ON THE NUMERICAL RELATIONS OP GRAVITY AND MAGNETISM. 



133 



more powerful magnets, lying nearly in the line of the earth's axis, has been objected to 

 on the ground that it is difficult to understand how any conceivable magnetic intensity, 

 by its simple induction, could produce so great a disturbance as is daily observed.* But 

 if Barlow's theory — that the magnetism is superficial — is correct,f and the reasonable 

 grounds which have led to its general adoption have never been controverted, his idea that 

 it is also in some manner induced, must be regarded as extremely probable. The argu- 

 ments that have been plausibly urged against an induction analogous to that which takes 

 place between ordinary magnets, have no weight if the inducing force is simple gravita- 

 tion, and the induced effect is a simple reaction. From the fact that magnetism, like 

 gravity, is a central force, varying inversely as the square of the distance, Secchi's conclu- 

 sions derive a new interest, and lend encouragement to those who are endeavoring to find 

 additional evidences of kinetic unity. 



The daily magnetic variations corresponding, as has been shown, both in character and 

 amount, with the variations of gravitating force, the question may naturally arise, whether 

 a like correspondence can be found in the annual and secular changes. The unknown 

 meteorological agency which is gradually shifting the isothermal lines on which I have 

 supposed the terrestrial magnetism primarily to depend, the want of a sufficiently precise 

 determination of the monthly and annual perturbation-values, and the variety of elements 

 (many of which are still undetermined) that should enter into our equations, render it 

 impossible at present to arrive at quantitative results, but the evidence of a qualitative cor- 

 respondence is most satisfactory. 



Among the conclusions which General Sabine has deduced from his discussions are the 

 following: 



1. "That the easterly and westerly disturbances have different hours for their principal 

 development, — that these hours differ most widely in different parts of the globe, — and 

 that we may already perceive some indications of a connection existing between the epochs 

 of greatest development and differences of geographical longitude." (St. Helena Obs., Vol. 



II, p. cxvii.) 



2. " The evidence afforded by each of the three observational elements in regard to 

 annual variation is to one and the same effect. January and June are the months of mini- 

 mum disturbance ; September and April the months of maximum disturbance. The 

 aggregate value of the disturbances in the equinoctial months is about three times as 

 great as in the solstitial months. Of the two equinoctial months, the value is somewhat 

 higher in each element in September than in April; and of the two solstitial months, 

 December is higher than June, also, in each of the three elements." (Toronto Obs., Vol. 



III, p. lxx.) 



* See, inter alia, Phil. Mag. [4] 15, 192 and 27, 384. f Phil. Transactions, 1831. 



