rhm- li> i,cMi,;r C. .Martin 

 BLUEGELLS AXD MOSS AMONG THE ASHES! MIDDLE BAY, SEPTEMBER 12, I912 



toward easterly, so that Mount Wilson 

 lay decidedly too far to the south for the 

 most favorable communication. 



HOW THE sun's HEAT IS MEASURED 



Before taking up the question of the 

 reasonableness of the hypothesis that the 

 dust from Alount Katmai was distributed 

 all over the higher atmosphere and re- 

 mained there for months in suspension, 

 we may consider for a moment the exact 

 effects which were observed with our 

 ajiparatus and the nature of the appa- 

 ratus with which these effects were ob- 

 served. 



In the tirst ])lace we have the pyrhcli- 

 ometer. an instrument for measuring the 

 heating effect of the sun at the earth's 

 surface. In the second place we have 

 the spectro-bolometer, that wonderful de- 

 vice of Langley for observing the exces- 

 sively minute heating effects of the rays 

 of the solar spectrum. Imagine that you 

 have before you a very intense solar 

 spectrum, and that it is still early morn- 

 ing, with the sun perhaps an hour and a 

 half high. 



If you had a thin, delicate blackened 

 thermometer, you could carry it along in 

 the spectrum from the extreme ultra- 

 violet to far beyond the red. and detect 



varying degrees of temperature rise, pro- 

 portional to the heat produced by each 

 spectral ray. It would make no differ- 

 ence whether these lay between the violet 

 and the red and were visible to the eye, 

 or were the short Avave-length photo- 

 graphic rays beyond the visible end of 

 the violet spectrum, or the long wave- 

 length rays lying beyond the visible end 

 of the red. All would produce their just 

 and ])roportional heating effects upon this 

 delicate thermometer. At each of the 

 Fraunhofer absorjition lines the ther- 

 mometer would fall slightly. 



The "A" band of oxygen would pro- 

 duce a comparatively great decrease of 

 temperature, and beyond the red there 

 would be still more prominently the great 

 bands, due to the water vapor in tlie 

 earth's atmos])here. 



Suppose now that several hours later 

 vou repeated the experiment. You would 

 find that, excepting in these great water- 

 vapor bands, practically every part of 

 the spectrum was hotter than before, and 

 that the change had been greatest in the 

 violet end. Knowing the altitude of the 

 sun above the horizon at each time of 

 observation, you could compute the thick- 

 ness of the layer of air traversed by the 

 solar beam. 



183 



