case, little or no cold will be obsciwcd upon them above that of the atmosphere, as the
frequent application of V'arm air must quickly return a heat equal, or nearly so, to tJiat
which they had lost by radiation. A slight agitation of the air is suflicient to produce
some effect of this kind ; though, as has already been said, such an agitation, wlicn tho
air is very pregnant with moisture, will render greater the quantity of dew ; ono requisite
for a considerable production of this fluid being more increased by it, than another is
diminished.
^ 1335. I t has been remarked, that the hurtful effects o f cold occur chiefly in holloiv places.
I f this be restricted to what luippens on tho serene and calm nights, two reasons, Irom
different som*ces,_ are to be assigned for it. The first is, that the air, being stiller in such
a situation than in any other, the cold, from radiation iu tlic bodies contained in it, will
l)c less diminished by rcncivcd applications of warmer air ; the second, that, from tho
longer continuance of the same air in contact with the ground, in depressed places
than in others, less dew will be deposited, and therefore less heat extricated, during its
formation.
1336. An observation, closely connected with the preceding, namely, that, in clear and
still nights, frosts are less severe upon the hills than in the neighbouring plains, has excited
more attention, chiefly from its contradicting what is commonly regarded an established
fact, that the cold of the atmosphere always increases with the distance from the earth.
Rut, on the contrary, the fact is certain, that, in very clear and still nights, the air near
to the earth is colder than that which is more distant from it, to the height of at least
220_fect, tliis being the greatest to which experiments relate. If, then, a hill be supposed
to rise from a plain to the height of 220 feet, having upon its summit a small flat surface
covered with grass; and if the atmosphere, during a calm and serene night, be admittcci
to be 10° warmer there than it is near the surface of the low grounds, which is a less
difference than ivhat sometimes occurs iu such circumstances, it is manifest that, shoultl
both the grass upon the hill, and that upon the plain, acquire a cold of 10° by radiation,
the former will, notwithstanding, be 10° warmer than the latter. Hence, also, the tojis
of trees are sometimes found dry, when the grass on the ground’s sui-flice has been found
covered with dew.
1337. A very slight covering will exclude much cold. “ I had often,” observes Dr. Wells,
“ ill the pride of half knoudedgc, smiled at the means frequently employed by gardeners,
to protect tender plants from cold, as it appeared to me impossible that a thin mat, or
any such flimsy substance, could prevent them from attaining the temperature of the
atmosphere, by which alone 1 thought them liable to bo injured. But, when I liad
learned that bodies on tlie surface of the earth become, during a still and serene night,
colder than the atmosphere, by radiating thcir heat to tho heavens, I perceived immediately
a just reason for the practice which I had before deemed useless. Being desirous,
however, of acquiring some precise information on this subject, I fixed, perpendicularly,
in the earth of a grass-plot, four small sticks, and over tlicir upper extremities, which
were six inches above the grass, and formed the corners of a square, the sides of which
wcre_ two feet long, drew tightly a very thin cambric handkerchief. In this disposition
of things, therefore, nothing existed to prevent the free passage of air fi-om the exposed
grass, to that which was sheltered, except the four small sticks, and there was no substance
to radiate heat downwards to the latter grass, except the cambric handkerchief.
The temperature of the grass which was thus shielded from the sky, was, upon many
nights afterwards, examined by me, and was alv'ays found higher than that of neighbouring
grass, which was uncovered, if this was colder than the air.”
1338. The covering has most effect when placed at a little distance above the plants or
objects to be sheltered. A diflercncc 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. “ I found, for example, upon one night,” says Dr. Wells, that “ the wai-nith of
grass, sheltered by a cambric handkerchief raised a few inches iu the air, was 3° greater
than that of a neighbouring piece of grass, which was sheltered by a similar handkerchief
actually in contact with it. On another night, the diflerence between the temperatures
of two portions of grass, shielded in the same manner as the two above mentioned,
from the influence of the sky, was 4°. Possibly, 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.”
1339. Heat produced by walls. “ WaUs,” Dr. Wells continues, “ as flu-as warmth is
concerned, are regarded üs useful, during a cold night, to the plants which touch them
or arc near to them, only in two ways : first, by the mechanical shelter which they aflord
against cold winds ; and, secondly, by giving out the heat which they had acquii-ed
during tho day. I t appearing to me, however, that, on clcai- and calm nights, those on
which plants frequently receive much injury from cohl, walls must be beneficial in a
third way, namely by preventing, in part, the loss of heat which the plants would sustain
rom radiation, if they were fully exposed to tho sky, the following experiment was made
for the purpose of dctormmmg tho justness of this opinion. A cambric haiidkerciiief
having been placed by means of two upright sticks, perpendicularly to a grass-plot and
at right angles to the course of the air, a themiometcr was laid upon the grass, close to
tho lower edge of tho handkerchief, on its windward side. The thei-mometcr thus
situated, was several nights compared with another, lying on tho same grass-plot, but on
a part of it fully exposed to tho sky. On two of these nights, the air being cleai- and
calm, the grass close to the handkerchief was found to be 4° wai-mcr than the fully
exposed grass. On a third, the difference was 6°. An analogous fact is mentioned by
Garstin, who says that a horizontal surface is more abundantly dewed than one which
IS perpendicular to the groimd.”
