350 
THE COTTAGE GARDENER. 
[March 6. 
Now that glass is so reduced in price, we hope to see it 
| supersede extensively all other materials for the protec¬ 
tion of wall-trees; and we will to-day endeavour to 
dispel a remnant of ignorance which we did not believe 
lingered in the mind of the oldest jobbing gardener in 
the least frequented nook of all the British Isles—an 
ignorance thus expressed to us a few days since—“ Glass 
cannot be so warm as woollen netting !" If either the 
i woollen net or the glass was touching the body to be 
kept warm, there would be some truth in this opinion ; 
j but when they are at a slight distance from that body, 
j so as to inclose air between it and the exterior cold, the 
: contrary is the truth. We will explain this once more, 
j although we have done so as follows both in these pages 
and elsewhere:— 
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 radiation of heat from a body keeps 
it warmer. For example,—a thermometer placed upon 
a grass-plat, exposed to a clear sirs'-, fell to 35°, but 
another thermometer within a few yards of the preced¬ 
ing, but with the radiation of the rays of heat from tire 
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 perpendicular to the horizon; for when 
the mercury in a thermometer, 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 appa¬ 
rently such slight shelter to our wall-fruit when in blos¬ 
som. A sheet of canvas, 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 
; an - at night before the return of day begins to re-elevate 
I the external temperature. 
The colder the body surrounding another body, the 
more rapid the radiation from the latter ; for it is a law 
ot 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 cooling body is expanded, and becoming specifi¬ 
cally lighter, ascends, and is replaced by a colder por¬ 
tion. This, in its turn, becomes heated and dilated, and 
gives place to another colder portion, and thus the pro¬ 
cess goes on until the cooling body 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 ; j 
but if there be a brisk wind, a constant current of air at ' 
the lowest temperature then occurring is brought in con- 1 
stant 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 preventing the sweeping con¬ 
tact of cold air at such times. 
We could give some further illustrations of this, but 
are spared the necessity at present by the following com¬ 
munication from G. Sparkes, Esq., of Bromley, Kent:— 
“ The amelioration of climate produced by a glass cover¬ 
ing, without artificial heat, having of late been prominently 
brought before the public, it may, perhaps, interest some 
of your readers to learn the result of some observations 
made and recorded from the 1st February, 1850, to the | 
31st January, 1851. 
“ The structure within which the observations were made, j 
is a lean-to, with low front wall; roof at 45°, glazed with 
thick crown; air damped occasionally with hot water; aspect ! 
S.S.E., hut rather shaded in the morning and afternoon. It 
was only close, to the floor that I was able to find for the 
thermometer a place screened both from the direct and 
reflected rays of the sun, and also from any warm vapour. 
“ A glass structure has a two-fold office. First, to excite 
growth by increased heat; and, secondly, to preserve fife by 
protection from cold. It is obvious that a thermometer 
placed near the floor can only indicate the extent to which 
the latter office is fulfilled. 
“ The external temperature was not taken from actual 
observation, but from the Chiswick tables. Considering 
the elevation of this spot, and its proximity to Greenwich, 
the temperature of which is .44° below Chiswick, it is rea¬ 
sonable to suppose that at least half a degree ought to be 
added to the differences recorded below:— 
Difference during the three months of spring between the 
mean external and internal temperature . 8.9° 
Summer. 5.5° 
Autumn .. .. 5.5° 
Winter . 3.6° 
“ These differences will at first sight appear trifling ; less, 
indeed, in winter, than the ordinary difference between 
London and Penzance. Yet within the house it only froze 
six times during the year; and the lowest point the ther¬ 
mometer ever reached was 29° ; out of doors it fell to 14°, 
and on the same night, even in Cornwall, to 23°. After a 
warm sunny day in spring everything within is compara¬ 
tively safe, however low the external temperature may fall. 
A difference of 14° is common, and on one occasion 19° 
were observed. 
“ The daily maximum within the house is seldom much 
greater than that without; and this equality reduces the 
average difference. Thus if, on any day, the maximum of 
the two thermometers is 60°, and the minimum at night is, 
indoors, 50°, and outside 40°, the mean temperature of the 
twenty-four hours is recorded as 55° in the one case, and 
50° in the other; so that the difference seems only 5° 
instead of 10°. Taking, for instance, four consecutive weeks 
in August, 1850, the average maximum inside the house 
was 72.3; the average minimum 58.5; the mean 05.4. At 
Chiswick, during the same period, the maximum was 71.9 ; 
the minimum 47.0; the mean 59.7. The difference is 
almost entirely in the minima. It may here be noted as a 
singular fact, verified in each of the twelve months, that the 
average minimum inside the house closely approximates, 
and is generally all but identical with, the outside mean at j 
Chiswick. 
“ The average difference is reduced by warm, dull, damp 
weather from the south-west. In this case, the air is warmed 
not by sunshine, but partly from the heat given out by the 
condensation of vapour, and partly from having passed over 
the warm current known as the gulpli stream. When, 
therefore, the external air becomes itself the source of heat, 
it is obvious that the inside of a greenhouse may, for a time, 
be the coldest spot in the garden. It is fortunate, however. 
