SAMUE;Iv PIERPONT LANGLIiY. II 



He now steadily pressed on in both these directions. As to scien- 

 tific astronomy, his resourceful work while in charge of the Coast 

 Survey party, observing the eclipse of 1869 in Kentucky, led Pro- 

 fessor Winlock of Harvard to invite him to join the Government 

 expedition for observing the eclipse of 1870 in Spain, and as to 

 observator}- work applied to practical affairs, he published, shortly 

 after his return to America, an article in the American Journal of 

 Science, proposing a more complete transmission of time, not only 

 to railways, but to makers and regulators of clocks and watches in 

 various city centers. 



About the year 1873 he concentrated the purely scientific side of 

 his work upon observation and study of the sun. He had for 

 some time previously given all attention possible to this field, and 

 now for seven years he was engaged in minute telescopic study 

 and in drawing the details of the sun's surface, and especially of 

 sun-spots. This demanded, first, intense and long-continued ob- 

 servations ; next, genius in catching the essential phenomena ; and, 

 finally, infinite patience in recording them. Photography had not 

 yet been pressed into the service of such work, and his skill and 

 accuracy proved of special value. Astrophysicists still declare that 

 his drawings made at the Allegheny Observatory prior to 1875 are 

 even now to be regarded as among the most trustworthy evidence 

 regarding the sun's surface, and to this hour the standard illustra- 

 tion of a sun-spot which appears in most of the astronomical text- 

 books is the one drawn by Langley with his own hand in 1873. 



Since that time vast progress has been made in this work ; but we 

 have the authority of Professor Hale, who has since advanced astro- 

 physics by the aid of the forty-inch reflector of the Yerkes Observa- 

 tory and of the Carnegie telescope on Mount Wilson, California, 

 that, in his own observations of sun-spots, the better they have been 

 seen, the more nearly have they appeared as in Langley's drawings. 



In 1875 Langley's researches went deeper, and he began to con- 

 centrate his thought and work upon the measurement of the heat 

 spectra of the sun and other sources of radiation; but here, too, he 

 had to break a way through obstacles. No heat-measuring appa- 

 ratus known up to that time could enable him to discriminate accu- 

 rately between the temperatures of various narrow portions of the 

 spectrum ; the thermopile, which was then the main reliance for the 

 closest observation in this field, was utterly inadequate; he must 

 invent new instruments; and in 1879 and 1880 he produced his in- 

 vention of the bolometer. 



This was a vast advance upon all previous instruments for the 



