VELOCITY OF ELECTRICITY. 87 



be satisfactorily compared with the results deduced from ob- 

 servations on aberration and on the satellites. 



The attention of physicists has been powerfully attracted 

 to the experiments on the velocity of the transmission of 

 electricity, recently conducted in the United States by Walk- 

 er during the course of his electro-telegraphic determina- 

 tions of the terrestrial longitudes of Washington, Philadel- 

 phia, New York, and Cambridge. According to Steinheil's 

 description of these experiments, the astronomical clock of 

 the Observatory at Philadelphia was brought to correspond 

 so perfectly with Morse's writing apparatus on the tele- 

 graphic line, that this clock marked its own course by points 

 on the endless paper fillets of the apparatus. The electric 

 telegraph instantaneously conveys each of these clock times 

 to the other stations, indicating to these the Philadelphia 

 time by a succession of similar points on the advancing pa- 

 per fillets. In this manner, arbitrary signs, or the instant 

 of a star's transit, may be similarly noted down at the sta- 

 tion by a mere movement of the observer's finger on the stop. 

 "The special advantage of the American method consists," 

 as Steinheil observes, " in its rendering the determination of 

 time independent of the combination of the two senses, sight 

 and hearing, as the clock notes its own course, and indicates 

 the instant of a star's transit (with a mean error, according 

 to Walker's assertion, of only the 70th part of a second). A 

 constant difference between the compared clock times at 

 Philadelphia and at Cambridge is dependent upon the time 

 occupied by the electric current in twice traversing the 

 closed circle between the two stations." 



Eighteen equations of condition, from measurements made 

 on conducting wires of 1050 miles, "gave for the velocity of 

 transmission of the hydro-galvanic current 18,700 miles,* 

 which is fifteen times less than that of the electric current 

 in Wheatstone's rotatory disks. As in Walker's remarkable 

 experiments two wires were not used, but half of the con- 



* Steinheil, in Schumacher's Astr. Nachr., No. 679 (1849), s. 97-100; 

 Walker, in the Proceedings of the American Philosophical Society, vol. 

 v., p. 128. (Compare earlier propositions of Pouillet in the Comptes 

 Rendus, t. xix., p. 1386.) The more recent ingenious experiments of 

 Mitchel, Director of the Observatory at Cincinnati (Gould's Astron. 

 Journal, Dec, 1849, p. 3, On the Velocity of the Electric Wave), and the 

 investigations of Fizeau and Gounelle at Paris, in April, 1850, differ 

 both from Wheatstone's and Walker's results. The experiments re- 

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

 ences between iron and copper as conducting media. 



