SAMIKl. I'lKKl'ONl' I.ANCl.EY. 829 



excessively fine metallic filaiiunt for the resistance submitted to radiant 

 energy. This filament responded to extraordinarily small increments 

 of heat. I well remember his enthusiasm, when on a visit to Cam- 

 bridge, he showed me the modification of the balance which he called 

 a bolometer and said " I have found a means of overcoming all my 

 difficulties." A new instrument often marks the beginning of a new 

 epoch in science, Langley opened a great field of investigation in that 

 portion of the solar spectrum which extends into darkness beyond the 

 visible red — the portion called the infra red; and mapped lines and 

 absorption bands in a region eight to ten times the extent of the visible 

 spectrum. 



With his bolometer he undertook an investigation of the heat of the 

 moon; but could not distinguish between the heat given oflF by the 

 body of the moon and that due to reflection of the sun's rays. He 

 made journeys to Mt. Whitney where the height and steadiness of the 

 atmosphere promised to enable him to determine the constancy of 

 the radiation of the sun. He laid the foundation of the subsequent 

 refined measurements of Dr. Abbot. When Langley was called to 

 the Smithsonian, as Director he founfled an astrophysical observatory 

 in connection with the Institution which has become renowned as a 

 centre of investigation of radiant energy. 



Langley obtained by his investigations with the bolometer an en- 

 during place in the history of science which, however, was to be greatly 

 increased by his later work on the aeroplane. My acquaintance 

 with him began on a camping out expedition in ^Slaine. He impressed 

 me as a man wrapped in heavy thought. One evening Professor 

 Alfred ]M. Mayer, who was of the party, expressed the con\'iction that 

 a scientific man could acquire in half an hour the practical expe- 

 rience which had taken our guide twenty years to obtain; and he 

 and Langley took lessons in paddling a canoe. There was no wind 

 and the lake, on the shores of which we were encamped was placid. 

 Langley, taking with him a copy of Maxwell's Matter and Motion, 

 paddled across the lake. A thunder cloud presently arose and Langley 

 endeavored to return; but there was no stone in the bow of the canoe; 

 and it did not occur to him to shift his position to the middle of the 

 canoe. He had to summon the guide. Later we were together in 

 London, and on one occasion while riding in the suburbs, he broke 

 a moody silence by remarking, "How absurd it is to be carried by this 

 horse — a mass of flesh and bones, nine hundred pounds in weight, 

 I have an engine, which weighs only four pounds and develops two 

 horse power." 



