266 



C. S. Cook — Spectrum of Aqueous Vapor. 



made. The readings taken showed no difference within the 

 limits of reading of the instruments. How closely the two 

 readings agreed is shown in the diagram. Figs. 3 and 4. The 

 shading indicates rain-fall, the heavier shading corresponding to- 

 rapid precipitation. The full lines represent observations^ at 



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the summit, the broken lines those at the base of the moun- 

 tain. Unfortunately in the first instance as shown in figure 2, 

 very few observations were taken at the base so that the com- 

 parison is not satisfactory. 



An observation made on the mountain top August 6, 1887, 

 gives quite direct evidence on the. relative efficacy of damp air 

 and storm cloud in producing a spectrum. As the chart 

 shows, it had rained steadily during the previous night and 

 from 9 to 12 of the forenoon, and during two hours of this 

 my assistant's record says that it "rained furiously." At 

 12.15 the rain ceased falling. As usual the intensity of the 

 vapor line fell off rapidly (see fig. 4). Fifteen minutes 

 after the rainfall ceased, the clouds in which the mountain had 

 been immersed broke away for a few minutes. We saw far 

 above us bright cirro-cumulus clouds, thin and much broken, 

 showing considerable blue sky through the breaks. The zenith 

 reading with the spectroscope was 22, an exceedingly large 

 reading for a zenith reading on a nearly cloudless sky. I 

 believe it is the highest reading ever obtained on such a sky.. 



