REVIEWS. 
415 
author’s hook are not less interesting than that from which we have 
quoted, hut our limited space forbids our entering upon them. The hook 
is full of instructive reading, and is withal a most attractive volume. 
TERRESTRIAL MAGNETISM.* 
W HATEVER may he thought of this author’s views by physicists com- 
petent to follow him through a difficult line of argument in a subject 
where few ordinary persons are qualified! for the task, there can he little 
doubt, we think, that he has performed his task ably. So far as we have 
examined his views, they are throughout fortified by an amount of learning 
and patience in investigation which might have made the author sufficiently 
courageous to have appended his name. And we think so all the more 
because we know how important is a name in making a book be read and 
carefully attended to. Of course, if the author bore a name unadorned or 
at least undistinguished in physics, it would be a different affair. It would 
cause his book to convince some persons of the error of his views. But' 
really we do not think that this is the case. To us the author of the book 
appears, as no doubt he is, as a writer to whom physics has been a subject 
of careful and prolonged study, and who has by this tune obtained dis- 
tinction in his subject. However, be that as it may, there is no doubt 
that the theory he puts forward is modestly set forth and is further sup- 
ported by a multitude of facts in the science of physics. It will of course 
yet remain undecided as to how our earth became originally magnetised ; 
but, granting its magnetisation, we think it is much more coincident with 
facts as they are to suppose a condition of electricity intervening between 
the earth and the sun, and that this is the parent of the earth’s magnetism, 
than that the latter should be derived from any kindred condition in the 
sun itself. Indeed, the author’s illustration of the position of the earth 
in regard to the sun and the impossibility of its poles corresponding to 
the two of the earth, appears to us adequately clear. In fact, it seems 
probable, from analogy, that if the two poles should act on one of the 
earth’s, they would thus destroy each other. Further, as the author has 
shown (and this seems to us to be a more powerful argument), the motion 
of the sun round its own axis should produce profound changes in the 
magnetisation of the earth every twelve-and-a-half days, or half the sun’s 
own period of rotation. This and other facts which he adduces sufficiently, 
we think, disprove the view of the sun’s direct activity as a magnetiser. 
In the further elaboration of his views we shall not follow the author. 
Nor do we desire to express any opinion on their accuracy or force. The 
subject is one which can only be followed out by r an experienced physicist, 
and by him only with considerable time at his disposal. The book is well 
got up, and contains abundant maps and plans illustrating the variations 
* “ A Treatise on Terrestrial Magnetism.” Edinburgh: William Black- 
wood, 1871. 
