18 82.] On the Sim's Surface and the JEarttis Magnetism. 407 



Here it is necessary to remark that the indications recorded by 

 Broun are those of a differential instrument (the Bifilar), which is 

 not well adapted for recording long-period variations of the horizontal 

 component in a trustworthy manner ; but is, on the other hand, 

 admirably suited for short-period variations. As it is with this latter 

 species of change we have now to deal, we may therefore trust without 

 hesitation to the instrumental records given by Broun. 



4. The state of the sun's surface as regards spotted area for the 

 years 1844 and 1845, will be found in an appendix to the Report of 

 the Solar Physics Committee. The information there given is 

 derived from Schwabe's sun pictures, and the whole amount of spotted 

 area for each observation day is expressed in millionths of the sun's 

 visible hemisphere. 



5. The value of the comparison of these two records, terrestrial 

 and solar, is lessened by the fact that in the four observatories Sunday 

 was always a blank day, while bad weather caused the catalogue of 

 Hofrath Schwabe to present many blank days, and even groups of 

 days, during which it would be hazardous to estimate the spotted area 

 by interpolation. 



The comparison has been made in the following manner. The 

 various days exhibiting a maximum of horizontal force have been 

 taken as central epochs and compared with two days before and two 

 days after, and this comparison has been extended to sun-spots, as 

 well as to magnetic force. The method will be seen from the follow- 

 ing specimen. 



Horizontal force. Sun-spots area. 



Central date.... (1) (2) (3) (4) (5) .. (1) (2) (3) (4) (5) 

 Jan. 4,1844.. 12 -41 15 "47 16 -25 13 '22 (13 -48) .. (81) 30 36 0] 

 „ 16, „ .. 15-06 16-76 17-02 15 '75 15-18 .. 210 150 210 (202) (195) 



April 24, 1844 . 18 -53 18 -11 19 '68 14 "77 9 "88 .. 480 420 516 396 390 

 „ 28, „ . 9-88 12-67 15-47 15-01 13 '85 .. 390 594 420 270 270 



From what we have said, it will be inferred that the central dates 

 represent the dates of (3), whether for horizontal force or sun-spots. 

 The method is analogous to that adopted by Professor Loomis, when 

 comparing together sun-spots and disturbances. 



A comparison precisely similar was next made for days of magnetic 

 minima. 



6. The following results have been obtained from this comparison. 



For magnetic maxima there were in all 59 cases in which the com- 

 parison was reasonably good, while for magnetic minima there were 

 49 such cases. 



The mean of these is expressed as follows : — 



2 e 2 



