MOUNT WILSON OBSERVATORY. 



229 



Photographs of spectra taken with the new vertical spectrograph 

 of the Snow telescope include : 



Venus 54 



Sun 17 



Sky 31 



Sun with interferometer 25 



SUN-SPOT ACTIVITY. 



The calendar year 1918 showed a marked decline in sun-spot activity, 

 314 groups having been observed as against 450 during 1917. There 

 were no spotless days during the year, although about the middle of 

 June there were several days on which only two groups were recorded. 

 In contrast with this quiescent period, the first two days of June, judged 

 by the number of individual groups, were the most active of the year. 

 Sixteen spot-groups were recorded on each of these days. The fol- 

 lowing table gives the average number of groups observed per day 

 during each month: 



The average latitude of all groups observed during the year was 13°, 

 2° less than the average for 1917. 



SUN-SPOT POLARITIES. 



The results of the investigation referred to in the last annual report 

 have been pubhshed by Messrs. Hale, Ellernmn. Nicholson, and Joy 

 in Contribution No. 165 on 'The MagneScPolarity of Sun-spots." 

 This paper describes the methods of observation employed in obtaining 

 a daily record of the polarities of all spots with the 75-foot spectro- 

 graph of the 150-foot tower telescope; the precautions that must be 

 taken to avoid error in the case of spots near the Umb; the method of 

 measuring the inclination of the lines of force in sun-spots, in planes 

 parallel and at right angles to the line of sight; and a method of classi- 

 fying sun-spots on the basis of their magnetic properties. The paper 

 includes a magnetic classification of 970 spots observed during the 

 years 1915, 1916, and 1917, and discusses the results of the observations 

 made during this period. About 60 per cent of all sun-spots are found 

 to be binary groups, the single or multiple members of which are of 

 opposite magnetic polarity. Unipolar spots are very seldom observed 



