VELOCITY OF ELECTRICITY. 117 



which is fifteen times less than that of the electric current in 

 Wheatsto no's rotatory discs. As in Walker's remarkable expe- 

 riments two icircs were not used, but half of the conduction, 

 to use a conventional mode of expression, passed through 

 the moist earth, we should seem to be justified in concluding 

 that the velocity of the transmission of electricity depends upon 

 the nature as well as the dimensions 68 of the medium. Bad 

 conductors in the voltaic circuit become more powerfully heated 

 than good conductors ; and the experiments lately made by 

 Riess 59 show that electric discharges are phenomena of a very 

 various and complicated nature. The views prevailing at the 

 present day regarding what is usually termed " connection 

 through the earth" are opposed to the hypothesis of linear, 

 molecular conduction between the extremities of the wires, 

 and to the conjectures of the impediments to conduction, of 

 accumulation, and disruption in a current; since what was 

 formerly regarded as intermediate conduction in the earth is 

 aio w conjectured to belong exclusively to an equalisation or 

 restoration of the electric tension. 



Although it appears probable, from the extent of accuracy 



Fizeau and Gounelle at Paris, in April, 1850, differ both from 

 Wheatstone's and Walker's results. The experiments recorded 

 in the Comptes rendus, t. xxx. p. 439, exhibit striking differ- 

 ences between iron and copper as conducting media. 



5S See PoggendorffWmwfoa, bd. Ixxiii. 1848, s. 337, and 

 Pouillet, Comptes rendus, t. xxx. p. 501. 



' 9 Riess in Poggend. Ann., bd. 78, s. 433. On the non-con- 

 duction of the intermediate earth see the important experiments 

 of Guillemin Sur le courant dans une pile isolee et sans commu- 

 nication entre les poles in the Comptes rendus, t. xxix. p. 521. 

 " Quand on remplace un fil par la terre, dans les telegraphes 

 electriques, la terre sert plutot de reservoir commun, que de 

 moyen d' union entre les deux extremites du fil." " When the 

 earth is substituted for half the circuit in the electric tele- 

 graph, it serves rather as a common reservoir than as a means 

 of connexion between the two extremities of the wire." 



