1918] Gravitation and the Principle of Relativity 215 



WEEKLY EVENING MEETING, 



Friday, February 1, 1918. 



J. H. Balfour Browne, K.C. LL.D., Vice-President, 

 in the Chair. 



Professor A. S, Eddington, F.R.S. 



Gravitation and the Principle of Relativity. 



There were many difficulties to encounter in entering the room just 

 now. To begin with, we had to bear the crushing load of the 

 atmosphere, amounting to 14 lbs. on every square inch. At each 

 step forwards, it was neaessary to tread gingerly on a piece of ground 

 moving at the rate of 20 miles a second on its way round the sun. 

 We were poised precariously on a globe, apparently hanging by our 

 feet, head outwards into space. And this acrobatic feat was performed 

 in the face of a tremendous wind of ether, blowing at I do not 

 know how many miles a second literally through us. We do not 

 claim much credit for overcoming these difficulties — because we 

 never noticed them. But I venture to remind you of them, because 

 I am about to speak of some other extraordinary things that may be 

 happening to us of which we are quite unconscious. 



Not to go too far back in history, the present subject arises from 

 a famous experiment performed in the year 1887, known as the 

 Michelson-Moiiey experiment. The apparatus was elaborate, but the 

 principle of the experiment is not very difficult. If you are in a 

 river, which will be the quicker — to swim to a point 50 yards up stream 

 and back again, or to a point 50 yards across stream and back again ? 

 Mathematically the answer is, perhaps, not immediately obvious, 

 because the net effect of the current is a delay in both cases. But I 

 think that anyone who has swum in a river will have no hesitation 

 about the answer. The up-and-down journey takes longer. Now we 

 are in a river —of ether. There is a swift current of ether flowing 

 througn this room ; or, if we happen to be at rest in the ether at the 

 present moment, six months hence the earth's orbital motion will be 

 reversed, and then there must be a swift current. Michelson divided 

 a beam of light into two parts ; he sent one half swimming up the 

 stream of ether for a certain distance, and then by a mirror back to 

 the starting-point ; he senl the other half an equal distance (as he 

 thought) across the stream and back. It was a race ; and with his 

 apparatus he could test very accurately which part got bacii first. To 

 his surprise, it was a dead-heat. Clearly the two paths could not 

 really have been eqaal, the along-stream path must have been a little 



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