MOUNT WILSON SOLAR OBSERVATORY. 



255 



PRESSURE IN THE SOLAR ATMOSPHERE. 



Mr. St. John's investigations on the relative wave-lengths of arc 

 and solar lines, begun three years ago with the 60-foot tower telescope, 

 have been continued to great advantage with the 75-foot spectrograph 

 of the 150-foot tower. It soon appeared, however, that a detailed 

 study of individual lines in various parts of the arc would be necessary 

 before a satisfactory discussion of the measures could be undertaken, 

 Some of the results of this work may be found in another part of this 

 report. Pending its completion, it has seemed worth while to publish 

 the data in hand, because of their bearing on Einstein's suggestion that 

 the Fraunhofer lines may be shifted toward the red by the effect of 

 solar gravitation, and because of Evershed's paper on this subject. 



The lines, 163 in number, are distributed among the five pressure 

 groups as follows: 



The results for the groups are decidedly characteristic and clearly 

 show that the Fraunhofer lines fall into the identical groups into which 

 the arc lines are classified by pressure shift, suggesting an intimate 

 relationship. 



According to Freundlich, the magnitude of the Einstein displacement 

 to the red, independent of element or class of line, is 0.009 or 0.010 a for 

 these wave-lengths. There is, however, no indication of a general 

 displacement to the red of the order required; in fact, 40 per cent of the 

 lines are displaced to the violet and 80 per cent of the displacements to 

 the red are much under the magnitude predicted. According to the 

 anomalous dispersion theory, as interpreted by Professor Julius, the 

 Fraunhofer lines must be displaced to the red. The data are equally 

 opposed to this theory. 



Groups a and b contain the lines for which the precision of measure- 

 ment is the highest. On the assumption that pressure in the solar 

 atmosphere is alone effective in producing these displacements, they 

 indicate an anomalous distribution of pressure, in which the greatest 

 pressure is at the highest level. It is evident that pressure, though 

 clearly involved, is not the only cause. Assuming that the displace- 

 ments are due only to a downward motion and that pressure in the solar 

 atmosphere does not differ greatly from zero, we have the other extreme 



