Cuar. VII., § 7.] 
means of a simple connection with an astronomical 
clock. A separate marking apparatus under 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. 
Tt has been successfully applied to the determination 
of longitudes, and more recently to ail kinds of astro- 
nomical observations, by Mr Bond in America, and 
by Mr Airy at Greenwich (231). 
4. Chronoscopes, or instruments for the measure- 
ELECTRICITY.—M. JACOBI—CAVENDISH. 
987 
electro-dynamic machines on a small scale. 
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 
They M. Jacobi's 
were (probably) first mechanically applied by M. dal ™**bines. 
ment of excessively short intervals of time, such as Spencer of Liverpool—we are also indebted for one Flectro- 
Scopes: the flight of military projectiles, and even the trans- of the simplest and most elegant applications of elec- Ybor P 
Seat : ‘ aes * . yPe 5 
mission of sensation and motion along nervous tricity, the Galvano-plastic art, or Voltatype. In —MM. Ja- 
fibres.—Such instruments have been constructed on this, advantage is taken of the perfectly metallic state cobi and 
a great variety of principles. Those of M. Pouillet, in which the base of a metallic salt is deposited at SP°°°” 
Mr Wheatstone, and Mr Siemens, deserve especial the negative pole of a voltaic combination. In the 
mention. case, for example, of the decomposition of sulphate 
(866.) III. Of electro-magnetism used as a moving of copper, the sulphuric acid unites with the positive 
Electro- power, we need say little. No one can witness the wire, or remains suspended, while the metallic cop- 
magnetism astonishing experiment of the sudden creation of per is slowly and homogeneously deposited on the 
as a prime 
magnetic power sufficient to sustain one or two tons surface of any object (rendered conducting by the 
mover, 
by the voltaic dissolution of a few grains of zine, 
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 
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- 
: sufficiently gigantic arrangement, the success has 
hitherto not been encouraging, rous to be noticed here. Daniell’s invention of the 
; (867.) The rotations of Mr Faraday and Dr Ritchie were Constant Battery evidently suggested the Voltatype. 
§ 7. CavENDISH—CovuLomMB—Eaperimental Laws of the Distribution of Statical Electricity ;— 
Mathematical Theory of the same.—Potsson—Mathematical Theory of Statical Electricity 
and of Magnetism generalized. Green; Professor William Thomson. 
(869.) Having thus brought down the history of galvanic Perhaps the most elaborate of the memoirs not 871.) 
Gaocy ot or voltaic electricity, and that of the wonderful dis- strictly chemical which Cavendish published were Caven- 
Electricity. 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 
those on electricity, The Franklinian hypothesis of dish’s 
a single fluid in excess or in defect of its average se 
state producing the phenomena then known as elec- ments. 
Cavendish, the last century.” 
of the quantitative laws which regulate the distribu- 
tion of statical electricity on bodies charged with it. 
‘« JEpinus and Coulomb,” says Dr Whewell,! ‘* were 
two of the most eminent physical philosophers of 
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 Cavenpisu, of whose general 
labours I have already given some account in the 
Second section of the chapter on Heat. The labours 
of ZEpinus belong rather to the period embraced in 
the previous Dissertation, where they have been re- 
ferred to by Sir John Leslie. 
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 
1 History of the Inductive Sciences, vol. iti., p. 34, 2d edition. 
