circum- 
stances, until the whole of a given mass is reduced to the low tem- 
perature of 42°, at which point water attains its maximum den- 
sity. The further descent of cold through this process would then 
cease ; but the refrigeration occasioned by it must affect all soils, 
to a greater or less degree, which hold water in excess, i. e. when 
in a state of stagnancy near to the surface. Those soils only can 
be exempt from this chilling influence which are not naturally 
retentive of water, or which are artificially and deeply drained. 
Thus, excess of water conduces to the production of cold in 
soil, by means of several independent, vigorous, and ever active 
properties. 
On the other hand, when a soil is naturally so porous, or is 
brought into such condition by art, that rain-water can sink down 
into the earth, it becomes a carrier, an alert pun eyor, instead of 
a robber of heat ; and tends to raise, permanently, the tempera- 
ture of the mass of useful soil ; and this more particularly and 
beneficially during the vegetative season. Rain-water, at that 
time, conveys downwards the more elevated superficial heat of 
the soil, and imparts it to the subsoil in its course to the drains ; 
it leaves the soil in a fil state to receive fresh doses of rain, dew, 
and air, and in a better condition to absorb and retain heat, at the 
same time that it promotes, in other wa^ s, its fertility and produc- 
tiveness ; but a consideration of the chemical effects attributable 
to the continual circulation and renewal of water and air is foreign 
to the present discussion. 
In order to render the change of water perfect, and its action 
uniform throughout a field, all drains should be deeper than the 
active or worked soil, and covered. If drains are open, much of the 
rain precipitated on the surface necessarily passes into them, before 
it l.as permeated the whole mass; consccjuenily, it carries off with 
it l.eat, which would have been usefully employed in warming the 
