1869.] 



Dr. B. Stewart on the Kew Magnetic Curves. 



463 



hollows ; and the following remarks will serve to show that the study of 

 these may be attended with considerable advantage. 



The labours of General Sabine have been instrumental in showing that 

 there are at least two forces concerned in producing disturbances ; and this 

 conclusion is confirmed by the appearance of the Kew curves, from which 

 it may be seen that no disturbance of any magnitude is due to the action 

 of a single force merely varying in amount and not in direction ; for if 

 this were the case the distance at any moment of a point in the curve of 

 one of the elements from its normal position should bear throughout such 

 a disturbance an invariable proportion to the distance of a corresponding 

 point in the curve of another of the elements from its normal ; but this 

 is by no means the case. 



It becomes therefore a question of interest to endeavour to find the 

 elementary forces concerned in producing a disturbance ; and it is thought 

 that this knowledge may to some extent be attained by a study of those 

 small and rapid changes of force which are denoted by peaks and hollows. 

 For if several independent forces are at work, it may be thought unlikely 

 that at the same moment a sudden change should take place in all ; there 

 is thus a probability that sudden changes of force, as exhibited in peaks 

 and hollows, are changes in one of the elementary forces concerned, which 

 may thus enable us to determine the nature of that force. Even if the 

 change is not a very abrupt one, provided that we confine ourselves to 

 such peaks and hollows as present a similar appearance for all the curves, 

 we may suppose that we are observing changes in one only of the ele- 

 mentary disturbing forces ; for it is unlikely that two or more independent 

 forces, changing independently, should produce similar appearances in all 

 of the three curves. Thus what we have to look a 

 for is similar appearances ; and the precise meaning / \ 



attached to this expression will be rendered clear 

 by means of the annexed graphical representation. Declmatlon - 



We see here that (time being reckoned horizon- 

 tally) we have a disturbance commencing at the Hor - force « 

 same moment in each of the three elements, that 

 for the declination being throughout three times as Vert, force, 

 large, and that for the annexed force twice as 

 large as the corresponding vertical-force disturbance. 



In a paper communicated by me to the Royal Society, and published in 

 the Transactions (1862, page 621), it was stated that, as a rule, small and 

 abrupt disturbances at Kew tend either to increase at the same moment 

 both components of magnetic force and the westerly declination, or to 

 decrease these elements, as the case may be. As in the Kew curves of 

 1862, increasing ordinates represented decreasing horizontal force, decreas- 

 ing vertical force, and decreasing declination, the above statement is the 

 asme as saying that, as a rule, peaks and hollows in one element correspond 

 to peaks and hollows in the other two. 



