described in a tonner aiticle of a chain transmitting' waves. Atten- 

 tion was drawn to the displacement of a link and to its rotation. Now 

 for the analogy: To seem at all satisfactory the first thing that wonld 

 strike one would be to pay attention to two motions, to the velocity of 

 displacement of the link and to its rotation. This wonld lead to inter- 

 minable difticulties in carrying out the analogy. We can not liken 

 electric strain to a velocity in this direct and simple way, because what 

 are we to do with a change in the strain which produces the same ef- 

 fects as a continuous current? A change in the strain is all very well, 

 it would be like a change in the velocity, but what about a continuous 

 change in the velocity : We can hardly sui)pose a velocity continually 

 increasing forever; we are evidently landed in immediate difficulties. 

 It is better therefore to be content to liken the electric strain to a dis- 

 placement of the chain link. It seems most likely that it really is a 

 peculiar motion in the ;ether, but we nnist be content for the present 

 with the analogy. If we want to drive it further, we must suppose 

 stress in the chain that draws the link back to be due to a motion in 

 the chain or of things fastened to it, and then the changed motions 

 produced by a displacement of the chain might be analogous to the . 

 peculiar motions accompanying electric strain. It would lead us too 

 far to work out this analogy. 



Returning to the simpler case of the diidsacement of the link 

 representing electric strain, and the velocity of its rotation representing 

 magnetic strain, see how the actions near a Hertzian generator may 

 be likened to what takes place when a wave is being sent along a 

 chain. While the conductors are being slowly cliarged we nnist sup- 

 pose electric strain to be produced in all the surrounding space. This 

 is a comparatively slow action, and as the rate of propagation is 

 very rapid, the electric strain will rise practically simultaneously 

 in the whole neighborhood, and that it does so is a most important 

 fact to be taken account of in all our deductions from these experi- 

 ments. This slow charging must be rei)resented by a slow raising 

 of one end of the chain, which raises the rest of it to a great distance 

 apparently sinniltaneously if the raising be done slowly. Suddenly 

 the air-gap breaks. This might be represented by lifting the chain 

 with a weak thread, and by having the end of the chain fastened 

 to a pretty sti'ong spring. When the thread broke the sjn-ing would 

 pull the chain back (piickly, would pass its position of equilibrium, and 

 thus commence a series of rapid vibrations on each side of this posi- 

 tion; the vibrations would gradually die away owing to the energy of 

 the spring being gradually spent, i)artly on friction in itself, and partly 

 in sending waves along the chain. In actually performing the experi- 

 ment, an india-rubber tube or limp thin roi)e is better than a chain 

 when hung horizontally, as the chain is so heavy; when it can be hung 

 vertically, a chain does very well. In the description it simplifies 

 matters to describe a chain, because it is easier to talk of a link than of 



