4 o6 THE POPULAR SCIENCE MONTHLY. 



needle then moves, and the amount of this motion measures the 

 amount of heat disturbance. The sensitiveness of the instrument is 

 from ten to thirty times greater than that of the most delicate ther- 

 mopiles possible, and its constancy specially tits it for its work. The 

 years 1879 and 1880 were given to perfecting this new and powerful 

 instrument. Some of its first results were to show, by direct experi- 

 ment, that the maximum of heat in the normal spectrum was in the 

 orange, not the infra-red (then an interesting fact) ; and that the 

 solar-constant,* as determined by previous methods, was decidedly 

 too small. The most suitable methods of determining this important 

 constant were pointed out. 



In 1881 Professor Langley organized an expedition to the top of 

 Mount Whitney, in California, for the purpose of applying these new 

 methods under the most favorable conditions. The expenses of the 

 expedition were jointly borne by the United States Signal Service 

 and by the private subscription of a wealthy gentleman in Pittsburg, 

 who had now for some years taken the greatest interest in the re- 

 searches of the observatory, and whose liberality had provided many 

 of its instruments. 



His name ought to be here mentioned. He has materially aided 

 science in the most liberal and thoughtful way ; but, against his ex- 

 pressed wish that he should be nameless in this connection (as he is 

 in hundreds of other kind deeds), I have no right to contend. 



The most important single result of the previous experiments with 

 the bolometer had been the establishment of the fact of selective ab- 

 sorption of the solar rays by the earth's atmosphere. The results of 

 this action are so important that I may be permitted to quote from 

 Professor Langley an elementary exposition of them. He says : " Our 

 observations at Allegheny had appeared to show that the atmosphere 

 had acted with selective absorption to an unanticipated degree, keep- 

 ing back an immense proportion of the blue and green, so that what 

 was originally the strongest had, when it got down to us, become the 

 weakest of all, and what was originally weak had become relatively 

 strong, the action of the atmosphere having been just the converse of 

 that of an ordinary sieve, or like that of a sieve which should keep 

 back small particles analogous to the short wave-lengths (the blue and 

 green), and allow freely to pass the large ones (the dark-heat rays). 

 It seemed from these observations that the atmosphere had not merely 

 kept back a part of the solar radiation, but had totally changed its 

 composition in doing so not by anything it had put in, but by the 

 selective way in which it had taken out, as if by a capricious intelli- 

 gence. The residue that had actually come down to us thus changed 

 in proportion was what we know familiarly as ' white ' light, so that 

 white is not ' the sum of all the radiations,' as used to be taught, but 



* The amount of heat received from the sun's rays, falling perpendicularly on a 

 square metre of the upper surface of the earth's atmosphere, in a minute of time. 



