542 Notices of Books. [OGtober, 
they are caused to rotate about a vertical axis, they will become 
ellipsoidal, and that a substance is then said to be polarised. 
The theory of induction developed therefrom, which we are told 
is very different from Faraday’s theory, is equally hypothetical. 
The actual subject of induction (p. 31) receives but slight dis- 
cussion, and the author passes on at once to electrical machines. 
The action of the Holtz machine is described, but in so compli- 
cated a form that we are convinced no general reader could 
follow it fora moment. That important instrument, the Leyden. 
jar, is neither figured nor described, because it is so well known. 
The third chapter treats of magnetism, the fourth of galvanic 
batteries. This latter contains a very fair account of the exist- 
ing batteries, including several of the new forms, and at the end 
of the chapter alist of 11 batteries is given, with the various 
purposes for which they are most suitable. The fifth chapter, 
occupying more than 50 pages, is on the very important subject 
of electrical measurement ; the author has evidently given much 
attention to the matter, and the account which he gives is valu- 
able, and will be appreciated by those engaged in practical tele- 
graph work, as also will the following chapter on conductivity 
and resistance. In the seventh chapter the current is discussed, 
and this is commenced by a statement of Ohm’s law. At 
curious diagram on p. 204 attempts to explain the action of the 
galvanic current by a mechanical analogy. This chapter is 
appropriately followed by one on electromotive force, wherein 
will be found (p. 217) a table giving the value of no less than 
29 units. Final chapters treat of electrolysis, electro-magnetism, 
and electro-metallurgy. A short dictionary of terms con- 
cludes the work. The Electric Telegraph oddly enough 
receives only a few pages of notice at the conclusion of 
the book. One of the first things we look for now-a-days in a 
work on Electricity is a definition of that much-used (and by 
beginners much-abused) word Potential. Mr. Sprague does not 
employ it, because, according to his view, ‘‘at present it has 
really no definite and accepted meaning”; in support of this 
assertion he quotes the definitions of Latimer Clark and 
Fleeming Jenkin, and shows that they are at variance. He con 
siders that the word is almost always used in place of tension or 
electromotive force. Again, the Farad is a term which does 
not always receive the same definition. 
The book bears evidence of much thought, but it is somewhat 
patchy, and very jerky and variable in style. We do not think 
that it can be useful to the general reader, or indeed to anyone 
but the advanced student. To him we recommend a perusal 
of the work. It is often eminently suggestive, and if he be 
already well grounded in electrical matters, he will find much in 
Mr. Sprague’s book to excite his thinking faculties. 
