652 THE POPULAR SCIENCE MONTHLY. 



It is true we can see these in the visible spectrum, but you remem- 

 ber we propose to explore the invisible also, and since to this the dark 

 is the same as the light, it will feel absorption lines in the infra-red 

 which might remain otherwise unknown. 



I have spent a long time in these preliminary researches ; in indi- 

 rect methods for determining the absorption of our atmosphere, and 

 in experiments and calculations which I do not detail, but it is so 

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

 and so little is realized of the labor 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- 

 ward 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 the 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 analyze 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 analyze 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 can not 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 afterward 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 



