Notices 7rspecting New Books. d-TS 



nous and the rods emitted sparks. In tliis case, as Professor 

 ' Henry observes, the electricity of the machine must be con- 

 sidered as free electricity; and as the bodies on which they 

 fell were all in their natural state, the spark is immediately 

 thrown off' as a lateral discharge. Whether insulated or not, 

 the electricity of the body is evidently acted on by induction, 

 before the spark can be distributed over it or the earth. 

 Hence, when sparks of about an inch long are thrown on the 

 upper end of a lightning-rod, or other metallic body passing 

 into the earth, the induction upon the rod and earth requiring 

 a short time for its development, a spark is thrown off' upon 

 any adjacent conductor in a state to receive it. Such experi- 

 ments, therefore, apply only to small quantities of electricity 

 suddenly thrown upon conductors in a neutral state. This, 

 as I have shown, (13, figure 6,) is a distinct case from that, in 

 which a charged surface throws off' its redundant electricity 

 upon an opposite surface eager to receive it through a con- 

 ducting-rod sharing in the electrical state of that surface, and 

 which is consequently prepared already by induction to dis- 

 charge it. One might be led to infer, from the particular 

 description given by the author of this experiment, page 235, 

 that sparks had been obtained from a lightning-rod at the time 

 of its conveying a discharge of lightning. It may not be amiss 

 to add, that Professor Henry did not consider these experi- 

 ments as applicable to lightning-rods; and that in accordance 

 with the opinion of Biot, he thinks the spark observable at 

 the time of discharging ajar — that is, Mr. Sturgeon's 7/cw fact 

 — is entirely owing to a small quantity of redundant electricity 

 always existing on one side of the jar, as I have already stated, 

 {J",) and not to the "wJwle charge. 



I am, Gentlemen, yours, &c. 

 Plymoutli, Nov. 5, 1839. W. Snow Harris. 



LXXII. Notices respecting New Booh. 



Remarks on the Classification of the Different Branches of Human 

 Knowledge. By J. W. Lubbock, Esq., Treas. R.S., SfC, Vice- 

 Chancellor of the University of London. Second Edition. London 

 and Cambridge, 1839. 8vo. pp. 42. and xxii. 



THE advancement of knowledge and the continually increasing 

 number of those Avho apply themselves to its improvement and 

 extension, are daily rendering the subject of classification, whether 

 as relating to the objects of nature or to the different branches of 

 science and learning, more and more important. Arrangement, con- 

 sidered as forming itself an object of science, has not been long at- 

 tended to in this country, even with respect to those branches of na- 



