212 J. hovering — Measuring the Velocity of Ehctricity. 



When experiment undertakes to deal with such amazing rates 

 of transmission as those of light or electricity, one of two things 

 is indispensable ; it must possess the means, either of operating 

 over enormous distances of space, or of measuring excessively 

 small intervals of time. When the propagation of light is under 

 consideration, there is a free choice between the two methods. 

 If we choose the first, which may be called the direct 

 method, astronomy will supply ample spaces, and no extraordinary 

 nicety of measurement in the other element is demanded. But the 

 practicability of the second method, even when the spaces traversed 

 by the light do not exceed the limits of the physical laboratory, 



af/S: 



has been demonstrated by Fizeau, Foucault i 



If we turn now from the propagation of light to that of elec- 

 tricity, it is obvious that nothing less than the largest lines of 

 telegraph wire furnish the conditions required by the first method. 

 On the 28th of Februarv, and again on the 7th of March, 1869, 

 the late Professor Winlock, of the Harvard College Observatory, 

 sent electrical signals from Cambridge to San Francisco, and 

 thence by other lines to Canada, and back again to Cambridge, 

 over a loop of wire measuring 7200 miles. This long journey was 

 performed by electricity in about two-thirds of one second ; and 

 no small portion of this brief interval was lost in bringing into 

 action the thirteen repeaters which were interpolated * "" 



? of the signals comes out as j 

 result. When the signals are sent eastward, the apparent ditter- 

 ence of longitude exceeds the real difference of longitude by the 

 transmission time. When the signals are sent westward, the 

 apparent is less than the true longitude by the same quantity. 

 The average of the two values is the true difference of longitude, 

 and half the difference of the two values is the transmission time 

 of electricity. For example, in the campaign conducted by oflS- 

 cers of the United States Coast Survey, in 1869-70, for the deter- 

 mination of transatlantic longitudes, I obtained the following 

 results. The total transmission time between Brest, France, and 

 Duxbury, Mass., by the way of St. Pierre, was '816 of one second. 

 The total distance by cable is 332 y nautical miles; the distance 

 from Brest to St. Pierre being 2680 nautical miles, and that from 

 St. Pierre to Duxbury 749 nautical miles. When the differences 

 of length, caliber and materials as between the two branches ot 

 the cable are all taken into account, I find that the transmission 

 time between Brest and St. Pierre was -639 of a second, and 

 between St. Pierre and Duxbury -177 of a second, so that the two 

 branches were traversed, one at the rate of about 4000 nautical 

 miles a second, the other at the rate of 4230 nautical miles a second. 

 Wheatstone's remarkable experiments on the velocity of fric- 

 tion electricity, first published in 1834, offer an example of the 

 second method of measuring great velocities. In this case, the 

 experiment was made upon a length of only one quarter of a mile; 

 and the exceedingly small fraction of time required by electricity 



