660 HENKY A. KOWLAND 



its grasp at the distance of many millions of miles and prevents it from 

 departing forever from its life-giving rays. Science is full of this half 

 knowledge, and the proper attitude of the mind is one of resignation 

 toward that which it is impossible for us to know at present and of ear- 

 nest striving to help in the advance of our science, which shall finally 

 allow us to answer all these questions. 



The electric current is an unsolved mystery, but we have made a very 

 great advance in understanding it when we know that we must look out- 

 side of the wire at the disturbance in the medium before we can under- 

 stand it: a view which Faraday dimly held fifty years ago, which was 

 given in detail in the great work of Maxwell, published sixteen years 

 since, and has been the guide to most of the work done in electricity 

 for a very long time. A view which has wrought the greatest changes 

 in the ideas which we have conceived with respect to all electrical 

 phenomena. 



So far, we have considered the case of alternating electric current in 

 a wire connecting the inner and outer coatings of a Leyden jar. The 

 invention of the telephone, by which sound is carried from one point to 

 another by means of electrical waves, has forced into prominence the 

 subject of these waves. Furthermore, the use of alternating currents 

 for electric lighting brings into play the same phenomenon. Here, 

 again, the difference between a current of water and a current of elec- 

 tricity is very marked. A sound wave, traversing the water in the tube, 

 produces a to and fro current of water at any given point. So, in the 

 electrical vibration along a wire, the electricity moves to and fro along 

 it in a manner somewhat similar to the water, but with this difference: 

 the disturbance from the water motion is confined to the tube and the 

 oscillation of the water is greatest in the centre of the tube, while, in 

 the case of the electric current, the ether around the wire is disturbed 

 and the oscillation of the current is greatest at the surface of the wire 

 and least in its centre. The oscillations in the water take place in the 

 tube without reference to the matter outside the tube 1 , whereas the elec- 

 tric oscillations in the wire are entirely dependent on the surrounding 

 space, and the velocity of the propagation is nearly independent of the 

 nature of the wire, provided only that it is a good conductor. 



We have, then, in the case of electrical waves along a wire, a disturb- 

 ance outside the wire and a current within it, and the equations of 

 Maxwell allow us to calculate these with perfect accuracy and give all the 

 laws with respect to them. 



We thus find that the velocity of propagation of the waves along a 



