NO. 4 SITE FOR SOLAR-RADIATION STATION MOORE 3 



strip exposed to the radiation. When the current is adjusted so that 

 this condition prevails, no current will pass from the thermocouples 

 to the galvanometer. From the measured current which passed through 

 the strip and the constant of the instrument, the intensity of the 

 radiation can readily be computed. 



In order to lessen the weight of the equipment to be carried up the 

 high and rugged peaks, no theodolite was included, but instead, the 

 altitude of the sun was obtained by measuring the length of a shadow 

 cast upon a level surface over a triangle lo centimeters in height. With 

 low sun the angle was increased by 45° in order to get a shorter and 

 sharper shadow. From the length of the shadow the elevation of the 

 sun (neglecting refraction) was obtained, and from this the air-mass 

 was read from a Bemporad " air mass versus altitude " plot. 



When sky conditions permitted, 10 observations were made at 

 approximately equal spacing of air mass, between air mass 5.60 (about 

 10° solar altitude) and nearly meridian sun. The logarithm of the 

 solar calories per square centimeter per minute was plotted versus 

 the air mass through which the solar beam had passed. Such a plot 

 should be nearly a straight line (slightly convex toward the origin), 

 with a sky of uniform transparency. If such a line were not obtained 

 in the plot, it would indicate nonuniformity of transparency in the 

 atmosphere above the peak during the observations. 



Disregarding small changes in the heat radiated l)y the sun, the main 

 terrestrial causes of nonuniformity in atmospheric transparency are 

 changes in invisible water vapor, haze, dust, smoke, and ozone. Visible 

 clouds should, of course, be included, but solar observations are never 

 undertaken through clouds. The changes of radiation due to changes 

 in ozone in the path of the beam are small and were not taken into 

 account in the work of this expedition. But the first four agencies 

 mentioned are very important causes of nonuniformity of trans- 

 parency of the atmosphere, and instruments were carried to measure 

 fairly accurately their effect. 



Invisible water vapor is perhaps of first importance, and for 

 determining the uniformity of water vapor prevailing in the path of 

 the solar beam a new (and as yet unnamed) instrument was included 

 in the equipment. The instrument is essentially a spectroscope, so 

 arranged that the dift'erence in radiation of the selective absorption 

 water-vapor band called p, and the radiation of the region immediately 

 adjacent to this band, is indicated by a small, sensitive thermocouple. 

 In addition, by suitably interposing a shutter the difference between the 

 p band and zero radiation (given by the shadow of the shutter), is 

 obtained. Figure i represents the solar-radiation curve as measured 



