CHAPTER IV.

RADIATION.

WE have hitherto been speaking of the temperature of the air, but have not alluded to the source whence that temperature is derived, or to any means of measuring such an agency. The heat which affects our atmosphere must be derived from one of three sources, the earth itself, the stars, or the sun.

Although the earth possesses internal heat, as is shown by the fact that its temperature increases with the depth below the surface, at the rate of about one degree for every fifty feet, we are justified in disregarding this internal heat as an agency which may exert influence on our climate, for the simple reason that the earth is not growing appreciably colder. If the earth gave out heat to the atmosphere around it to any measurable extent, its own temperature must necessarily fall, and if that fell the mass of the earth would grow colder and would contract. Now the effect of any contraction would be to make the earth revolve more quickly on its axis, or to make the day shorter. Astronomers, however, tell us that the day has certainly not become shorter within historic times; in fact, that the tendency is for it to grow surely but very slowly longer, owing to the retarding action of the tides. The earth's temperature, therefore, cannot have been

appreciably reduced within the last three thousand years, and we may assume that the earth, as an independent source of heat, may be practically disregarded.

1

The relation between the earth, considered as an independent source of heat, and the sun, is thus stated by Dr. Haughton: The heat received from the interior of the earth, at present, is sufficient to melt a layer of ice one quarter of an inch in thickness all over the surface of the globe; while that received from the sun would melt a layer forty-six feet in thickness; being thus 2,208 times greater than the heat derived from the interior.'

It need scarcely be said that the amount of benefit in the way of heat we receive from the stars is infinitesimal, for the heat which reaches us even from the moon, which is much nearer than the stars, is so small in amount as to be hardly measurable even by the most delicate apparatus.

We see, therefore, that we must regard the sun's rays as the sole source of the heat which renders the earth habitable, and radiation from the earth into space as the sole way in which the heat is again lost.

Solar Radiation.-Our first problem, therefore, is to measure the amount of solar heat when it has arrived within our atmosphere. For this purpose various plans have been proposed, but none of them can be described as thoroughly satisfactory. It is not sufficient to expose a thermometer to the rays of the sun, for, if the bulb be bright, much of the heat which strikes it will be radiated from it again.

It would go beyond the scope of this work to de

1 Six Lectures on Physical Geography, p. 77.

scribe particularly Pouillet's Pyrheliometer, one of the first instruments devised to measure solar radiation. A more recent instrument for the same purpose is Herschel's Actinometer, which consists of a glass tube furnished with a bulb, like a thermometer, but filled with a highly-coloured blue liquid, an ammoniacal solution of copper, of which the expansion affords a measure of the sun's action, and provided with an arbitrary scale. The instrument is placed inside a blackened box, covered by a lid, removable when an observation is to be taken. The front of the box is glazed, to protect the bulb from being cooled by convection, as it would be if the air had free access to the bulb. The instrument is taken out and exposed in the sunshine for a minute, and then in the shade for an equal interval. These operations are repeated, and the mean of all the indications is taken as the measure of the desired effect. An arrangement is provided by which the column is brought to the zero point before each observation by means of a screw. The instrument is very difficult to manage, for it is so sensitive that a very slight elevation of temperature fills the entire tube with liquid. It is now rarely used.

One of the most recent forms of actinometers, which has been found to yield good results, is that invented by Professor Balfour Stewart, F.R.S., ('Nature,' vol. xiii. p. 118). The arrangement adopted is to enclose a large-bulb thermometer in a cubical castiron chamber of such massive material that its tem perature will remain constant for some time. The sun's rays are admitted to the thermometer bulb through a hole in the chamber wall. The mode of making the observation with this instrument is gene

rally similar to that employed with Herschel's instru

ment.

The use of a box, blackened inside and out and provided with a glazed lid, to increase the effect of the sun's heat on an inclosed thermometer is a very old device. The reason of its efficacy is that while the glass is diathermanous to the rays of heat coming from the sun, it is nearly athermanous to rays coming from a cooler body, such as the contained thermometer, and it therefore does not allow the heat which enters the box to escape with the same facility as it passed in.

A familiar illustration of this property of glass is afforded by the use of glass fire-screens, which allow a view of the fire while stopping its heat, whereas, as everyone knows, the glass of a window stops comparatively little of the sun's heat. The reason is that the temperature of the fire is infinitely lower than that of the sun's surface, and accordingly the so-called 'dark' heat rays from the fire cannot pass through the glass, while those emanating from the sun are freely transmitted.

In such a box as has been described the tempera ture has been known to rise above the boiling-point of water, but it is obvious that such a method of observation can lay but little claim to scientific accuracy.

The instrument generally adopted in this country for measuring the intensity of solar radiation is termed the Black Bulb Thermometer in vacuo, and its construction was first suggested by Sir John Herschel. This instrument in sunshine will show a temperature far above that of an ordinary thermometer placed beside it.

It consists of a sensitive maximum thermometer

1 Diathermanous bodies are those which allow heat to pass through them, Athermanous those which do not.

having the bulb and about an inch of the stem coated with dull lamp-black. The whole is then inclosed in a glass tube, of which one end is blown out into a bulb of about 24 ins. diameter, in the centre of which the bulb of the thermometer is fixed. The glass jacket, so constructed, is then exhausted of air by a good air-pump, and permanently closed.

It is evident that as the action of this instrument depends in some measure on the completeness of the vacuum, it is not sufficient simply to test the thermometer which is to be inclosed in the envelope. It is necessary to have some independent method of gauging

FIG. 9.

Black Bulb Thermometer in vacuo.

the amount of rarefaction which has been attained. This may be done in various ways, some makers introducing a mercurial pressure gauge into the vacuum chamber, while others test the state of the vacuum by the passage of the electric light, by soldering platinum wires into the tube, as shown in the figure.

The instrument is then freely exposed to the sun and air by fixing it horizontally above the ground at the same height as that at which the shade thermometers are placed. This is usually four feet. It must be at a distance from walls or trees, and from any

The object of coating part of the stem is to prevent the temperature of the blackened bulb being lowered by contact with the cooler glass of the unblackened stem.