572 Professor Oliver Lodge [April 1, 



if the earth carries any ether with it, or if the ether, being in motion 

 at all, is not equally in motion everywhere throughout every trans- 

 parent substance, then such a lag or negative aberration must occur : 

 in precise proportion to the amount of the carriage of ether by 

 moving bodies. 



On the other hand, if the ether behaves as a perfectly frictionless 

 inviscid fluid, or if for any other reason there is no rub between it 

 and moving matter, so that the earth carries no ether with it at 

 all, then all rays will be straight, aberration will have its simple and 

 well-known value, and we shall be living in a virtual ether stream 

 of 19 miles a second, by reason of the orbital motion of the earth. 



It may be difficult to imagine that a great mass like the earth 

 can rush at this tremendous pace through a medium without dis- 

 turbing it. It is not possible for an ordinary sphere in an ordinary 

 fluid. At the surface of such a sphere there is a viscous drag, and a 

 spinning motion difluses out thence through the fluid so that the 

 energy of the moving body is gradually dissipated. The persistence 

 of terrestrial and planetary motions shows that ethereal viscosity, if 

 existent, is small ; or at least that the amount of energy thus got rid 

 of is a very small fraction of the whole. But there is nothing to 

 show that an appreciable layer of ether may not adhere to the earth 

 and travel with it, even though the force acting on it be but small. 



This, then, is the question before us : — 



Does the earth drag some ether with it ? or does it slip through the 

 ether with perfect freedom ? (Never mind the earth's atmosphere ; the 

 part it plays is not important.) 



In other words, is the ether wholly or partially stagnant near 

 the earth, or is it streaming past us with the opposite of the full 

 terrestrial velocity of nineteen miles a second ? Surely if we are 

 living in an ether stream of this rapidity we ought to be able to 

 detect some evidence of its existence.* 



It is not so easy a thing to detect as you would imagine. We 

 have seen that it produces no deviation or error in direction. 

 Keither does it cause any change of colour or Doppler effect ; that is, 

 no shift of lines in spectrum. No steady wind can affect pitch, 

 simply because it cannot blow waves to your ear more quickly than 

 they are emitted. It hurries them along, but it lengthens them in 

 the same proportion, and the result is that they arrive at the proper 

 frequency. The precise effects of motion on pitch are summarised in 

 the following table : — 



Clianges of Frequency due to Motion. 



Source approaching shortens waves. 

 Receiver approaching alters relative velocity. 



Medium flowing alters both wave-length and velocity in exactly compensatory 

 manner. 



* The word "stationary" is ambiguous. I propose to use "stagnant," as 

 meaning stationary with respect to the earth, i. o. as opposed to stationary in 

 space. 



