CONCEENED IN PEODUCING THE GEEATEE MAGNETIC DISTTJEBANCES. 629 
16. Let us now endeavour to ascertain if any use may be made of this fact to throw 
light on the nature of the forces concerned in producing disturbances. 
17. Two distinct suppositions may be made regarding the nature and mode of action 
of disturbing forces. 
1°. We may suppose that forces of every imaginable variety of character are con- 
cerned together in producing disturbances. 
2°. We may suppose a disturbance occasioned by one or more groups of forces the 
elements of which are bound together by a certain law. 
With respect to the first of these hypotheses, it is refuted by the discussion of disturbing 
forces given by General Sabine for the diflerent Colonial Observatories and for Kew. as 
well as by the results in Tables I., II., and III. The second hypothesis must, therefore, 
represent the mode of action of the forces concerned. 
18. And, first of all, it may safely be affirmed that no disturbance of any magnitude is 
due to the action of a single force, merely varying in amount ; 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 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 true. 
Since, therefore, a disturbance is not a phenomenon due to the action of a single force, 
and since at the same time it does not represent the action of a number of different forces 
promiscuously huddled together, it becomes a question of interest to ask ourselves how 
we may find the elementary forces concerned. 
19. * A little consideration will show that this is likely to be obtained by the study of 
small and rapid changes of force. For if several forces are at work, it is unlikely that at 
the same moment a sudden change should take place in all ; there is thus a high proba- 
bility that a sudden and rapid change is a change in one of the elementary forces con- 
cerned, and which will therefore 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 be satisfied that 
we are observing changes taking place in one only of the elementary disturbing forces ; 
for it is inconceivable that two or more independent forces changing independently 
should produce similar appearances in all the three curves. This may be illustrated by 
an example. Suppose, for instance, a disturbance takes place in declination which at the 
end of one minute has raised the curve one-tenth of an inch, while at the end of the 
second minute it has fallen again to its original level. Suppose also that a change, 
precisely the same in nature and amount, takes place in the horizontal force, while in 
the vertical force the change is only one-twentieth of an inch. Here then we have in our 
curves three isosceles triangles which, although the last is not, strictly speaking, similar 
to the other two, yet all three convey the idea of only one force acting. Suppose now 
another force to have been superimposed which afiected the declination very much more 
than either of the force-components, but which did not begin to work until the end of 
the first minute. The result would be that the declination peak would be much further 
