136 
On the Iiifluence of Water 
He then ascertained by experiment that 
" A difference in temperature of some magnitude was always observed, 
on still and serene nights, between bodies sheltered from the sky, by 
substances touching them, and similar bodies which were sheltered by a 
substance a little above them." " Possibly," he continues, " experience 
has long ago taught gardeners the superior advantage of defending 
tender vegetables from the cold of clear and calm nights, by means of 
substances not directly touching them, though I do not recollect ever 
having seen any contrivance for keeping mats, or such like bodies, at a 
distance from the plants which they were meant to protect." 
It is a common practice in France to cover transplanted veg^e- 
taljles by linen sheets placed over sticks about two feet high. All 
the spare linen of my own house has been occasionally borrowed 
for this purpose ; and I have laid my friends equally under con- 
tribution, until tender plants were sufficiently rooted, and strong 
enough to bear complete exposure to the heat of the sun and the 
cold of the night. 
May it not possibly be of advantage to the agriculturist to pro- 
tect his potatoes, turnips, or other stored roots, from frost, by 
means of impermeable portable cloths stretched at a convenient 
height, instead of with earth, straw, &c. placed upon them ? 
When substances touch each other, heat is conducted from the 
mass, and finally radiated awa}' into space ; cold results, and the 
roots are frost-bitten. The experiment may be worth a trial. 
Mr. Graburn has communicated to me a remarkable pheno- 
menon connected with hoar-frost, which is, perhaps, generally 
known to farmers, but, if not, the mention of it will convey a 
useful warning. He has remarked that the passage of a flock of 
sheep across a clover field covered with boar- frost, particularly 
young spring clover, is certainly followed by the destruction of 
every leaf over which the animals have passed. He further aptly 
observes, " You might trace the footsteps of a thief across a clover 
field covered with hoar-frost, at noon the day following, by the 
withering of the grass in his track." Knowing, as we do, that 
hoar-frost is a great protection to the leaf against further accession 
of cold, we might be disposed to attribute the death of the leaf, 
indirectly, to the shaking off of the frozen dew ; but it is pos- 
sible that the proximate cause is purely mechanical, and the 
withering the direct effect of injury from the tre.id, when the 
leaves are so crisp as to be in a state to be bruised by a suffi- 
cient weight pressing on them. The cause would be manifested 
by ascertaining whether the leaf, under the circumstances, would 
perish if the hoar were carefully brushed off it, and not trampled. 
Experiments on the Temperature of Soils. 
Schublers Experiments. — This subject appears to have attracted 
the attention of several German philosophers, who have investi- 
