658 



THE POPULAR SCIENCE MONTHLY. 



Now, the shaded curve above the spectrum represents the amount 

 of energy in the sun's rays at the foot of the mountain, and was ob- 

 tained in this way : Fix your attention for a moment on any single 

 part of the spectrum, for instance, that whose wave-length is 60. If 

 the heat in this ray, as represented by the 

 bolometer at the foot of the mountain, was 

 (let us suppose) 2, on any arbitrary scale 

 we draw a vertical line, two inches, or two 

 feet high over that part of the spectrum. 

 If the heat at another point, such as 40, 

 were but \, a line would be drawn there 

 a quarter of an inch high, and so on, till 

 these vertical lines mark out the shaded 

 parts of the drawing, the gaps and depres- 

 sions in whose outline correspond to the 

 " cold bands " already spoken of. Again, 

 if on top of the mountain we measure all 

 these over once more, we shall find all are 

 hotter, so that we must up there make all 

 our lines higher, but in very different pro- 

 portions. At 60, for instance, the heat 

 (and light) may have grown from 2 to 3, 

 or increased one half, while above 40 the / / 



heat (and light) may have grown from 



or increased 



times. 



mountain measurements give another spec 



trum, the energies in each part of which are defined by the middle 

 dotted line, which we see indicates very much greater energy whether 

 heat or light in the blue end than below. Next, the light or heat 

 which would be observed at the surface of the atmosphere is found 

 in this way. If the mountain-top rises through one half the absorbing 

 mass of this terrestrial atmosphere (it does not quite do so, in fact), 

 and, by getting rid of that lower half, the ray 60 has grown in bright- 



