The Solar Activity 1833-1900. 



287 



it ; whether, in fact, it is in the nature of a cycle of variations within 

 the Sun, we have, at present, no way of deciding." 



In the investigations on periods of solar activity most workers have 

 relied simply on Wolf's numbers, which are given by him back to the 

 year 1749. Any one acquainted with these knows that from the time 

 systematic observations of the Sun's surface were commenced by Hof rath 

 Schwabe (1833), these numbers agree very closely with the actual facts ; 

 but before that date, the numbers are based, not on facts alone (which 

 were not very numerous), but on a system of " meaning,"* suggested 

 by the results of the observations from 1833 to 1876. 



Although then Dr. Wolf was able to present us with a curve dealing 

 with the spotted area from 1749, it was decided for the present commu- 

 nication to limit the discussion to those relative numbers which are 

 based on the actual systematic observations since 1833. This neces- 

 sarily restricted the investigation to a comparatively short number of 

 years, namely, sixty-six (1833-1899), but it was thought that any 

 variations detected, if greater than any which might be justifiably 

 considered errors of observation, would be based on sound facts, and 

 not on uncertain data. 



The important magnetic results obtained from a discussion of the 

 Greenwich Observations by Mr. William Ellis,f placed at my disposal 

 a most valuable check on any variation that might be obtained from 

 the sunspot curves, Mr. Ellis having shown that the curves for the 

 magnetic elements are in almost exact accord with those of the sun- 

 spots obtained by Dr. Wolf. In this connection Mr. Ellis writes % : 



" Considering that the irregularities in the length of the sunspot 

 period so entirely synchronise with similar irregularities in the magnetic 

 period, and also that the elevation or depression of the maximum 

 points of the sunspot curve is accompanied by similar elevations and 

 depressions in the two magnetic curves, it would seem, in the face of 

 such evidence, that the supposition that such agreement is probably 

 only accidental coincidence can scarcely be maintained, and there 

 would appear to be no escape from the conclusion that such close cor- 

 respondence, both in period and activity, indicates a more or less 

 direct relation between the two phenomena, or otherwise the existence 

 of some common cause producing both. The sharp rise from minimum 

 ejioch to maximum epoch, and the more gradual fall from maximum 

 epoch to minimum epoch, may be pointed out as characteristic of all 

 three curves." 



* For Wolf's method of " meaning" see c Astronomisclie Mittheilungen,' von 

 Rudolf Wolf, Zurich, 1876, p. 39 et seq. 

 f 'Roy. Soc. Proc.,' vol. 63, p. 64. 

 t Ihid., p. 70. 



