410 Notices of Books. [July, 



the laws of falling bodies. " Mere observation of lightning 

 and its effects could never have led to the discovery of their re- 

 lation to the phenomena presented by rubbed amber. A modifi- 

 cation of the course of Nature, such as the bringing down of 

 atmospheric electricity into our laboratories, was necessary. 

 Without experiment we could never even have learned the ex- 

 istence of terrestrial magnetism." 



These are specimens of the exceedingly beautiful and unique 

 illustrations of our authors. But these again are surpassed by 

 the description of the laws by which the experimentalist should 

 be controlled in the deduction of results. In all cases, to quote 

 further, when a particular agent or cause is to be studied, ex- 

 periments should be arranged in such a way as to lead, if possible, 

 to results depending upon it alone. Or, if this cannot be done, 

 they should be arranged so as to increase the effects due to the 

 cause to be studied till these so far exceed the unavoidable con- 

 comitants, that the latter may be considered as only disturbing, 

 not essentially modifying, the effects of the principal agent. 

 Thus, in order to find the nature of the action of a galvanic 

 current upon a magnetised needle, we may adopt either of these 

 methods. For instance, we may neutralise the disturbing 

 effects of the earth's magnetism on the needle by properly placing 

 a magnetised bar in its neighbourhood. This is an instance of 

 the first method. Or by increasing the strength of the current, 

 or by coiling the wire many times about the needle, multiply the 

 effects of the current so that those of the earth's magnetism 

 may be negligible in comparison. In some cases, however, the 

 latter mode of procedure is utterly deceptive — as, for instance, 

 in the use of multiplying condensers for the detection of very 

 small electromotive forces. In this case the friction between 

 the parts of the condenser often produces more electricity than 

 that which is to be measured, so that the true results cannot be 

 deduced. We thus see that it is uncertain which of these 

 methods may be preferable in any particular case ; and, indeed, 

 in discovery, he is the most likely to succeed who, not allowing 

 himself to be disheartened by the non-success of one form of ex- 

 periment, carefully varies his methods, and thus interrogates in 

 every conceivable manner the subject of his investigations. A 

 most important remark, due to Herschel, regards what are 

 called residual phenomena. When, in an experiment, all known 

 causes being allowed for, there remain certain unexplained effects 

 (excessively slight it may be), these must be carefully investi- 

 gated, and every conceivable variation of arrangement of 

 apparatus, &c. tried ; until, if possible, we manage so to 

 exaggerate the residual phenomenon as to be able to detect its 

 cause. It is here, perhaps, that in the present state of science 

 we may most reasonably look for extensions of our knowledge ; 

 at all events we are warranted by the recent history of Natural 

 Philosophy in so doing. Thus, to take only a very few instances, 



