400 SCIENTIFIC WRITINGS. 



In the year 1875 an opportunity occurred of 

 bringing into use my modified method, proposed in 

 1845, of measuring the velocity of propagation of 

 electricity in suspended wires. The experiments, which 

 were instituted with a double iron wire, 7 '8 7 miles 

 long, yielded a velocity of propagation of 150,300 

 miles, a result which satisfactorily agrees with Kirch- 

 hoff 's calculated result, regard being had to retardation 

 by the condenser action of the wires and to the self- 

 induction. Before the performance of these experiments, 

 very carefully carried out by Dr. Frolich, I inclined 

 to the opinion that the actual velocity of electricity 

 in conductors would be immeasurably large, as an 

 experiment, which I made with a caoutchouc tube 

 more than a hundred feet long filled with water, did 

 not show any perceptible difference in position of the 

 spark marks. The velocity of propagation of electricity 

 could accordingly not depend mainly on the specific 

 resistance of the traversed conductor, and I regarded 

 it therefore as probable that the very different values 

 found by Wheatstone, Fizeau, Gounelle, and others, 

 had only been expressions for the retardation by the 

 charge of the conductors employed. This doubt was 

 removed by the experiments described, for the further 

 prosecution of which I have unfortunately never found 

 time and opportunity. 



I was led into a sphere of inquiry entirely new 

 for me by an observation of the activity of Vesuvius 

 in May 1878. It struck me that from the brightly 

 glowing opening, at the apex of the lava cone, which 



