341 
THE COTTAGE GARDENER AND COUNTRY GENTLEMAN, September 4, 1360. 
it of increasing most freely, and even at the expense of others, 
those organs which are most necessary, the leaves of such abun¬ 
dantly supplied branches are increased both in number and size, 
whilst the blossom is proportionately diminished in number, or 
is obliterated entirely. A plant propels its sap with greatest 
force perpendicularly; so much so, that the sap rising in a Vine 
branch growing in a right line from the root with a force capable 
of sustaining a column of mercury twenty-eight inches high, will, 
if the branch be bent down to a right angle, support barely 
twenty-three inches ; and if bent a few degrees below the hori¬ 
zontal, the column sustained will not be more than twenty-one 
inches. This is the reason why, at such angles, gardeners find 
the trained branches of their wall trees rendered more productive 
of blossoms, and furnished with a smaller surface of leaves. A 
similar effect is produced by training a branch in a waving form, 
for two-thirds of its length are placed horizontally. Other 
modes of interrupting the rapid flow of the sap by checking its 
return have been previously noticed; among which modes are 
ligatures and wounds round the bark. 
Light and heat are so combined, and so equally essential for 
the ripening of fruit, that they may be considered conjointly. 
They are both diminished in ungenial summers ; and in such, 
fruit ripens indifferently, or not at all, being, if it does ripen, 
deficient in colour as well as flavour. In our latitudes, how¬ 
ever, warmth is more deficient than light for the maturing of 
exotic plants; therefore, by securing to them a higher tempe¬ 
rature, we have the Peach, the M elon, the Mango, and the Pine 
Apple as richly-flavoured and even superior in excellence to that 
which they attain in their native climes. 
It must be remembered, in considering this branch of our 
subject, that all cooling is occasioned either by the heat being 
conducted from a body by a colder, which is in contact with it, 
or by radiating from the body cooled, though circumstances 
accelerate or retard the radiation ; and whatever checks the ra¬ 
diation of heat from a body keeps it warmer. For example,—a 
thermometer placed upon a grass plat, exposed to a clear sky, 
fell to 35°; but another thermometer, within a few yards of the 
preceding, but with the radiation of the rays of heat from the 
grass checked by no other covering than a cambric pocket- 
handkerchief, declined no lower than 42°. No difference of 
result occurs, whether the radiating surface be parallel or per¬ 
pendicular to the horizon; for when the mercury in a ther¬ 
mometer, hung against an openly exposed wall, fell to 38°, 
another thermometer against the same wall, but beneath a web 
of gauze stretched tightly at a few inches distance, indicated a 
temperature of 43°. 
These results explain the beneficial operation of apparently 
such slight shelter to our wall fruit when in blossom. A sheet 
of canvass, or of netting, prevents the direct radiation of heat 
from the wall—the cooling goes on more slowly, and is not 
reduced to that of the exterior air at night before the return of 
day begins to re-elevate the external temperature. 
The colder the body surrounding another body, the more 
rapid the radiation from the latter; for it is a law of heat that it 
has a constant tendency to be diffused equally, and the greater 
the diversity of temperature between two bodies in contact with 
each other, the greater is the rapidity with which the progress 
towards equilibrium goes on. This is one reason why a tem¬ 
perature of 32° with a brisk wind attending it, will injure plants 
to a far greater extent than a temperature many degrees lower 
with a still atmosphere; but it is aided by the operation of 
another law of heat—viz., that aeriform bodies convey it from a 
cooling body, as a wall or a tree, by an actual change in the 
situation of their own particles. That portion of the air which 
is nearest to the body cooling is expanded, and becoming specifi¬ 
cally lighter, ascends, and is replaced by a colder portion. This, 
in its turn becomes heated and dilated, and gives place to 
another colder portion; and thus the process goes on until the 
body cooling is reduced to the same temperature as the air. In 
a still atmosphere this goes on slowly, the air in contact with the 
wall and tree rises very gradually as it imbibes warmth from 
them; but if there be a brisk wind, a constant current of air at 
the lowest temperature then occurring is brought in constant 
contact with them, and the cooling is rapid in accordance with 
the law of equilibrium just noticed. A shelter of netting, or 
even the sprays of evergreens are of the greatest service in pre¬ 
venting the sweeping contact of cold air at such times. 
