Velocity of Light. 259 



aberration-observations 166,196 geographical miles in a second, we 

 get a difference of 83,804 geographical miles in a second for the 

 greater velocity of the electric current. 



This result appears to contradict the previously mentioned view of 

 William Herschel, which regarded the light of the sun and of the 

 fixed stars as perhaps the effect of an electro-magnetic process, — a 

 perpetual Aurora. I say appears to contradict ; for it cannot be 

 deemed impossible that, in the different luminous bodies of space, 

 there may be several magneto-electric processes very different in kind, 

 and in which the light produced by the process may have a different 

 rate of propagation. To this possible conjecture must be added the 

 uncertainty of the numerical result obtained with Wheatstone's appa- 

 ratus, which result he himself regards as " not sufficiently established 

 and as requiring fresh confirmation," in order to be compared satisfac- 

 torily with the deductions from aberration, and satellite-observations. 



Later experiments made by Walker in the United States of North 

 America on the velocity of the propagation of electricity, on the oc- 

 casion of his telegraphic determination of the longitudes of Washing- 

 ton, Philadelphia, New York, and Cambridge, have excited a lively 

 interest in the minds of physical enquirers. According to Stein- 

 heil's description of these experiments, the astronomical clock of the 

 observatory at Philadelphia was connected with Morse's writing ap- 

 paratus on the line of telegraph in such a manner, that the clock's 

 march noted itself by points on the endless strip of paper of the appa- 

 ratus. The electric telegraph carries each of these points instantane- 

 ously to the other stations, and gives them the Philadelphia time by 

 similar points on their moving strips of paper. Arbitrary signals, 

 or the instant of the passage of a star, may be noted in the same 

 manner by the observer, by merely touching or pressing an index 

 with his finger. The material advantage of this American method 

 consists, as Steinheil expresses it, '* in its making the determination 

 of time independent of the connection of the two senses, sight and 

 hearing ; as the clock's march notes itself, and the instant of the star's 

 passage is given direct (to within a mean error of the 70th part of a 

 second, as Walker states) by the movement of the observer's finger. 

 A constant difference between the compared clock-marks of Phila- 

 delphia and Cambridge is produced by the time which the electric 

 current requires to traverse twice the closed circuit between the two 

 stations." 



Measurements made with conductors 1050 English statute miles^ 

 or 968 geographical miles, in length, gave, from 18 equations of 

 condition, the rate of propagation of the hydrogalvanic current at 

 only 18,700 statute or 16,240 geographical miles in a second ; t. c, 

 fifteen times slower than the electric current in Wheatstone's rotating 

 disc apparatus I As in Walker's remarkable experiments two wires 

 were not used, but half the conduction, according to the common 

 expression, took place through the moist body of the earth, it might 



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