samu£;l pijjrpont langley. 



27 



scientific periodical was on "A New Form of Solar Eyepiece," and 

 it is characterized by a careful study of practical detail. It is evi- 

 dently the work of one who had laboriously constructed the instru- 

 ment, overcoming- its early defects by careful trial, and not that of a 

 student who suggests a new method by theory alone. His second 

 article, "On the Allegheny System of Electric Time Signals," is also 

 a remarkable one. All the trains of the great system of the Penn- 

 sylvania Railroad were run for many years on time furnished by 

 him. His method of furnishing and distributing time was one of 

 the first and one of the best that has been adopted. Its introduction 

 provided an important addition to the resources of the observatory. 

 He now entered upon serious scientific work. The dense smoke of 

 the adjacent city of Pittsburg rendered observation of the stars diffi- 

 cult, and he wisely selected the sun as a subject for study. In this 

 work the smoke, in some respects, proved to be an advantage, since 

 it cut off the heat and rendered the image of the sun steadier than 

 in a clearer atmosphere. From 1874 to 1890 he was a frequent con- 

 tributor to the scientific journals, sending to them an average of 

 about three articles a year. His early studies related to the structure 

 of the sun, and his artistic skill enabled him to produce the finest 

 engraving ever made of the marvelously complex structure of a sun- 

 spot. He took part in eclipse expeditions in 1869, 1870, 1878, and 

 1900. His studies of the sun at first related to its structure, but 

 later to its spectrum. Recognizing the limitations of the human 

 eye, he endeavored to find a substitute for it, and this led to his in- 

 vention of that wonderfully delicate instrument, the bolometer, 

 through which his name is best known to the scientific public. It 

 is easy to render the principle of this instrument clear, even to those 

 unfamiliar with science, but difficult even for the professional 

 physicist to realize the immense care and labor required to bring it 

 to, its final perfection. Whatever may be our belief regarding the 

 nature of a current of electricity, there is no better way of describing 

 some of its properties than to go back to the time of Franklin and 

 treat it like a current of water. Suppose, then, that two pipes, laid 

 side by side, are carrying water from one vessel to another, and that 

 they are connected at their centers by an intermediate, or cross, pipe. 

 If the two principal pipes are exactly alike, evidently there will be no 

 flow through the cross-pipe; but even a slight obstruction of one 

 will cause some of its water to pass through the cross-pipe into the 

 other pipe. Substituting electricity for water, we have Wheat- 

 stone's bridge, of which the bolometer is a modification. A delicate 

 galvanometer is substituted for the cross-pipe, and an excessively 