It is not altogether immaterial of what substance netting is 
formed. Worsted is to be preferred, not only becauso it is the 
most durable, but because it is the best preventive of a wall’s 
cooling. We have found the thermometer under a hemp net 
sink during the night from two to four degrees lower than that 
under a net of worsted, the meshes being small and of equal size 
in both nets. This can only be because worsted is a known worse 
conductor of heat, than hemp, and, not absorbing moisture so 
easily, is not so liable to the cold always produced by its drying. 
Snow is a protection to plants for the three foregoing 
reasons—it prevents heat radiating from them—protects them 
from the chilling blasts—and is one of the worst conductors of 
heat. We have never known the surface of the earth below a 
cover ng of snow colder than 32°, even when the temperature of 
the a:r above has been 28°. A similar protection, though less 
effectual, is afforded by straw. 
Strange as it may appear, yet it is nevertheless true, that a 
shelter is more beneficial in preserving the temperature of trees, 
when from three to six inches from them, than when in imme¬ 
diate contact with their surfaces. When a woollen net was sus¬ 
pended four inches from the wall, on which a Peach tree was 
trained, the thermometer fell very slowly, and the lowest degree 
it reached was 38° ; when the same screen was twelve inches off, 
it fell to 34°; and when drawn tightly over the tree, it barely 
kept above 32°, the temperature of the exterior air. When at 
twelve inches from the wall, it permitted the too free circulation 
of the air; and when in immediate contact with the polished 
bark of the Peach, perhaps another law of cooling came into 
operation. That law is that polished surfaces radiate heat 
slowest. Thus, if two glass bottles, equal in size and thickness 
of glass, and of the same shape, be filled with warm water, and 
one of the bottles be covered with an envelope of fine muslin, this 
bottle will give out heat to the surrounding air with much 
greater rapidity than the other bottle: so that in a given time 
the bottle with the envelope will be found colder than the one 
which has no covering. 
In the uniformity of temperature being sustained by the 
equivalent radiation and absorption of the bodies at the surface 
of the earth, we find the solution of many iuteresting natural 
phenomena. The production of dew and frost are to be thus 
acco nted for. In the absence of the sun, the surface of the 
earth losing, by radiation, a great quantity of heat, should have 
its temperature considerably lowered, were it not, that the canopy 
of clouds which generally lie above it radiate in return, and thus 
mainain the temperature almost the same. If then the clouds be 
absent, all the heat radiated by the earth is lost in the planetary 
spac •, and the temperature of its surface brought many degrees 
belo .v that of the atmosphere. The stratum of air which lies in 
cont set with the surface of the ground is then cooled, by contact, 
and a portion of the watery vapour, which it had possessed in 
its elastic form, is deposited as liquid water. If the temperature 
of the air be itself low, and the night very clear, the cooling may 
proceed so far that the drops of dew at the moment of their 
deposition shall be frozen, and thus form frost. The truth of 
tills explanation is demonstrated by the fact, that it is only on 
the surfaces of good radiators, and during clear starlit nights, 
that he dew or frost is found. If a plate of polished metal be 
laid n the centre of a rough board, and exposed to the air of a 
frostv night, the rough surface will bo found in the morning 
cove ed with copious frost; but on the bright metal no trace 
will >e deposited. It is thus, that by lightly covering a thin 
layer of water with straw to increase the radiating power, a sheet 
of ic • may be obtained in a single night between the tropics, 
wher) the actual temperature of the air may have continued far 
above the freezing-point. That the cooling effect is produced by 
the loss of heat in its radiant form, and not by the contact or 
diffusion of the particles of the air, may be proved by the inter¬ 
position of a screen of any substance which intercepts the passage 
of ra iiant heat, when the deposition of the dew or frost instantly 
ceases, and the surface C00I3 no more.—( Kane’s Elements of 
Chemistry .) 
“And mark here a beautiful adaptation,” says Professor 
Johnston. “ Different substances are endowed with the property 
of radiating their heat, and of thus becoming cool with different 
degre j s of rapidity ; and those substances which in the air become 
cool Qrst also attract first, and most abundantly, the particles of 
failin' dew. Thus in the cool of a summer’s evening the grass 
plat is wet, while the gravel walk is dry ; and the thirsty pasture 
and every green leaf are drinking in the descending moisture, 
while the naked land and the barren highway are still unconscious 
of its fall. 
“How beautiful is the contrivance by which water is thus 
evaporated or distilled as it were into the atmosphere—largely 
