MOUNT WILSON SOLAR OBSERVATORY. 163 



(5) The displacements in the case of calcium and titanium and probably 

 the enhanced lines of iron appear to increase in size in direct proportion to 

 the wave-length of the lines. In the case of nickel and the arc lines of iron 

 the increase toward longer wave-lengths is more rapid than in direct propor- 

 tion to wave-length. These differences may readily be accounted for on the 

 basis of differences of level among the various elements. 



(6) It seems probable that the greater length of the path in the lower 

 strata of the sun's atmosphere at the limb as compared with the center, and 

 the relatively greater pressure in these lower strata, may account in large 

 measure for the displacements observed. 



The investigation will be continued with the spectrograph of the 150-foot 

 tower telescope. 



The General Circulation of the Calcium Vapor in the Solar Atmosphere. 



Mr. St. John carried out during the year an extensive investigation on the 

 circulation of the calcium vapor in the sun's atmosphere. Some of the prin- 

 cipal results may be summarized as follows : 



( 1 ) The calcium- vapor producing the absorption line K3 in the solar spec- 

 trum has a descending motion over the general surface of the sun amounting 

 to about I.I km. per second in the mean. The calcium vapor producing the 

 bright line K, has an ascending motion of 2.0 km. per second in the mean. 

 The motions are essentially radial in their nature. 



(2) The close agreement of the wave-lengths of Kg and K3 at points near 

 the pole and the equator argues strongly against the existence of currents of 

 appreciable velocity parallel to the sun's surface. 



(3) The narrowness of the absorption lines H3 and Kg, the lower pressure 

 suggested by the slightly shorter wave-length of K3, the continued increase 

 in absolute width in passing from center to limb, as well as direct observa- 

 tions with a radial slit, all point to a high level, small depth, and extreme 

 tenuity for the absorbing layer. 



(4) A possible cause of the high radiating power of the emitting layer 

 may be found in its increased temperature resulting from the transformation 

 into heat of the mechanical energy set free by the loss of velocity in the 

 opposing upward and downward currents. 



Mr. St. John is continuing the investigation, and extending it to include 

 measures of the bright H and K lines in the spectrum of the chromosphere. 



Spectroscopic Observations oe the Rotation of the Sun. 



No extended series of observations on the rotation of the sun has been 

 made during the year, but occasional photographs have been taken for the 

 purpose of detecting a possible variation in the rate of rotation. The meas- 

 urement of five of these confirms the results obtained in 1908 and indicates 

 no appreciable change in the interval. 



