216 Professor A. S. Eddington [Feb. 1, 



shorter to compensate for the greater hindrance of the current. That 

 objection was foreseen, and the apparatus, which was mounted on a 

 stone pier floating in mercury, was rotated through a right angle, so 

 that the arm whicli was formerly along the stream was now across the 

 stream, and vice versa. Again, the two portions of the beam arrived 

 at the same moment ; so this time the other arm bad become the 

 shorter— simpl J by altering its position. In fact these supposedly 

 rigid arms had contracted when placed in the up-and-down stream 

 position by just the amount necessary to conceal the effect which was 

 looked for. 



That is the plain meaning of the experiment ; but we might well 

 hesitate to accept this straightforward interpretation, and try to evade 

 it in some way, were it not for s<>me theoretical discoveries made 

 later. It has gradually appeared that matter is of an electrical nature, 

 and the forces of cohesion between the particles, which give a solid 

 its rigidity, are electrical forces. Larmor and Lorentz discovered 

 that this property of contraction in the direction of the ether current 

 was something actually inherent in the formula for electrical forces 

 written down by Maxwell many years earlier and universally adopted ; 

 it only waited for some mathematician to recognize it. It would be 

 going too far to say that Maxwell's equations actually prove that 

 contraction must take place ; but they are, as it were, designed to 

 fall in line with the contraction phenomenon, and certain details left 

 vague by Maxwell have since been found to correspond. 



We are then faced with the result that a material body experi- 

 ences a contraction in the direction of its motion through the ether. 

 According both to theory and experiment the contractioji is the same 

 for all kinds of matter— a universal property. One reservation should 

 be made ; the experiment has only been tried with solids of labora- 

 tory dimensions, which are held together by cohesion. There is at 

 present no experimental evidence that a body such as the earth whose 

 form is determined by grcivitation will suffer the same contraction ; 

 we shall however assume that the contraction takes place in this case 

 also. 



I am going to ask you to suppose that we in this room are 

 travelling through the ether at the rate of 161,000 miles a second, 

 vertically upwards. Let us be bolder and say that that is our velocity 

 through the etht^r — because no one will be able to contradict us. No 

 exjjeriment yet tried can detect or disprove that motion ; Ijecause all 

 such experiments give a null result, as the Michelson-^Iorley experi- 

 ment did. With that speed the contraction is just one-half. This 

 pointer, which I hold horizontally, is 8 feet long. Now [turning it 

 vertically] it is 4 feet long. But, you may say, it is taller than I am, 

 and I must l-e approaching 6 feet. No, if I lay down on the floor I 

 sh-^uld be, but as I am standing now I am under 8 feet. The con- 

 traction affects me just as it did the pointer. It is no use bringing 

 a standard yard-measure to measure me, because that also will con- 



