National Geographic : 1913 Feb
Photo by George C. Martin BLUEBELLS AND MOSS AMONG THE ASHES: MIDDLE BAY, SEPTEMBER 12, 1912 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 Mount 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 apparatus and the nature of the appa ratus with which these effects were ob served. In the first place we have the pyrheli 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 wave-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 proportional heating effects upon this delicate thermometer. At each of the Fraunhofer absorption 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 the earth's atmosphere. Suppose now that several hours later you 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.