70 
POPULAR SCIENCE REVIEW. 
the facts of electricity and magnetism, and of the theories which hare been 
advanced concerning them. It is quite refreshing to go over its pages 
after putting down English translations of French works on the subject, 
and other works too evidently, though not admittedly, borrowed from our 
continental neighbours. For in its pages we find accounts of the researches 
and speculations of our own philosophers, and proper importance given to 
their labours. Not that there is any confessed or apparent effort of the 
author in this direction, but merely that it is a genuine compilation by a 
man versed in the literature of his subject. We have often felt pained to 
see English students with textbooks in their hands which leave the vast 
stores of philosophical knowledge contributed by their countrymen to the 
general stock unnoticed, and to know that there was no truly English 
textbook sufficiently comprehensive to offer in place of them. 
The work is arranged in ten parts, one of which, filling a quarter of the 
volume, is devoted to the electric telegraph. The chapter in the first part, on 
atmospheric electricity, will be found very interesting. In it and the chapter 
on marine telegraphy, Professor William Thomson’s beautiful electrometers 
are described, viz. the divided ring electrometer, the common house electro- 
meter, and the portable electrometer. One important use of the first and last 
of these is the testing the insulating power of the insulating coat of short 
lengths of marine telegraph cable. This testing consists iu estimating the 
rate at which the tension of a given charge of electricity communicated to 
the wire diminishes. We shall not attempt to describe these electrometers, 
but content ourselves by stating that the indicating needle is kept in metallic 
communication with the inside of a charged Leyden jar, by which its at- 
traction and repulsion for the two manifestations of electricity in bodies is 
rendered much more sensitive and much more certain under the slight leak- 
age of electricity which always takes place ; that the needle oscillates over 
two half rings of brass (the divided ring), one in connection with the earth, 
and the other with the charged cable or other body to be tested ; and lastly, 
that for continuous observation of the electric tension of the atmosphere, the 
motions of the needle are recorded by the photographic action of the light 
of a lamp reflected by a mirror connected with the needle, upon a cylinder 
covered with photographic paper and rotated by clockwork. 
Dr. Noad gives us full accounts of even the most recent additions to our 
knowledge of the subject of which he is treating. His account of the tele- 
graph is exceedingly interesting. The descriptions of the many instruments 
he has to notice are usually very clearly drawn out, and a comprehension of 
the text is materially simplified by the introduction of numerous woodcuts. 
The enunciations of fundamental principles, and the definitions of terms, 
are, as a rule, usually unsatisfactory in a scientific point of view. Even these, 
however, are generally compiled from the writings of others, as stated by 
Dr. Noad himself. But the compiling here, as elsewhere, is a remarkable 
proof of the comprehensive and philosophical way in which the author 
handles his subject. And we venture to say that seldom has a scientific 
writer been more happy in throwing into a treatise on a given subject the 
most important of the investigations and the most correct of the views of 
the workers at it. Every student of physical science will undoubtedly be 
glad to possess it. 
