RADIATION THERMOMETERS.
=82. Solar and Terrestrial Radiation considered.=--The surface of the
earth absorbs the heat of the sun during the day, and radiates heat into
space during the night. The envelope of gases and vapour, which we call
the atmosphere, exerts highly important functions upon these processes.
Thanks to the researches of Professor Tyndall, we are now enabled to
understand these functions much more clearly than heretofore. His
elaborate, patient, and remarkably sagacious series of experiments upon
radiant heat, have satisfactorily demonstrated that _dry_ air is as
transparent to radiant heat as the vacuum itself; while air _perfectly
saturated_ with aqueous vapour absorbs more than five per cent. of radiant
heat, estimated by the thermal unit adopted for the galvanometer
indications of the effect upon a thermo-electric pile.
Aqueous vapour, in the form of fog or mist, as is well known, gives to our
sensation a feeling of cold, and interferes with the healthy action of the
skin and the lungs; the cause being its property of absorbing heat from
our person.
Air containing moisture in an invisible state likewise exerts a remarkable
influence in radiating and absorbing heat. By reason of these properties,
aqueous vapour acts as a kind of blanket upon the ground, and maintains
upon it a higher temperature than it would otherwise have. "Regarding the
earth as a source of heat, no doubt at least ten per cent. of its heat is
intercepted within ten feet of the surface." Thus vapour--whether
transparent and invisible, or visible, as cloud, fog, or mist--is
intimately connected with the important operations of solar and
terrestrial radiation. Cloudy, or humid days, diminish the effect upon the
soil of solar radiation; similar nights retard the radiation from the
earth. A dry atmosphere is the most favourable for the direct transmission
of the sun's rays; and the withdrawal of the sun from any region over
which the air is dry, must be followed by very rapid cooling of the soil.
"The removal, for a single summer night, of the aqueous vapour from the
atmosphere which covers England, would be attended by the destruction of
every plant which a freezing temperature could kill. In Sahara, where 'the
soil is fire and the wind is flame,' the refrigeration at night is often
painful to bear. Ice has been formed in this region at night. In
Australia, also, the _diurnal range_ of temperature is very great,
amounting, commonly, to between 40 and 50 degrees. In short, it may be
safely predicted, that wherever the air is _dry_, the daily thermometric
range will be great. This, however, is quite different from saying that
when the air is _clear_, the thermometric range will be great. Great
clearness to light is perfectly compatible with great opacity to heat; the
atmosphere may be charged with aqueous vapour while a deep blue sky is
overhead; and on such occasions the terrestrial radiation would,
notwithstanding the 'clearness,' be intercepted." The great range of the
thermometer is attributable to the absence of that protection against gain
or loss of heat which is afforded when aqueous vapour is present in the
air; and during such weather the rapid abstraction of moisture from the
surface of plants and animals is very deleterious to their healthy
condition. "The nipping of tender plants by frost, even when the air of
the garden is some degrees above the freezing temperature, is also to be
referred to chilling by radiation." Hence the practice of gardeners of
spreading thin mats, of bad radiating material, over tender plants, is
often attended with great benefit.
By means of the process of terrestrial radiation ice is artificially
formed in Bengal, "where the substance is never formed naturally. Shallow
pits are dug, which are partially filled with straw, and on the straw flat
pans containing water which had been boiled is exposed to the clear
firmament. The water is a very powerful radiant, and sends off its heat
into space. The heat thus lost cannot be supplied from the earth--this
source being cut off by the non-conducting straw. Before sunrise a cake of
ice is formed in each vessel.... To produce the ice in abundance, the
atmosphere must not only be clear, but it must be comparatively free from
aqueous vapour."
Considering, therefore, the important consequences attending both
terrestrial and solar radiation, it appears to us that observations from
radiation thermometers are of much more utility in judging of climate than
is usually supposed. These observations are very scanty; and what few are
upon record are not very reliable, principally from bad exposure of the
instruments, while the want of uniformity in construction may be another
cause. Herschell's actinometer and Pouillet's pyrheliometer, instruments
for ascertaining the absolute heating effect of the sun's rays, should,
however, be more generally employed by meteorologists. In comparing
observations on radiation it should be kept in mind, that "the difference
between a thermometer which, properly confined [or shaded], gives the true
temperature of the night air, and one which is permitted to radiate freely
towards space, must be greater at high elevations than at low ones;"[6]
because the higher the place, the less the thickness of the vapour-screen
to intercept the radiation.
