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 thermopiles possible, and its constancy specially fits 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 experiment, 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,[1] 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 researches 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 expressed 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 absorption 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, keeping 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 intelligence. 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.
