1862.] 197 



and it therefore becomes possible to give the elements of this force, 

 which will thus characterize the disturbance. 



The author then attempts, by means of comparing similar appear- 

 ances, to represent the force at work for each disturbance between the 

 beginning of 1858 to the end of 1860. The great disturbance of 

 August to September 1859 is here remarkable as one in which two 

 independent disturbing forces seem to have acted at once, one of 

 these being of the normal type, in which all the elements were raised 

 or depressed together, while in the other the declination was raised 

 when both elements of the force were depressed. 



It will be observed that this method of analysis does not completely 

 determine the disturbing force, but merely fixes the line of its 

 resultant action, along which the force itself may be either positive 

 or negative ; or, again, there may be two nearly opposite forces acting 

 against one another, the visible disturbance denoting merely the 

 difference in strength between the two ; and there is some reason to 

 think that this last supposition represents the true state of the case. 



For while the definite relation which exists between the peaks of 

 the two force-components shows that all disturbing forces affect these 

 in nearly the same way, yet sometimes, though very rarely, in the 

 general progress of the curve one of the elements will be above the 

 normal while the other is below it. Now, this may be accounted for 

 in the following manner. Suppose we have a disturbance producing 

 an elevation in the horizontal force represented by +40, and one in 

 the vertical force represented by +20. This will be of the normal 

 type. Suppose now that at the same time we have another force 

 nearly similar, whose action on the two force-elements is represented 

 by 39 21. This is also sufficiently near the normal type. The 

 result of these two disturbances superimposed will be + 1 and 1, 

 showing that the one element is raised above its normal position, 

 while the other is depressed below it. This idea of two opposite 

 forces acting simultaneously in disturbances is that entertained by 

 General Sabiue from other considerations. 



VOL. XII. 



