194 [June 19, 



This inconvenience is more than compensated for by the fact that 

 the angular elements of every face, and its relation to another face, 

 can at once be calculated from the symbols of the faces by very easy 

 formulae. The magnitude of the indices are also shown to be much 

 diminished by using approximations bringing every pole to its place 

 on the sphere of projection within 5 or 6 minutes an approximation 

 not greater than that constantly used to make observations tally with 

 the calculated symbols. 



XXI. "On the Forces concerned in producing the larger 

 Magnetic Disturbances." By BALFOTJR STEWART, ESQ., 

 M.A., F.R.S. Received June 14, 1862. 

 (Abstract). 



The author begins by alluding to a previous communication made 

 to the Royal Society, containing an account of the great magnetic 

 storm of August 28-September 7, 1859, in which he had shown 

 that the first effect of this great disturbance was to diminish in 

 intensity both components of the earth's magnetic force at Kew, 

 during a period of about six hours. Such an effect, he argues, can 

 scarcely be supposed due to any combination of earth-currents, of 

 which the period is only a few minutes. 



But another appearance is noticeable on the photographic curves 

 which regard the progress of this great disturbance. 



While the great wave of force had a period of about six hours, 

 there were superimposed upon it smaller disturbances having a 

 period of a few minutes, and therefore comparable in this respect 

 with earth-currents. 



These smaller disturbances are of very frequent occurrence, and 

 show themselves in the Kew magnetograph curves as serrated appear- 

 ances, occasionally magnified into peaks and hollows. 



Two hypotheses may be entertained regarding them. 



1st. They may be conceived to represent small and rapid changes 

 in the intensity of the whole disturbing force which acts upon the 

 magnet ; and since (as stated above) this force cannot be supposed 

 due to earth- currents, so neither can its variations be caused by these. 



2nd. The peaks and hollows may be supposed due to the direct 

 action of earth-currents upon the magnets. 



