Experiments with the thermopile show that the heat radiated by the solar disk varies, like the light, very considerably from the center to the edges. The first observations of this kind were made by Professor Henry, at Princeton, in 1845, and have since been repeated by many others, Secchi and Langley especially. According to Langley, the heat emitted from a point about 20" from the limb is only one half that from the same extent of surface at the center of the disk; the diminution of heat being notably less than that of light, as shown by Vogel's observations. Langley's table runs as follows, the first column giving the distance from the center of the disk, and the second the intensity of radiation shown by the thermopile:
| Distance from Center. | Heat-Radiation. | ||
| 0∙00 | 100 | ||
| 0∙25 | 99 | ||
| 0∙50 | 95 | ||
| 0∙75 | 86 | ||
| 0∙95 | 62 | ||
| 0∙98 | 50 | ||
Besides this regular variation of the radiation from center to edge, Secchi in 1852 found, or thought he found, a notable difference between the radiation from the equator of the sun and that from the higher latitude, the difference being at least one sixteenth between the equator and latitude 30°. The northern hemisphere he also found to be a little hotter than the southern. Later investigators (Langley especially) have failed to find any such difference; and on the whole it seems probable that Secchi was mistaken; though this is not certain, as it would be quite unsafe to assert that the actual condition of the sun's surface may not have changed between 1852 and 1876.
In connection with the absorption of the solar atmosphere, Langley has ventured some interesting speculations. After showing that variations in the number and magnitude of sun-spots can not directly produce any sensible effect upon terrestrial temperatures, he calls attention to the fact that even slight changes in the depth and density of the sun's absorbing layer would make a great difference; and he raises the question whether we may not find here the explanation of glacial and carboniferous periods in the earth's history. It is quite certain that, were the envelope removed, the solar radiation would be at least doubled, and perhaps increased in a much higher ratio; while any considerable increase of its thickness would so diminish our heat-supply as to give us perpetual winter.
As yet our means of observation have not sufficed to detect with certainty any variations in the amount of heat emitted by the sun at different times. That there are such variations is almost certain, since the nuclei of sun-spots radiate much less heat as well as light than neighboring regions of the solar surface, and the faculæ more; this has been directly determined with the thermopile. The whole amount
