234 CARNEGIE INSTITUTION OF WASHINGTON. 



When the measures are discussed by zones for the purpose of de- 

 tecting deviations from the uniform field presupposed in deriving the 

 above elements, it is found that the inclination of the magnetic axis is 

 sensibly constant between latitudes 45° N. and 45° S. and equal to 

 about 4°. The polar field-strength, however, seems to change rapidly 

 with the latitude, the result from the equatorial zone being nearly 

 twice that found for the zones 10° to 45°. 



SOLAR ROTATION. 



The differences between the rotation values for the earlier and later 

 portions of the 20-year period now covered by spectrographic observa- 

 tions remain an outstanding feature of the problem. An extended 

 series of observations taken under uniform conditions seems still the 

 most promising mode of attack. Such a series, for which the 150- 

 foot tower telescope is used, is now in its sixth year at Mount Wilson. 

 These observations by Dr. St. John show practically the same period 

 of rotation for each of the six years from 1914 to 1919. 



An important modification in method made during the year con- 

 sists in observing the center of the disk simultaneously with the two 

 limbs. The positions of the solar lines in spectra of the sun's center 

 are remarkably constant, but the differences found by comparing the 

 east and west limbs with the center often differ by 10 or 15 per cent of 

 the differences at the equator. The observations also show that 

 high values at one limb are not correlated with high values at the 

 other, as would be the case if the variations at the limbs were due to 

 changes in the rotation period of the reversing layer or to conditions 

 in the terrestrial atmosphere. The observations of the present year 

 confirm the earlier evidences of local disturbances in the reversing 

 layer, and indicate further that they are frequent at all latitudes, and 

 that the motions of the vapors are tangential. Evidence is accumu- 

 lating which tends to show that the motions in the reversing layer 

 around spots and pores are similar in character. Simultaneous 

 observations at the center and limbs provide a valuable means of 

 checking results and of investigating the hemispheres separately, and 

 give also as a by-product the limb-efTect at different latitudes. The 

 means from 172 observations on a group of 10 hues give, within the 

 precision of measurement, equal values for the displacements between 

 limb and center in both hemispheres and at all latitudes. The result 

 is of importance in the intensive study of the limb-effect to be under- 

 taken at Mount Wilson. 



Further tests at Mount Wilson of the effect of integrated sky-light 

 in line displacements at the sun's limb show that the integrated Hght 

 just outside the solar image is not of sufficient intensity to affect the 

 photographic plate during the exposure time employed for solar- 

 rotation observations, that it requires marked cloudiness over and 



