SKETCH OF PROFESSOR S. P. LANGLEY. 405 



condition. Four years of labor on this subject had not failed to sug- 

 gest many other researches. 



A detailed study of the distribution of the heat of the solar sur- 

 face was begun about this time, by means of the thermopile, and was 

 quickly rewarded by the discovery of an unknown thermo-chroic ac- 

 tion in the sun's atmosphere, such that it transmits the light less readi- 

 ly than the heat, owing to the difference in wave-length. An inter- 

 esting consequence of this action is that, if, at any time, the sun's 

 atmosphere should grow thicker, the color of the sun would tend 

 toward red ; if thinner, then toward blue. These changes, which are 

 quite possible, suggest interesting explanations of some of the phe- 

 nomena of the variable stars. The glacial epochs on the earth may be 

 connected with changes in the solar atmosphere. 



In 1877 we find another outcome of the series of measures of the 

 heat from various parts of the sun's disk, and especially from the 

 umbrae, etc., of sun-spots. The periodic changes in the spotted area 

 of the solar disk, which had long been known, induced the inquiry 

 whether changes in the amount of spotted surface bore any relation to 

 changes of temperature on the earth's surface. 



The result of the extremely delicate measures of Professor Langley 

 led plainly to the conclusion that the direct effect of sun-spots on 

 terrestrial temperature is sensible ; that, when the spotted area is a 

 maximum, the temperature is on that account lower, and the converse ; 

 but that the total direct effects of the periodic changes in the spotted 

 area on the earth's mean temperature are extremely small, not more 

 than a change of three tenths of 1 C. in eleven years, and not less 

 than one twentieth of 1 C. The indirect effects are not here con- 

 sidered. 



A thermopile used in connection with the most sensitive galvanom- 

 eters is an extremely delicate instrument ; and Allegheny Observatory 

 now possessed a most complete outfit of this sort. 



But the most important and pressing questions in solar physics 

 demanded a means of measurement of heat still more delicate. When 

 it w r as a question to measure the heat radiation from the different 

 parts of the sun's disk, the thermopile was adequate. But if the heat 

 from one of these parts is spread out into a heat-spectrum several feet 

 or even yards long, it becomes necessary to devise new means of 

 measuring the minute differences between the various parts. Such a 

 device is the bolometer, which consists of two systems of extremely 

 thin steel or platinum strips. Through these two systems an electric 

 current passes. A sensitive galvanometer connected with both sys- 

 tems keeps its needle steady when the currents are equal. 



If one system is now exposed to heat radiations while the other is 

 protected from them, the temperature of the first is raised, its electric 

 resistance is increased, and the battery-currents through the two sys- 

 tems and the galvanometer no longer balance. The galvanometer- 



