1904] Sitnspot Variation in Latitude, 1861 — 1902. 145 



the equator, as would probably be the case according to Sporer's law. 

 With 5° zones it is possible to detect the presence of two maxima in 

 one or other of the hemispheres, all of which have a trend towards the 

 equator in succeeding years. Still more detail is displayed in the 

 3° zones, and here is apparent a spot distribution and movement which 

 is practically masked in the two preceding sets of curves. 



The advisability of adopting 3° zones for the present investigation 

 being thus apparent, the whole series of observations from the year 

 1861 — 1902 was treated in the above manner, the points plotted and 

 the curves drawn as shown in the figure previously referred to. 



This reduction was rendered comparatively easy by the fact that 

 the Astronomer Royal has quite recently published* the values of the 

 amount of sunspot area for each degree of latitude for each hemisphere 

 from the year 1874 — 1902. For information previous to that date use 

 was made of the detailed observations of the positions and areas of 

 sunspots collected by Sporer in the publications already referred 

 to, and curves for each year were drawn. For this period the curves 

 employed were of a less degree of accuracy than those drawn from the 

 Greenwich reduction, as the number of days of observation throughout 

 a, year was not so great. 



Advantage was taken of the fact that Sporer's observations and 

 those reduced at Greenwich overlapped during the four years 1874 — 

 1877, and a comparison of the curves from each series was rendered 

 possible. The close similarity of these in each case showed that the 

 reduction of Sporer's observations exhibited the chief features of the 

 movements of centres of spot-activity as indicated by the Greenwich 

 curves. 



By thus employing Sporer's observations, curves for each of the 

 42 years from 1861 — 1902 inclusive were drawn in the manner 

 described above, and these were placed vertically one under the other, 

 like those shown in fig. 1 for the years 187 9- -1883. 



In this way it was possible to trace the varying positions, as regards 

 changes of latitude, of the centres of action, or maxima points of the 

 curves, from year to year, just as was previously attempted in the case 

 of the prominences. These centres of action were then connected 

 by lines passing from one yearly curve to the next. It is worthy of 

 remark that very little difficulty was met with in deciding the maxima 

 points to be joined. There was always, throughout the whole period, 

 a most distinct march of these points individually towards the 

 equator, and the method of placing the curves one beneath the other 

 rendered such movement at once obvious to the eye. There was only 

 •one instance where it seemed necessary that a march from lower to 

 higher latitudes ought to be considered. This was in the southern 

 hemisphere, in the years 1889 and 1890 (see Plate 5, Curve A). There 

 * ' Monthly Notices R. A. S.,' vol. 63, pp. 452— 44ft 



