196 [June 19, 



concludes that peaks and hollows represent small and rapid changes 

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

 magnet. 



It is then shown that use may be made of these peaks and hollows, 

 if we wish to analyse the forces concerned in producing disturbances. 

 Let us suppose that several independent forces are concerned. It is 

 very unlikely that a small and rapid change takes place at the same 

 instant in more than one of these. The measurement therefore of 

 simultaneous abrupt changes for the three elements may enable us to 

 determine the character of one of the elementary disturbing forces at 

 work. 



It is not even necessary to confine ourselves to very rapid changes, 

 provided we take peaks or hollows which present a similar appearance 

 for all the elements, as such can only be produced by the action of a 

 single force. 



The author then shows that a peak of the horizontal force always 

 corresponds to a peak of the vertical force, and not to a hollow, and 

 that, when similar peaks are compared together, the horizontal-force 

 peak is always as nearly as possible double in size that of the vertical 

 force. 



This curious fact would imply that the resolved portion of the 

 disturbing force which acts in the plane of the magnetic meridian is 

 always in nearly the same direction. The dip of this resolved por- 

 tion will be about 17i. 



It is also found that a declination peak corresponds to a peak of 

 either force, except in the case of the great disturbance of August to 

 September 1859, during the most violent portion of which a peak of 

 the declination corresponded to a hollow in either force. The length, 

 however, of a declination peak does not bear an invariable ratio to 

 that of a force peak this ratio varying much from one disturbance 

 to another, but not much from one part to another of the same dis- 

 turbance. In this last case, however, the variation of the ratio, 

 although not great, is yet greater than that of the ratio between the 

 two force peaks ; so that it is somewhat difficult to obtain similar 

 peaks when comparing the declination curve with that of either 

 force. 



It thus appears that the force which acts upon the magnets does 

 not vary much from one part to another of the same disturbance, 





