500 Bev. S. J. Perry [May 24, 



As it is most important, before referring to any conclusions that 

 may be drawn from our observations, to test severely the fidelity of 

 which solar drawings are capable, I will throw upon the screen a 

 number of sun-pictures drawn at Stonyhurst, and enable you to 

 contrast them with sketches of the same spots made by experienced 

 astronomers in England, and at Brussels, Palermo, and Kalocsa, and 

 also with photographs taken at Dehra Dun and at Meudon ; and I 

 think these few examples will amply suffice to vindicate a high 

 character for solar drawings. But 1 have not been satisfied with 

 this ready comparison, and lately this solar work has been put to a 

 more rigid test of accuracy by placing side by side the measurements 

 of areas obtained from drawings and photographs ; and here again 

 the hand-sketch by projection seems to bear well the scrutiny. 



The method of projection not only permits the area covered by 

 spots and faculsB to be accurately determined, but it also furnishes 

 precise data for finding the heliographic co-ordinates of any mark 

 upon the solar surface. These, however, are now given so fully in 

 the annual publications of the Eoyal Observatory, and each in- 

 dividual spot can be so readily identified, that an indejjondent 

 calculation would be a mere waste of time and energy. The results, 

 therefore, dependent on position alone, are deduced immediately 

 from the Greenwich Tables. 



The decade of years which we are now considering, covers 

 almost an entire solar cycle, that period of eleven years the proof of 

 whose existence was the fruit of the unwearied labours of Baron 

 Schwabe, in his daily observations from 1826 to 1868. The present 

 cycle falls much below that which preceded it in the extent of its 

 sj)otted area, but it is remarkable for the duration of its maximum 

 period. The last minimum occurred about November 1878, and 

 therefore, if we accept the mean values 3 • 7 and 7 • 4 as the number of 

 years from minimum to maximum and from maximum to minimum 

 respectively, we obtain 1878 '9 -}-3*7 = 1882 6 as the date of the 

 maximum of the cycle, and 1882*6 + 7*4 = 1890*0 as the epoch of 

 the approaching minimum. Many facts seem to. support this con- 

 clusion. Thus, the greatest sun-spot area occurred on April 21, 

 1882, and the largest individual spot was at its maximum on 

 November 18 of the same year, when it covered almost one four- 

 hundredth of the visible hemisphere. The total spot area in April 

 reached, however, the much higher figure of one one-hundred-and- 

 eightieth, or about 6,000,000,0.0 square miles, which on the 

 following day had diminished to something under 4,000,000,000, 

 showing the marvellous activity of the solar forces at that epoch. 

 In the same year also the mean monthly amount of umbra was 

 greater in April and in November than at any other time of the 

 cycle, making it highly probable that the disturbances then pene- 

 trated more deeply below the surface of the photosphere. Again, if 

 we consider the mean latitude of the spotted area, its value at the 

 maximum seems also to favour the claims of 1882 ; and these are 



