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ANNUAL EEPOET SMITHSONIAlSr INSTITUTIOlSr, 1912. 



distant zero 55, 82.5, and 95 per cent, respectively, from the center 

 of the sun's disk. The intensities, as you see, diminish as we go 

 out toward the sun's edge in all wave lengths, but most rapidly for 

 the shorter wave-length rays. Hence not only does the height of the 

 energy curve alter, but its form changes, so that the place of maximum 

 intensity shifts toward the red as we approach the sun's edge. This 

 is a peculiarity of spectra of heated bodies at successively lower tem- 

 peratures. It is quite reasonable to conclude that the light we see 

 near the edge of the sun comes from regions of lower temperature than 

 that which we get fi'om the center of the disk. This is probably 

 because our ^dew is cut off before it penetrates very deeply, at the 

 edge, because we are there looking very obliquely. Thus we see only 



1680 



1890 



Fig. 7.— Sun-spots and the earth's temperature and magnetism. 



I. Temperature departures for the United States. 

 II. Sun-spot numbers (Wolfer). 



III. Magnetic declination / mean, diurnal rangeA . 



IV. Magnetic horizontal force \ Ellis, Chree. / 



superficial layers at the edge, and much deeper and hotter ones at the 

 center. But all these temperatures are very much higher than any 

 we loiow on the earth. The electric arc, for instance, is said to be at 

 a temperature of about 3,800° absolute Centigrade. From the form 

 of the solar energy curve, the value of the solar constant, and other 

 data we estimate that the solar temperatures range from about 6,000° 

 absolute Centigrade upward. 



As the temperature of the earth depends almost directly on the 

 quantity of solar radiation, we are immediately interested to inquire 

 if the sun, like so many others of the stars, is variable, and if so, what 

 corresponding variations are produced in the temperature, cloudiness, 

 and rainfall of the earth. It has been known for over half a century 

 that the sun has a slight variability of an average period of about 1 1 



