680 I^rof. 0. Lodge on the Motion of the Ether, dec. [April 1, 



small changes had to be made, and gradually the spurious shifts have 

 been reduced and reduced, largely by the skill and patience of my 

 assistant, Mr. Davies, until now there is barely a trace of them. 



But the experiment is not an easy one. Not only does the blast 

 exert pressure, but at high speeds the churning of the air makes it 

 quite hot. Moreover, the tremor of the whirling machine, in which 

 some four or five horse-power is sometimes being expended, is but too 

 liable to communicate itself to the optical part of the apparatus. Of 

 course elaborate precautions are taken against this. Although the two 

 parts, the mechanical and the optical, are so close together, their 

 supports are entirely independent. But they have to rest on the same 

 earth, and hence communicated tremors are not absent. They are the 

 cause of all the slight residual trouble. 



The method of observation now consists in setting a wire of the 

 micrometer accurately in the centre of the middle band, while another 

 wire is usually set on the first band to the left. Then the micrometer 

 heads are read, and the setting repeated once or twice to see how 

 closely and dependably they can be set in the same position. Then 

 we begin to spin the disks, and when they are going at some high 

 speed, measured by a siren note and in other ways, the micrometer 

 wires are reset and read — reset several times and read each time. 

 Then the disks are stopped and more readings are taken. Then their 

 motion is reversed, the wires set and read again ; and finally the motion 

 is once more stopped and another set of readings taken. By this 

 means the absolute shift of middle band and its relative interpretation 

 in terms of wave-length are simultaneously obtained ; for the distance 

 from the one wire to the other, which is often two revolutions of a 

 micrometer head, represents a whole wave-length shift. 



In the best experiments I do still often see something like a fiftieth 

 of a band shift, but it is caused by residual spurious causes, for it 

 repeats itself with sufficient accuracy in the same direction when the 

 disks are spun the other way round. 



Of real reversible shift, due to motion of the ether, I see nothing. 

 I do not believe the ether moves. It does not move at a five-hundredth 

 part of the speed of the steel disks. I hope to go further, but my 

 conclusion so far is that such things as circular-saws, flywheels, 

 railway trains, and all ordinary niasses of matter do not appreciably 

 carry the ether with them. Their motion does not seem to disturb it 

 in the least. 



The presumption is that the same is true for the earth ; but the 

 earth is a big body, it is conceivable that so great a mass may be able 

 to act when a small mass would fail. I would not like to be too sure 

 about the earth. What I do feel already pretty sure of is that if 

 moving matter disturbs ether in its neighbourhood at all, it does so by 

 some minute action, comparable in amount perhaps to gravitation, and 

 possibly by means of the same property as that to which gravitation 

 is due — not by anything that can fairly be likeued to ethereal 

 viscosity. [0. L.] 



