CHAPTER VIII.

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.

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.

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.

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.

=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.

"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."_

=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_.