216 The Smithsonian Institution 



chroic action in the solar atmosphere, by reason of which, 

 owing to the difference in wave length, it transmits heat 

 more readily than light. Two years later, in 1876, another 

 discovery was announced as a result of his measures of the 

 heat from various parts of the sun's disk ; this was in regard 

 to the direct effect of sun-spots on terrestrial climates. Fol- 

 lowing up the observations made by Joseph Henry in 1845, 

 Mr. Langley found that sun-spots exercise a direct influence 

 on terrestrial climates by decreasing the mean temperature 

 of the earth at their maximum. This decrease, however, he 

 found to be so minute that it is doubtful whether it is directly 

 observed or discriminated from other changes. Its whole 

 effect is represented by the change in the mean temperature 

 of our globe in eleven years, not exceeding three- tenths and 

 not less than one-twentieth of one degree of the centigrade 

 thermometer; but this refers merely to the direct action by 

 the observation of the surface, and is not to be considered as 

 the only one. 



His early work upon solar heat was done with the aid of the 

 thermopile, an instrument which, though it had been effec- 

 tively used for nearly fifty years in the study of radiant 

 energy, was found by him not sufficiently sensitive and trust- 

 worthy to be used for the more minute work which he found 

 it desirable to undertake. It was equal to the task of meas- 

 uring the radiation from different parts of the sun's disk. 

 When, however, the heat from a given part had been spread 

 out into a heat-spectrum, some new means of measuring the 

 minute difference between the various parts was indispensa- 

 ble ; and this was specially the case with the spectra formed 

 by " gratings," now coming into general use, which, with the 

 great advantage of distributing the energy in a " normal " 

 spectrum, had the defect of giving extremely little heat for 

 examination. 



