NO. 1865 



OUR KNOWLEDGt: OF THli SUN — HALE 



349 



tion dates from a former epoch in the Sun's history, and that 

 it perhaps arose from the motion of the gases concerned in the for- 

 mation of the Sun from a nebula. After the lapse of some millions 

 of years, the effect of internal friction will tend to bring the veloci- 

 ties corresponding to different latitudes more and more closely into 

 harmony, and finally the Sun will rotate as a solid sphere. 



One of the most important results obtained by Adams is the dis- 

 covery that the lines of carbon and lanthanum, elements which lie 

 at a low level in the Sun's atmosphere, give values for the daily 

 rate about o°.i less than the mean values for all of the lines meas- 

 ured. Two lines of manganese, on the contrary, give systematically 

 high results. It seems probable that these differences are due to 

 differences in the level of the vapors of these elements in the solar 

 atmosphere, and that those substances which lie at high altitudes 

 complete a rotation in a shorter period than the vapors beneath them. 

 This supposition is confirmed by the fact that Adams's recent meas- 

 ures of the velocity of hydrogen, which rises higher above the solar 

 surface than any of the vapors included in the above investigation, 

 give very high values. Moreover, as the following table shows, the 

 rotational velocities of hydrogen in low and high latitudes are in 

 close agreement, and the equatorial acceleration characteristic of 

 lower levels does not exist. 



This important discovery leads us to inquire whether hydrogen 

 clouds in the solar atmosphere, if observed in projection against the 

 Sun's disk, would show daily motions corresponding to these results 

 obtained with the spectroscope. Fortunately, the spectroheliograph 

 permits these clouds to be photographed, as will be explained in the 

 next section of this lecture. 



