i8 THE POPULAR SCIENCE MONTHLY, 



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 Pro- 

 fessor 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. neat-Kadiation. 



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 lati- 

 tude, the difference being at least one sixteenth between the equator 

 and latitude 30. The northern hemisphere he also found to be a lit- 

 tle 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 varia- 

 tions in the number and magnitude of sun-spots can not directly pro- 

 duce any sensible effect upon terrestrial temperatures, he calls atten- 

 tion to the fact that even slight changes in the dej^th 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 con- 

 siderable 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 sui'face, and the faciiljB more ; this 

 has been directly determined with the thermopile. The whole amount 



