274 Professor S. P. Langley [April 17, 



often supposed that scientific investigation is a sort of happy guessing, 

 and so little is realised of the labour of preparation and proof, that I 

 have been somewhat particular in describing the essential parts of the 

 apparatus finally employed, and now we must pass to the scene of 

 their use. 



We have been compared to creatures living at the bottom of the 

 sea, who frame their deceptive traditional notions of what the sun is 

 like from the feeble changed rays which sift down to them. Though 

 such creatures could not rise to the surface, they might swim up to- 

 wards it ; and if these rays grew hotter, brighter, and bluer as they 

 ascended, it would be almost within the capacity of a fish's mind to 

 guess that they are still brighter and bluer at the top. 



Since we children of the earth, while dwelling on it, are always 

 at the bottom of a sea, though of another sort, the most direct method 

 of proof I spoke of, is merely to group as far as we can and observe 

 what happens, though as we are men, and not fishes, something more 

 may fairly be expected of our intelligence than of theirs. 



We will not only guess, but measure and reason, and in particular 

 we will first, while still at the bottom of the mountain, draw the 

 light and heat out into a spectrum, and analyse every part of it by some 

 method that will enable us to explore the invisible as well as record 

 the visible. Then we will ascend many miles into the air, meeting 

 the rays on the way down, before the sifting process has done its 

 whole work, and there analyse the light all over again, so as to be 

 able to learn the different proportions in which the different rays 

 have been absorbed, and by studying the action on each separate ray, 

 to prove the state of things which must have existed before this sifting 

 — this selective absorption — began. 



It may seem at first that we cannot ascend far enough to do much 

 good, since the surface of our aerial ocean is hundreds of miles over- 

 head ; but we must remember that the air grows thinner as we ascend, 

 the lower atmosphere being so much denser, that about one-half the 

 whole substance or mass of it lies within the first four miles, which is 

 a less height than the tops of some mountains. Every high mountain, 

 however, will not do, for ours must not only be very high but very 

 steep, so that the station we choose at the bottom may be almost 

 under the station we are afterwards to occupy at the top. 



Besides, we are not going to climb a lofty, lonely summit like 

 tourists to spend an hour, but to spend weeks ; so that we must have 

 fire and shelter, and above all we must have dry air to get clear 

 skies. First I thought of the Peak of Teneriffe, but afterwards some 

 point in the territories of the United States seemed preferable, par- 

 ticularly as the Government offered to give the Expedition, through 

 the Signal Service, and under the direction of its head, General 

 Hazen, material help in transportation and a military escort, if needed, 

 anywhere in its own dominions. No summit in the eastern part of 

 the United States rises much over 7000 feet ; and though the great 

 Kocky Mountains reach double this, their tops- are the home of fog 



