On the Forces concerned in producing Magnetic Disturbances. 481 



According to Mr. Walker's observations, however, on both these 

 occasions a current between Margate and Ramsgate was simultaneous 

 with one of the same name between Ramsgate and Ashford. 



Thus, if we adopt the second hypothesis, it would appear that 

 these lines ought to have been affected differently on these two 

 occasions, whereas by observation they were affected in the same 

 manner ; the conclusion is that this hypothesis does not represent 

 the truth. 



The author then shows that earth-currents observed simultaneously 

 with a very abrupt disturbance which commenced about 1 l h 1 7 m a.m., 

 September 1, 1859, would lead us to infer that the former are induced 

 currents due to sudden and rapid changes in the magnetism of the 

 earth. 



Referring now to the first hypothesis, which asserts that the peaks 

 and hollows represent small and rapid changes in the intensity of 

 the whole disturbing force which acts upon the magnet, it would 

 follow that these peaks and hollows should in this case comport 

 themselves with regard to the three elements of the earth's magnetism 

 in the same way as the whole disturbing force of which they repre- 

 sent the changes. Thus, if the tendency of the great body of the 

 disturbing force is to raise the curves for the three elements simul- 

 taneously, then a small peak in one element should correspond to a 

 peak, and not to a hollow, in the other two. But if, on the other hand, 

 the tendency of the disturbing force is to raise one of the curves 

 and lower the other two, then a peak in the first should correspond 

 to a hollow in the others. 



This is shown to be the case in the disturbances extending from 

 the beginning of 1858 to the end of 1860 ; and the author therefore 

 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 