=83. Solar Radiation Thermometer.=--"As the interchange of heat between
two bodies by radiation depends upon the relative temperature which they
respectively possess, the earth, by the rays transmitted from the sun
during the day, must be continually gaining an accession of heat, which
would be far from being counterbalanced by the opposite effect of its own
radiation into space. Hence, from sunrise till two or three hours after
mid-day, the earth goes on gradually increasing in temperature, the
augmentation being greatest where the surface consists of materials
calculated, from their colour and texture, to absorb heat, and where it is
deficient in moisture, which, by its evaporation, would have a tendency
to diminish it."[7] It is, therefore, important to have instruments for
measuring the efficacy of solar radiation, apart from those for exhibiting
the temperature of the place in the shade.
[Illustration: Fig. 63.]
Fig. 63 shows the arrangement of Negretti & Zambra's maximum thermometer,
for registering the greatest heat of the sun's direct rays, hence called a
_solar radiation thermometer_. It has a blackened bulb, the scale divided
on its own stem, and the divisions protected by a glass shield. In use it
should be placed nearly horizontally, resting on Y supports of wood or
metal, with its bulb in the full rays of the sun, resting on grass, and,
if possible, so that lateral winds should not strike the bulb; and at a
sufficient distance from any wall, so that it does not receive any
_reflected_ heat from the sun. Some observers place the thermometer as
much as two feet from the ground. It would be very desirable if one
uniform plan could be recognized: that of placing the instrument as
indicated in the figure appears to be most generally adopted, and the
least objectionable.
=84. Vacuum Solar Radiation Thermometer.=--In order that the heat absorbed
by the blackened bulb of the solar radiation thermometer may not in part
be carried off by the currents of air which would come into contact with
it, the instrument has been improved by Messrs. Negretti and Zambra into
the _vacuum solar radiation thermometer_, as illustrated by fig. 64.
[Illustration: Fig. 64.]
This consists of a blackened-bulb radiation thermometer, enclosed in a
glass tube and globe, from which all air is exhausted. Thus protected from
the loss of heat which would ensue if the bulb were exposed, its
indications are from 20 deg. to 30 deg. higher than when placed side by side with
a similar instrument with the bulb exposed to the passing air. At times
when the air has been in rapid motion, the difference between the reading
of a thermometer giving the true temperature of the air in the shade, and
an ordinary solar radiation thermometer, has been 20 deg. only, whilst the
difference between the air temperature and the reading of a radiation
thermometer in vacuo has been as large as 50 deg. It is also found that the
readings are almost identical at distances from the earth varying from six
inches to eighteen inches. By the use of this improvement, it is hoped
that the amounts of solar radiation at different places may be rendered
comparable; hitherto they have not been so; the results found at different
places cannot be compared, as the bulbs of the thermometers are under very
different circumstances as to exposure and currents of air. Important
results are anticipated from this arrangement. The observations at
different places are expected to present more agreement. Observers would
do well to note carefully the effect of any remarkable degree of intensity
in the solar heat upon particular plants, crops, fruit or other trees.
=85. Terrestrial Radiation Thermometer= is an alcohol minimum thermometer,
with the graduations etched upon the stem, and protected by a glass
shield, as shown in figure 65, instead of being mounted on a frame. The
bulb is transparent; that is to say, the spirit is not coloured.
[Illustration: Fig. 65.]
In use, it should be placed with its bulb fully exposed to the sky,
resting on grass, the stem being supported by little forks of wood. The
precautions required with this thermometer are similar to those for
ordinary spirit thermometers, explained at page 76.
[Illustration: Fig. 66.]