1340. Heat from a covering o f snow. “ The covering of snow,” the same antlior
obscrvcs, which eountnes in high latitudes enjoy during tlie winter, has been very
commonly thought to be beneficial to vegetable substances on the surface of the earth
as tar as tiicir temperature is concerned, solely by protecting them from the cold of tlie
atmosphere. But, were this supposition just, the advantage of the covering would be
p-catly circumsci-foed; since the upper parts of trees and of tall shmbs arc still exposed
to tlie influence of the air. Another reason, however, is furnished for its usefulness bv
wliat has been said above ; which is, that it pi-events the occurrence of the cold, wliich
bodies on the earth acquire, in addition to that of the atmosphere, hy tlic radiation of
their heat to tlie heavens during still and cleai- nights. The cause, indeed, of this
ailditional cold, does not constantly operate; bnt its presence, during only a few hours
migfit effectually destroy plants which now pass unhurt through the winter Aa-ain’
as things are, while low vegetable productions are prevented, by thcir covering of snow’
irom becoming colder than the atmosphere, in consequence of their own radiation the
parts oi trees and tall shrubs which rise above the snow ai-e little affected by cold from
tins cause ; for their uttcimost twigs, now that they arc destitute of leaves, avc mnch
smaller than the thermometers suspended by me in the air, which, in this situation very
seldom became more than 2° colder than tlie atmosphere. Tiic larger hranclics too
wJiicli if fully exposed to the sky, would become colder than tlie extreme parts, arc in
a great degree sheltered by them; and, in tho last place, the tmnks are .sheltered both
by the smaller and larger parts; not to mention that the trunks must derive heat bv
conduction through the roots from the earth kept warm by tho snow. In a similar iv.ar
IS partly to be explained tho manner in whieh a layer of earth or straw preserves
vegetable mattere from tlio injurious effects of cold in winter.” (Essay m Dew.)
J t tr icm opinions have been broached as ia tlw came o f dew, and it'h a s been
attributed to electricity and various other complicated causes; but all tho best writers
on the subject coincide with the opinion of Dr. Wells, that tho coldness of the earth’s
snrlaco produced by the sudden abstraction of caloric at night condenses the moisture
contained m the air, just m the .same manner as wc see drops of moisture condensed on
the outside of a cold decanter when it is brought from an ice-house or cold ccUar into
the moist and heated atmosphere of a dining-room.
1342. The nature o f light is only imperfectly known. The liglit which proceeds from
the sun appears to be of two distinct kinds, viz. the rays of heat and the rays of light
and both of these produce a chemical action on plants. Schcelc discovered that a glas!
mirror, held before the fire, reflected the rays of light, but not the rays of caloric- but
when a mctollic mm-or was placed in the samo situation, both heat and light were
reflected. The mirror of glass became hot in a short time, but no chang-e of temlieratnre
took place on the mctalhc mirror. This experiment shows that the glass muTor absorbed
the rays of caloric, and reflected those of light; while tho metallic mirror, suffering no
change of temperature, reflected both. I f a glass plate bo held before a bnriihig body
the rays of hght are not sensibly interrupted, but the rays of caloric arc intercepted- fo!
no .sensible heat is observed on the opposite side of the glass; hut when the glass has
reached a proper dcgi-cc of tcmporatiiro, the rays of caloric arc transmitted with the
same facility as those of light; and thus the rays of light and caloric may be separated
But ho curious experiments of Dr. Herschcl have clearly proved that the invisible ray!
winch are emitted by tho sun have tho greatest heating power. In thoso experiments
the different coloured rays wore thrown on tho bulb of a very delicate thei-momcter and
then- heating power was observed. The heating poivcr^ of the violet, green, and red
ra^ys were found to ho to each other as tho foUowing numbers: — violet, 16-0- green
22-4; rod, 55-0. The heating power of the most refrangible rays was least, and this
power increases as tho refraiigibility diminishes. The red ray, therefore, has the greatest
heating power ; a,nd the violet, which is tho most refrangible, the least. The illmuluat-
ing power. It has been already observed, is greatest in tho middle of the spcctrnm and
it diminishes towards both extremities; but the heating power, which is least at the