

314 Notices respecting New Books. 



circuit of an A Gramme machine sending a fairly strong cur- 

 rent can be broken by the use of this key without any visible 

 spark. 



The spring S, combined with the locking-arrangement, 

 makes it impossible for the key to be left partly on or off. If 

 not put so that the current flows without resistance, then the 

 spring S breaks the circuit altogether. 



Besides the advantage this key affords of being able to break 

 an entire electric-light circuit without spark, it is possible that 

 another advantage may be produced, viz. that the life of 

 incandescent lamps may be increased by the current passing 

 through them being always gradually diminished to nothing, 

 instead of being stopped as is at present the common practice; 

 but whether this second advantage exists, which, however, 

 seems extremely probable, time of course alone can prove. 



At any rate the existence of a Non-Sparking Key now makes 

 it possible for Insurance Companies to turn their attention to 

 the question of prohibiting sparking altogether, which hitherto 

 it has been almost useless contemplating. 



XXXVI. Notices respecting New Books. 



The Classification of the Cambrian and Silurian Rocks, being the 

 Sedgwick Prize Essay for the Tear 1882. By John E. Mabk, 

 M.A., F.G.S., Fellow of St. Johris College, Cambridge. 8vo. 

 Pp. 147. Deighton, Bell & Co. 



OF the many wars in which the " fighting geological " has been 

 engaged, none have been carried on with more persistence than 

 the almost continuous fight over the body of the ' Old Grey wacke," 

 which was broken up by those two paladins of geology, Sedgwick and 

 Murchison, some fifty years ago. If the author of this classification 

 be correct, the previous confusion of the Greywacke became more 

 elaborate in consequence of the errors of the founder of the Silurian 

 System, which was based, we are told, according to an American 

 geologist and an impartial judge, upon a series of stratigraphical 

 mistakes which are scarcely paralleled in the history of geological 

 investigation. 



In consequence of their conviction of the general inapplicability 

 of previous classifications, some authorities, in the light of more 

 recent studies of these rocks and* their contents, have desired to 

 abandon the historical classification in favour of one which they 

 consider more natural. To this the author demurs ; and the object 

 of the Essay is to show that the historical and natural classifica- 

 tions coincide, provided we accept the arrangement and subdivisions 

 proposed by Sedgwick rather than those of Murchison. The latter, 

 in consequence of his official position, was enabled to keep adding 



