In the experiments on radiant heat it has been noticed, that when such heat from whatever source has been once absorbed and again radiated it does not retain the peculiarities of its source. But heat of low refrangibility may be converted into that of higher refrangibility.
M. Berard ascertained that when light suffers double refraction in Iceland spar, the two pencils formed spectra which exhibited similar properties; in both, the calorific power differed at the two ends, and existed beyond the visible red rays. Also, on polarising by reflection from glass a beam of solar light, and receiving the reflected ray on a second glass, the latter being capable of turning round till the ray ceased to be reflected from it, he found that while the light was reflected, the heat was also reflected, and that when no light was reflected, there was no hest. The like effect was produced when, instead of a pencil of solar light a portion of radiant heat from a body not luminous was employed, and the inference is, that the particles of radiant heat are polarised by reflection, like thoso of light. This subject has however been since more completely investigated by Professor Forbes, (` Edinburgh Phil. Trans.', 1835.) Knoblauch, Pogg. Annul,' lxxiv.) has also obtained distinct evidence of the diffraction and interference of the rays of heat.
The subject of solar radiation, or of tho direct force of the sun's rays, so important to the agriculturist as well as to the philosopher, was treated by Mr. Daniell, in his ' Meteorological Essays,' 1823. On comparing a thermometer exposed to the action of the sun with one which gave the mean temperature of the air in the shade, Mr. Daniell observed that the power of solar radiation varies with the sun's decli nation ; the greatest intensity taking place in June, though the greatest mean temperature of the atmosphere does not occur till July.
lie observed also that the radiation varies at different hours of the day, increasing with the sun's altitude till a short time after it arrives on the meridian, and then diminishing till the evening. From the observations of Major Sabine at Sierra Leone, at Bahia, and at Port ltoyal, Daniell was led to conclude that the intensity of solar radios.
tion diminishes in proceeding towards the equator ; and the conclusion appears to have been subsequently confirmed by the observations of Scoresby and Parry, and of Richardson, in the Arctic regions. From the observations of Sabine on the mountains of Jamaica, Daniell considers that this radiation increases from the surface of the earth upwards. [AcTrsomsmen.] Since all bodies, even in vacuo, lose heat by radiation, it is easy to conceive that any part of the earth's surface, when not exposed to the direct action of the sun, must emit calorific rays of heat towards the heavens, and thus must become cooled. This is called terrestrial radiation, and the subject was particularly considered by Mr. Daniell, who, from observations continued during all the months of the year, found that the maximum depression of the thermometer, on account of radiation, varied from 10° to 17° between midwinter and mid summer ; but that the mean depression was the least in January and July, and the greatest in April. It is obvious however that numerous observations are yet to be made in different regions of the earth before any general theory respecting the extent and law of the variations of solar and terrestrial radiations can be formed. The progress that has been made on the subject of late years is noticed under METEO ROLOGY ; TEMPERATURE, TERRESTRIAL, DISTRIBUTION OF.