1 68 REPORTS O^ INVESTIGATIONS AND PROJECTS. 



between 1906 and 1908. If any such variation exists, it is confined to the 

 higher latitudes, and does not appear in the zones of greatest spot activity. 

 The resuhs are also opposed to a three-year period of variation, such as was 

 obtained by Halm from a comparison of his values with those of Duner. 



(3) The observations of 1908 confirm those of 1906-07 in showing that 

 dififerent lines give different velocities. Lines of lanthanum and cyanogen 

 give low velocities ; certain lines of manganese and iron give high velocities. 

 The investigation of two "enhanced" lines indicates a tendency toward low 

 values for lines of this type. In one of the cases this effect is very marked. 

 Lines considerably strengthened at the sun's limb give high values in general. 



(4) When lines give systematically large or small values for the rotational 

 velocity the differences from the mean become greater toward higher lati- 

 tudes. 



(5) The results given by the 1908 observations are satisfied within the 

 limits of accidental error by the equation given by Faye for the motion of the 

 sun-spots observed by Carrington. The fact that the observations of Duner, 

 Halm, and Adams are all satisfied by this equation indicates that it repre- 

 sents the law of rotation of the sun's reversing layer to within at least 10° of 

 the pole. 



(6) Comparison of the probable errors for the two series of observations 

 indicates a substantial gain in accuracy of measurement for the 1908 series 

 over that of 1906-07. 



(7) The motion of the reversing layer in the vicinity of solar vortices 

 may be seriously influenced by the motion of the vortices, and the rotational 

 velocities obtained from such regions are subject to large systematic errors. 

 It is important in taking observations for rotation to avoid all such disturbed 



areas. 



(8) A special study of the calcium line X 4227 shows that the rotational 

 velocity derived from this line is higher than that for the general reversing 

 layer, the difference at the equator amounting to 0.3°. Also, the decrease of 

 velocity with increasing latitude is much less marked than for the reversing 

 layer. At 75° of latitude the angular velocity of A 4227 is 1.5° greater than 

 for the reversing layer. 



(9) A special study of the Ha line of hydrogen shows that the rotational 

 velocities which it gives depend upon the distance from the sun's limb. Re- 

 sults obtained from Ha at the limb of the sun are considerably larger than 

 those for the reversing layer and show a comparatively small decrease in the 

 value of the angular velocity toward the pole. At the equator the difference 

 irom the reversing layer amounts to 0.6° and at 75° of latitude to 3.0°. 



(10) At a distance averaging 35'' inside the sun's limb the results ob- 

 tained from Ha are considerably smaller than the corresponding values at the 

 limb, although still much larger than for the reversing layer. They also 

 average somewhat larger than for A 4227. 



