ON HEAT AND MOISTURE. 
61 
greater disparity tlian is perfectly natural to the parts respectively. 
For, whatever temperature the branches may be exposed to, the 
fibres of the roots, from their extreme delicacy, require a much 
lower degree ; being equally averse to light, dry air, and extreme 
heat. Burning the roots is a frequent misfortune, both in hot¬ 
beds and hothouses ; and therefore it would appear that arti- 
V 
ficially-heated borders for trees is an unnecessary provision ; and 
in all cases, if the bottom of the border be sufficiently porous, the 
supply of subterranean heat will always be adequate to the wants 
of the working roots. 
That the deeper we pierce into the crust of the earth, the higher 
the temperature, is well known to miners, well-diggers, and even 
to the observant husbandman, who employs his sub-soil plough ; 
to the planter who trenches ; and even to our friends the florists, 
who invariably trench or break up deeply the natural bottom of 
their beds before laying on the surface compost. The good effects 
of so doing is known to every practical man. But the effects are 
not always attributed to the right cause; some declaring that it 
is because the roots have more scope ; others, that opening the 
ground so deeply admits more readily all atmospheric influences ;— 
both ideas being just as auxiliaries ; but the main and principal 
effect is giving facility to the ascending stream of humid heat 
from the interior of the earth. 
That such phenomenon is constantly in operation, is very visible 
at this time of the year (18th Dec.), if we only look at the gratings 
over drains, the mouths of pits or wells, or remark the tempera¬ 
ture of the water flowing from artesian wells. The snow upon 
newly trenched ground disappears sooner than it does upon the 
firm surface around; and it has often been observed, that snow is 
faster thawed by heat radiating from the earth, than from a milder 
change in the atmosphere. In a thaw, any solid body—as rocks, 
large stones, foundations of old buildings, or pieces of timber em¬ 
bedded under the surface—-may easily be discovered by the snow 
lying longer over them than it does on the surrounding surface, 
free from such obstructions. 
Ice-houses are warmer than the open air, and on this account 
they must be bad receptacles for preserving ice; and so those 
which are sunk deep in the ground are far less useful for their 
purpose than ice-houses built on the surface. But it is necessary 
to notice that it is not the heat of the place which dissolves ice ; it 
