132 Chronicles of Science. [Jan., 



universally admitted. This has been the course adopted by Doctor 

 Noad in the present "work, and the result is eminently satisfactory, 

 including as it does, within the limits of a moderate-sized volume, 

 all the information necessary to make the student acquainted with 

 the present state of electrical science, as well as with the successive 

 steps by which the knowledge has been obtained. Throughout 

 the book extracts are freely inserted from such works as the report 

 of the committee ' On the Construction of Submarine Cables,' 

 the ' Cantor Lectures ' of Mr. Fleeming Jenkin, and especially 

 from the beautiful ' Experimental Besearches " of Professor Fara- 

 day, with which every student of the science should be familiar. 

 Of late years the practical applications of electricity have 

 naturally received more attention than the theory of the science, 

 and thus we find that, while very few important laws are of recent 

 discovery, the branch relating to electric telegraphy, and more 

 especially to submarine cables, has immensely advanced. Most of 

 this advance is due, not so much to the discovery of new prin- 

 ciples, as to the better application of those already discovered, and 

 almost the whole of the present system of testing may be said to be 

 based upon two of these, viz. Ohm's law, and the law of derived 

 circuits. The former is expressed by the formula 



C ~ E' 



in which equation the current circulating, the electromotive force 

 of the battery, and the resistance of the circuit, are connected 

 together: while the latter law is simply this, that if two paths 

 are open for the electricity to travel by, the quantity circulating 

 in each will be inversely as the resistances of the two branches. 

 Wheatstone's bridge, or the electric balance, is one of the most 

 ingenious and valuable applications of these laws ; by its means the 

 resistance of any circuit may be found in terms of a known 

 resistance. A very clear description of this invention, and the prin- 

 ciples on which it is based, is given in the ' Text Book.' Most of 

 the information contained in the " Cantor Lectures," delivered by 

 Mr. Fleeming Jenkin at the Society of Arts, will be found here in 

 a condensed form, together with a full description of the im- 

 proved galvanometers and electronometers invented by Sir William 

 Thomson. 



Turning froin the scientific to the popular side of electricity, 

 as exemplified by the induction coil, we find that here also progress 

 has been chiefly in the direction of improved arrangement and 

 manufacture. In Dr. Noad's treatise on this instrument,* much of 

 the superiority of the modern coils is ascribed to the improved form 



* 'The Inductorium, or Induction Coil.' By Henry M. Noad, Ph.D., F.R.8., 



F.C.S., &c. Second edition. London : Churchill & Sons. 



