3 o SCIENCE PROGRESS 



moving together in their own plane an inch apart is not 

 increased or diminished by so much as ^wth part of their 

 velocity. It is true that it is not justifiable, therefore, to assume 

 that there is no viscous drag between the ether and the earth, 

 for the mass of the earth and its velocity are extremely large in 

 comparison with the corresponding quantities in the above 

 experiment ; but, combined with the previous consideration, 

 the negative result may be regarded as a weighty argument in 

 favour of a non-viscous ether. Accepting this position, it is 

 necessary to seek for another explanation of the results of the 

 attempts to measure ether-drift. 



A suggestion due to Prof. Fitzgerald was that the forces 

 between the molecules composing any body depend on the 

 angle they make with the direction of motion through the ether, 

 and therefore that the dimensions of the body vary with its 

 position relative to the motion. If it be supposed that the 

 length of any piece of material is shorter when parallel to the 

 velocity through the ether than when perpendicular to it, and 



that by the fraction | (y) of the length, an alternative explana- 

 tion of the negative results is obtained. In the case of the 

 Michelson-Morley experiment such a change in dimensions due 

 to rotation would alter the lengths of paths of the two beams in 

 just such a way as to compensate the time difference due to 

 the ether-drift. In the Trouton-Noble experiment, also, the 

 effect would be to alter the electrostatic capacity of the con- 

 denser, so that the total quantities of energy associated with it 

 would be equal in the two cases, thus eliminating the tendency 

 to turn. In the hands of Professors Larmor and Lorentz l sub- 

 stantial theoretical reasons for such a change of dimensions 

 have been adduced, and the theory has been extended to include 

 the relation of physical quantities other than length to the 

 direction of motion through the ether. 



Into this theory it is not here proposed to enter, this paper 

 being intended mainly as a discussion of actual experimental 

 results. The question arises, "Is it not possible, if such a 

 change of length really occurs, to devise investigations capable 

 of detecting it ? " It is true that the promoters of the complete 

 theory claim that motion through the ether will so affect all the 



1 H. A. Lorentz, Versuch einer Theorie der electrischen und optischen Ers- 

 heinungen in beivegien Korpem, Leiden, E. J. Brill, 1895. 



