28 SMITHSONIAN MISCELLANEOUS COLLECTIONS. 



small amount of electricity flowing one way or the other is thus 

 detected. If one of the wires carrying the current is heated, its 

 resistance is increased and the equilibrium disturbed. Heating the 

 other wire b}- a known amount, we may be sure that the two quanti- 

 ties are equal when the galvanometer shows that no current is pass- 

 ing. This gives but little conception of the years of work Langley 

 spent in improving the instrument in every detail, and which finally 

 led to a wonderful delicacy, so that in 1901 he stated that a change 

 in temperature of one-hundred-millionth of a degree Fahrenheit was 

 perceptible. The presence of a cow in a pasture could be detected 

 by the heat she radiated, even at a distance of a quarter of a mile. 

 Langley was thus placed in possession of an instrument of un- 

 heard-of delicacy. At Allegheny, and later at Washington, the use 

 he made of it was good indeed. The heat of sun-spots, of different 

 portions of the sun's disk, the temperature of the moon, the absorp- 

 tion of the earth's atmosphere, and the effect of elevation represent 

 in each case months or years of careful and laborious work. Prob- 

 ably his most important research was the extension of the red end 

 of the solar spectrum. Far beyond the limits of the eye, he found 

 rays greatly surpassing in energy those of the visual spectrum. 

 With infinite care he traced the spectrum, step by step, recording 

 hundreds of gaps, or dark lines, until he reached a point whose 

 wave-length was 5.3 /a, or seven times as great as that of the dark 

 red rays which form the limit to the eye. But the intensity of these 

 rays, previously only vaguely known, is so great that ninety-nine 

 per -cent of the energy radiated to us by the sun, or other sources of 

 heat, is contained in them, scarcely one per cent consisting of rays 

 visible to the eye. 



In the midst of these researches he was called to the position 

 which should be the greatest scientific prize in the country, that of 

 Secretary of the Smithsonian Institution. He would become the 

 scientific adviser of the President of the United States, would be 

 independent of political party, and not even controlled by a depart- 

 ment. A Board of Regents, including some of the highest officials 

 of the Government, would enable him to secure for his plans the 

 careful attention of the authorities. His field of work would be so 

 broad as to be limited only to the "increase and diffusion of knowl- 

 edge among men." As a patron, he would have the Government of 

 the United States, whose aid to certain departments of science has 

 been almost unlimited. Distinguished predecessors, high official 

 position, and the brilliant society he had always desired were united 

 in this position. All these dazzling attractions failed to induce him 



