Velocity of the Galvanic Currentin Telegraph Wires. 69 
being about three millimeters in diameter. We have every rea- 
son for believing that the velocity with which electricity is con- 
ducted, varies* with the conducting power of the medium, an 
should therefore naturally anticipate that this velocity would be 
found greater in copper than in iron. Wheatstone used in his ex- 
periment copper wire 1"":7 in diameter. Now the conducting 
power of iron is according to Lenz, Riess,{ and Pouillet,g¢ who 
determined it by different methods, less than ,’;'; that of copper 
at 0°-C. And when we take this circumstance into consideration, 
it appears a sufficient ground for believing that the results of 
' Walker and Wheatstone are not inconsistent with each other— — 
even without reference to the fact that the one used the galvanic 
current, and the other machine-electricity of the highest possible 
ension. 
Walker’s first results,|| derived from the longitude operations of 
the Coast Survey, led him to the opinion that the velocity with 
which the signals passed between Cambridge and Washington 
on the night of Jan. 23, 1849, was 18,690 miles a second, with a 
probable accidental error of 1000 miles. 
On the night of Oct. 31, 1849, a series of experiments was 
made for the express purpose of determining the time needed for _ 
¥ the transmission of signals. The results are published{ in No. 7 
zi of the Astr. Journ., with a detailed account of the methods which 
he used, and an analytical investigation of the effects of those 
circumstances which could interfere with the accuracy of his re- 
ul e measurements of all the registers gave him for the 
velocity on that night 16,000 miles a second,—differing less than 
1900 miles from his previous result and tending in general to con- 
firm it. The final result at which he arrived was the general 
16,000 to 19,000 miles a second. Later experiments with the 
chemical telegraph are described** in No. 14 of the Asér. Journ., 
and gives a still‘less velocity. . . 
Prof. Mitchel, of the Cincinnati Observatory, dissents from the 
view taken by Mr. Walker, and attributes the results obtained by 
im to the effect of various sources of error and uncertainty in 
the methods which Walker has used. He devised a special and 
Very curious apparatus for investigating the question,—and with 
the ingenuity and mechanical skill for which he is so eminent, 
SE TSR apa ne 
— Faraday. Researches, i, p. 423 + Pogg. Ann, xlv, p. 109. } Id, xlv, p. 20. 
see oe a 4y p> * ge. nn., 7 p- 
_ § Pouillet-Miiller, Lebrb. fe Physik, ii, p. 186. ‘. : 
Proc. Am. Phil. Soc., v, p. 74.  Astr. Journ, i, p. 49. 
Astr. Journ, i, p. 105. 
