1904] Sunsjjot Variation in Latitude, 1861 — 1902. 147 



in which the centres of spot activity occur, and give an idea of the 

 movements of these centres during each sunspot cycle. 



In this paper these curves have been called " spot-activity tracks," 

 but it is important to point out that this term is not necessarily 

 applied to the proper motion of any individual spot, but simply to the 

 changes of position of the regions in which they are most numerous. 

 "As, therefore, the term " spot-acti vity tracks" represents the different 

 positions of the regions of greatest spot-activity, so " prominence- 

 activity tracks " may be employed to indicate the equivalent variations 

 as regards the prominences which were shown in a previous paper.* 



These " spot-activity tracks" have possibly a terrestrial equivalent in 

 the variations from year to year of the positions of the " Zugstrassen," 

 or cyclone tracks of Koppen, it having been found that cyclones in 

 general, which move in the direction of the great mass of air carried by 

 primary currents, have a strong tendency to pursue somewhat the same 

 tracks according to the place of origin. 



For the sake of comparison, curves B, C, and D in each plate have 

 been added. Curves B show the variations of the mean heliographic 

 latitude of the total spotted area for each hemisphere as determined 

 and described in a previous paper. f 



Curves C illustrate the distribution and changes of position of the 

 centres of prominence activity. These curves are somewhat different 

 to those previously published,]: being so arranged that they form a 

 continuous series from the year 1870. The small circles in the years 

 1870 — 1871 represent Bespighi's observations, the curves from 

 1872 — 1881 those of Tacchini, and the remainder, up to the year 1902, 

 Bicco and Mascari's observations. The dotted curves previous to 1870 

 are intended only to give a rough idea of the prominence variations 

 based on a repetition of the observations of 1872 — 1885. § The last 

 curves, namely, those marked D in the plates, represent the variation 

 from year to year of the total spotted area on each hemisphere of the 

 sun, and special attention was drawn in a previous publication! | to the 

 great differences between the two hemispheres at the times of sunspot 

 maxima. The vertical broken and continuous lines indicate the epochs 

 of sunspot minima and maxima as determined by combining the 

 amount of spotted area on both hemispheres of the sun. 



Beverting now to the curves marked A, which form the special 

 subject of the present paper, the following general deductions may be 

 made : — 



1. From sunspot minimum to minimum there are three, but generally 



* ' Roy. Soc. Proc.,' vol. 71, p. 446. 



f ' Roy. Soc. Proc.,' vol. 71, p. 440. 



X ' Roy. Soc. Proc.,' vol. 71, pi. 6 and 7. 



§ ' Monthly Notices R. A. S.,' vol. 63, No. 8, p. 481. 



|| ' Roy. Soc. Proc.,' vol. 71, p. 216, footnote. 



