406 



Prof. B. Stewart. 



[Dec. 21, 



II. " On the Connexion between the State of the Sun's Surface 

 and the Horizontal Intensity of the Earth's Magnetism." 

 By Balfour Stewart, F.R.S. Received December 6, 

 1882. 



1 . The late John Allan Broun, in a paper published in the " Trans- 

 actions of the Royal Society of Edinburgh" (vol. xxii, Part 3), has 

 compared together the daily changes of the earth's horizontal magnetic 

 force at the four stations — Makerstoun (Scotland), Trevandrum 

 (India), Singapore, and Hobarton, and has come to the following 

 conclusions : — 



(1.) The daily mean horizontal force increases at the same time at 

 all the stations, and diminishes at the same time at all the stations. 



(2.) The proportional amounts of increase and diminution at the 

 stations are not very different from each other. 



(3.) If this holds for all the points of the earth's surface, we may 

 infer that the intensity of the magnetism of the whole earth is vari- 

 able, increasing, or diminishing from day to day. 



2. It is easy to see that the selection by Broun of the horizontal 

 force component in preference to the vertical force or the declination 

 was a happy choice. For since the magnetic system of the earth has 

 two pairs of force-foci, one pair of which is, perhaps, chiefly affected 

 by transient changes, the variations of declination at different stations 

 might not possess that uniformity which those of horizontal force 

 might be expected to exhibit ; and in like manner the changes of 

 vertical force might have opposite signs at opposite sides of the 

 equator of the variable magnetic system. 



On the other hand, at all stations in ordinary latitudes the hori- 

 zontal force might be expected to increase or diminish at the same 

 time, and nearly in the same proportion, whether the cause of this 

 change were due to only one of the two magnetic systems, or whether 

 it were shared by both. 



3. Assuming, therefore, as the most probable conclusion that these 

 changes of horizontal force represent changes in the intensity of the 

 magnetism of the whole earth, let us now endeavour to ascertain 

 whether they depend in any way on the state of the sun's surface. 



In order to determine this point, I have taken the daily means 

 given by Broun for the four stations above mentioned. 



These are for the years 1844 and 1845, the unit being the same 

 fraction of the horizontal force for each station. The next step has 

 "been to take the mean of these four daily means, and to assume that 

 this represents approximately at least the value from day to day of 

 the relative intensity of the magnetism of the earth. 