=86. AEthrioscope.=--The celebrated experimental philosopher, Sir John
Leslie, was the inventor of this instrument, the purpose of which is to
give a comparative idea of the radiation proceeding from the surface of
the earth towards the sky. It consists, as represented in fig. 66, of two
glass bulbs united by a vertical glass tube, of so fine a bore that a
little coloured liquid is supported in it by its own adhesion, there being
air confined in each of the bulbs. The bulb, _A_, is enclosed in a highly
polished brass sphere, _D_, made in halves and screwed together. The bulb,
_B_, is blackened and placed in the centre of a metallic cup, _C_, which
is well gilt on the inside, and which may be covered by a top, _F_. The
brass coverings defend both bulbs from solar radiation, or any
adventitious source of heat. When the top is on, the liquid remains at
zero of the scale. On removing the top and presenting the instrument to a
clear sky, either by night or by day, the bulb, _B_, is cooled by
terrestrial radiation, while the bulb, _A_, retains the temperature of the
air. The air confined in _B_, therefore, contracts; and the elasticity of
that within _A_ forces the liquid up the tube, to a height proportionate
to the intensity of the radiation. Such is the sensitiveness of the
instrument, that the smallest cloud passing over it checks the rise of the
liquid. Sir John Leslie says:--"Under a clear blue sky, the _aethrioscope_
will sometimes indicate a cold of fifty millesimal degrees; yet, on other
days, _when the air seems equally bright_, the effect is hardly 30 deg." This
anomaly, according to Dr. Tyndall, is simply due to the difference in the
quantity of aqueous vapour present in the atmosphere. The presence of
invisible vapour intercepts the radiation from the aethrioscope, while its
absence opens a door for the escape of this radiation into space.
=87. Pouillet's Pyrheliometer.=--"This instrument is composed of a shallow
cylinder of steel, _A_, fig. 67, which is filled with mercury. Into the
cylinder a thermometer, _D_, is introduced, the stem of which is protected
by a piece of brass tubing. We thus obtain the temperature of the mercury.
The flat end of the cylinder is to be turned towards the sun, and the
surface, _B_, thus presented is coated with lamp black. There is a collar
and screw, _C_, by means of which the instrument may be attached to a
stake driven into the ground, or into the snow, if the observations are
made at considerable heights. It is necessary that the surface which
receives the sun's rays should be perpendicular to the rays; and this is
secured by appending to the brass tube which shields the stem of the
thermometer, a disk, _E_, of precisely the same diameter as the steel
cylinder. When the shadow of the cylinder accurately covers the disk, we
are sure that the rays fall, as perpendiculars, on the upturned surface of
the cylinder.
[Illustration: Fig. 67.]
"The observations are made in the following manner:--First, the instrument
is permitted, not to receive the sun's rays, but to radiate its own heat
for five minutes against an unclouded part of the firmament; the decrease
of the temperature of the mercury consequent on this radiation is then
noted. Next, the instrument is turned towards the sun, so that the solar
rays fall perpendicularly upon it for five minutes; the augmentation of
heat is now noted. Finally, the instrument is turned again towards the
firmament, away from the sun, and allowed to radiate for another five
minutes, the sinking of the thermometer being noted as before. In order to
obtain the whole heating power of the sun, we must add to his observed
heating power the quantity lost during the time of exposure, and this
quantity is the mean of the first and last observations. Supposing the
letter _R_ to represent the augmentation of temperature by five minutes'
exposure to the sun, and that _t_ and _t^1_ represent the reductions of
temperature observed before and after, then the whole force of the sun,
which we may call _T_, would be thus expressed:--_T = R + 1/2(t + t^1)_.
"The surface on which the sun's rays here fall is known; the quantity of
mercury within the cylinder is also known; hence we can express the effect
of the sun's heat upon a given area, by stating that it is competent, in
five minutes, to raise so much mercury so many degrees in
temperature."--_Dr. Tyndall's "Heat considered as a Mode of Motion."_
[Illustration: Fig. 68.]
=88. Sir John Herschell's Actinometer=, for ascertaining the absolute
heating effect of the solar rays, in which _time_ is considered one of the
elements of observation, is illustrated by fig. 68. The actinometer
consists of a large cylindrical thermometer bulb, with a scale
considerably lengthened, so that minute changes may be easily seen. The
bulb is of transparent glass filled with a deep blue liquid, which is
expanded when the rays of the sun fall direct on the bulb. To take an
observation, the actinometer is placed in the shade for one minute and
read off; it is then exposed for one minute to sunshine, and its
indication recorded; it is finally restored to the shade, and its reading
noted. The mean of the two readings in the shade, subtracted from that in
the sun, gives the actual amount of expansion of the liquid produced by
the sun's rays in one minute of time. For further information, see _Report
of the Royal Society on Physics and Meteorology_; or _Kaemtz's
Meteorology_, translated by C. V. Walker; or the _Admiralty Manual of
Scientific Instructions_.