532 Notices of Books .^ [October, 



reasoning that the sun does not influence the earth by acting upon 

 it simply as a magnet, he then considers the only other known 

 way in which the sun can produce terrestrial magnetic effects — 

 namely, by bringing into existence a current of electricity. It is 

 then argued that, assuming there is a mutual attraction between 

 electricity and matter, and that therefore the matter of the sun 

 attracts the electricity of the earth and vice versa, there would be 

 a greater constant electrical density existing on the surface of 

 the earth immediately under the sun. This is, of course, allowing 

 the assumption that electricity is more or less free to move 

 among the particles of matter of which the earth is composed, 

 and that in a body moving through strata of electricity of varying 

 density, the same effects are produced as when a current is 

 caused to flow round the body. Now, as the earth moves from 

 west to east, there will, on these assumptions, be a current 

 circling from east to west, inducing a magnetic polarity north 

 and south. The magnetic poles would therefore be coincident 

 with the poles of the ecliptic, and would revolve around the 

 terrestrial poles in the same time. But this is not the case, and 

 the difficulty is surmounted by supposing that the earth is 

 analogous to a magnetised bar of hard iron, and that, conse- 

 quently, the changes in the magnetism are correspondingly 

 slower in proportion to the hardness of the ferruginous matter 

 and the strength of the current. The secular, annual, and 

 diurnal variations now become easy of illustration, because they 

 may be referred to so many combined causes. 



Of the relation of sun-spots to magnetic disturbances, the 

 writer remarks : — " Of the nature of this connection we have at 

 the present time no knowledge. It may be that this connection 

 is not direct, but results from an intermediate agency — viz., that 

 of the aurora ; for there appears to be a like variation in 

 the auroral displays, these being more frequent in proportion as 

 the number of sun-spots are greater, and vice versa. The aurora, 

 when present, always disturbs the magnetic needle ; the greater 

 or less amount of disturbance may, therefore, be dependent upon 

 the greater or less frequency of auroral exhibitions." 



The reasoning throughout is ingenious, but founded on so 

 hypothetical a basis, that it is clearly impossible to draw conclu- 

 sions. The arguments in favour of the theory, adduced in 

 the shape of calculated charts as compared with recorded obser- 

 vations, are very voluminous and certainly striking. The book 

 is well worth perusal as a very fair and clear statement of what 

 has been observed and advanced with regard to the magnetism 

 of the earth. The collection of charts, copies from those of Sir 

 Edward Sabine, would alone render the work of value. 



