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ASTRONOMY: ABBOT, FOWLE AND ALDRICH Proc. N. A. S. 
Among other instruments developed or improved for the work are the 
2-mirror coelostat; the reflecting galvanometer; the vacuum bolometer 
for which a full mathematical theory was derived; the pyranometer, an 
instrument for measuring the radiation received from the daylight sky; 
the honeycomb pyranometer or melikeron, a "black-body" instrument for 
observing nocturnal radiation ; the ballon pyrheliometer, 5 copies of which 
attached to sounding balloons, were raised to great altitudes in 1913 and 
1914 for observing the solar radiation very high up in the atmosphere; 
the slide wire extrapolator and special plate measuring instrument, both 
used for the reduction of observations of the solar constant of radiation. 
2. Value of the Solar Constant. — About 2000 observations of the in- 
tensity of solar radiation outside the atmosphere at mean solar distance, 
otherwise called the "solar constant," have been made according to the 
high and low sun method of Langley as systematized and perfected by use. 
These measurements have been made at Washington (sea level) ; Bassour, 
Algeria (1,160 meters); Hump Mountain, North Carolina (1,,500 meters); 
Mt. Harqua Hala, Arizona (1,729 meters); Mt. Wilson, California (1,735 
meters) ; Calama, Chile (2,250 meters) ; Montezuma, Chile (2,900 meters?) ; 
and Mt. Whitney, California (4,420 meters). The circumstances of ob- 
servation have been widely varied, ranging from days of high humidity in 
summer at Washington to less than a millimeter of precipitable water as 
in the atmosphere above Mt. Whitney; at temperatures from those of the 
Sahara Desert in Algeria to —25° C. at Hump Mountain, North Carolina; 
and clearness of the atmosphere, from the deep blue which prevails right up 
to the sun's disk on Mt. Whitney and Mt. Montezuma down to the white 
dusty sky which prevailed in the year 1912 at Mt. Wilson and in Algeria 
after the eruption of the volcano Mt. Katmai in Alaska. 
These varied circumstances appear to produce very little influence on 
the solar constant results, and the mean value, 1.94 calories per square 
centimeter per minute, is closely confirmed by the observations of the 
balloon pyrheliometer raised to an elevation of nearly 25,000 meters where 
the atmospheric pressure is reduced to one twenty-fifth of its value at sea 
level. 
We believe this value, 1.94 calories per square centimeter per minute, 
may be regarded as the datum mark for this epoch with which may be 
compared in future centuries the solar radiation at that time prevailing in 
order to determine what, if any, secular changes of it have occurred. 
3. Solar Variation. — To state the negative first, our observations have 
shown that the sun is not, like some of the other stars, widely variable but 
has at any rate not been found to vary in excess of 12 per cent in the last 
20 years. In general its variations do not exceed 5 per cent in a month. 
On the other hand, solar variations of from 1 to 5 per cent appear to 
occur at irregular intervals and with irregular magnitudes of variation; 
