Chase.] 42g [October. 



by innumerable currents of convection, or threads of ascending and 

 descending particles. It will also be evident that at every place 

 there are two principal sets of such double spirals, one with an axis 

 perpendicular to the earth's radius vector, producing a maximum 

 disturbance in the early afternoon, and the other more stable and 

 uniform, with an axis passing through the nearest poles of greatest 

 cold. In addition to the mutual perturbations of these two principal 

 polarizing currents, the rolling of the luni-tidal attraction-wave pro- 

 duces at every instant a greater or less derangement,* and I find that 

 the ratio of the lunar-barometric to the lunar-magnetic disturbance 

 (4.38-1:), is nearly identical with Mr. Welsh's determination of the 

 moment of magnetic inertia (4.4696; Phil. Trans., v. 153, p. 

 297). From a variety of considerations, it appears that the me- 

 chanical polarity or magnetic force thus engendered, is a third pro- 

 portional to two other forces, which may be called, respectively, cen- 

 tral and tangential. 



The communication which was presented at our last meeting, in its 

 exhibition of the first numerical relationship that has ever been 

 pointed out between the barometric and magnetic fluctuations, showed 

 that A : B : : B : M, a proportion in which A represents a central, 

 B a tangential, and M a magnetic force. 



I find a similar proportionality in each of Mr. Airy's summary 

 tables (Op. citat., p. 627, sqq). Thus in his "Table II, Algebraic 

 Sums of Magnetic Fluctuations (in terms of Horizontal Force) for 

 each Year, from 1841 to 1857, including all Days of Record of Great 

 3Iagnetical Disturbance," the Mean Disturbance is 



" Table III. Algebraic Sums of Magnetic Fluctuations (in terms 

 of Horizontal Force) for each Year, from 1841 to 1857, including 



* Besides the great disturbing agencies, whose effects may perhaps be de- 

 terminable by mathematical prediction, every transient local accumulation of 

 heat or cold will exert an influence. Everything that can produce currents or 

 eddies in the atmosphere, may also be presumed to affect the aether, and the in- 

 conceivable rapidity of the ajthereal motions, as manifested in the velocity of the 

 waves of light and heat, will account for the extreme sensitiveness of the mag 

 netic needle 



