CHAP. VII., 7.] 



ELECTRICITY. M. JACOBI CAVENDISH. 



189 



means of a simple connection with an astronomical 

 clock. A separate marking apparatus tinder the con- 

 trol of the experimenter enables him to interpolate a 

 dot or mark corresponding to the instant of any 

 event happening such, for instance, as the transit of 

 a star. This method has the great advantage of 

 leaving the observer at entire liberty to watch the 

 object without having to attend to the beats of the 

 clock, whilst it renders mistakes next to impossible. 

 It has been successfully applied to the determination 

 of longitudes, and more recently to all kinds of astro- 

 nomical observations, by Mr Bond in America, and 

 by Mr Airy at Greenwich (231). 

 (865 ; ) 4. Chronoscopes, or instruments for the measure- 



Chrono- ment of excessively short intervals of time, such as 

 the flight of military projectiles, and even the trans- 

 mission of sensation and motion along nervous 

 fibres. Such instruments have been constructed on 

 a great variety of principles. Those of M. Pouillet, 

 Mr Wheatstone, and Mr Siemens, deserve especial 

 mention. 

 (866.) HI- Of electro-magnetism used as a moving 



Electro- power, we need say little. No one can witness the 

 m astonishing experiment of the sudden creation of 



mover. l magnetic power sufficient to sustain one or two tons 

 by the voltaic dissolution of a few grains of zinc, 

 without having the idea suggested of a continuous 

 moving force. This enormous power is, however, 

 exerted through a space so excessively minute, that 

 its dynamical effect is always small ; and, though it 

 is, of course, possible to produce an engine by a 

 sufficiently gigantic arrangement, the success has 

 hitherto not been encouraging, 

 (867.) The rotations of Mr Faraday and Dr Ritchie were 



electro-dynamic machines on a small scale. They M. Jacobi's 

 were (probably) first mechanically applied by M.dal machin e8. 

 Negro in 1833, but more systematically by Mr. M. H. 

 Jacobi, of St Petersburg, the following year. It is 

 stated that the latter gentleman moved a boat on the 

 Neva with an electro-dynamic motor equivalent to 

 three-fourths of a horse power. He has also inves- 

 tigated the theory of these machines, and the most 

 advantageous circumstances of their employment. 

 The principle is the alternate attraction and repul - 

 sion of two temporary magnets (one of which re- 

 volves), the current of electricity being suddenly 

 changed at the critical part of the revolution. 



IV. To M. Jacobi almost simultaneously with Mr (868.) 

 Spencer of Liverpool we are also indebted for one Electr - 

 of the simplest and most elegant applications of elec- voUatvpe 

 tricity, the Galvano-plastic art, or Voltatype. In MM. Ja-' 

 this, advantage is taken of the perfectly metallic state cobi and 

 in which the base of a metallic salt is deposited at Spencer, 

 the negative pole of a voltaic combination. In the 

 case, for example, of the decomposition of sulphate 

 of copper, the sulphuric acid unites with the positive 

 wire, or remains suspended, while the metallic cop- 

 per is slowly and homogeneously deposited on the 

 surface of any object (rendered conducting by the 

 application of black lead or otherwise), of which it 

 forms a perfect mould, from which a fresh cast or 

 fac-simile in metal of the original object may be ob- 

 tained by a repetition of the process. To see the 

 veins of a leaf, or the delicate wing of an insect, thus 

 metallized, is certainly an astonishing thing ; and 

 the applications to the useful arts are far too nume- 

 rous to be noticed here. Daniell's invention of the 

 Constant Battery evidently suggested the Voltatype. 



7. CAVENDISH COULOMB <-Eocperimental Laws of the Distribution of Statical Electricity ; 

 Mathematical Theory of the same. POISSON Mathematical Theory of Statical Electricity 

 and of Magnetism generalized. Green ; Professor William Thomson. 



(869.) 

 Statical 

 theory of 

 Electricity 



(8/0.) 

 JEpinus, 

 Cavendish, 

 and Cou- 

 lomb. 



Having thus brought down the history of galvanic 

 or voltaic electricity, and that of the wonderful dis- 

 coveries connected with it, to our own time, I shall 

 in this section briefly notice the more intermitting 

 progress during the same period of our knowledge 

 of the quantitative laws which regulate the distribu- 

 tion of statical electricity on bodies charged with it. 



" ^Spinus and Coulomb," says Dr Whewell, 1 " were 

 two of the most eminent physical philosophers of 

 the last century." They laid the foundations of an 

 exact science of statical electricity ; and a third, and 

 still more eminent name, deserves to be connected 

 with theirs, that of CAVENDISH, of whose general 

 labours I have already given some account in the 

 Second section of the chapter on Heat. The labours 

 of ^Jpinus belong rather to the period embraced in 

 the previous Dissertation, where they have been re- 

 ferred to by Sir John Leslie. 



Perhaps the most elaborate of the memoirs not (g7l.) 

 strictly chemical which Cavendish published wereCaven- 

 those on electricity. The Franklinian hypothesis of dish ' s 

 a single fluid in excess or in defect of its average ^^ and 

 state producing the phenomena then known as elec- men ts. 

 trical, offered a tempting field to an experimental 

 philosopher well trained in the mathematical know- 

 ledge of the day ; and his paper on this subject shows 

 extreme care in its conception and execution. He 

 assumes, as a matter of necessity, the repulsion of 

 matter for matter at sensible distances, considered 

 apart from the electricity always combined with it in 

 greater or less quantity. The indifference of matter 

 under ordinary circumstances is held to arise from 

 the union with it of a sufficient amount of electricity 

 to neutralize the repulsion of the matter. In short, 

 the electric fluid is considered as a second kind of 

 matter repelling its own particles, and attracting those 



History of the Inductive Sciences, vol. ili., p. 34, 2d edition. 



