282 Professors Ayrton and Perry on the Ratio of the 



than the velocity of light, which is about 300 million metres per 

 second, and their mean value far lower. 



In 1872 a redetermination of the value of v was made by 

 Mr. Dngald. McKichan, in Sir Wm. Thomson's laboratory *, 

 using the same method that Sir William had previously em- 

 ployed ; but as certain improvements had since 1867 been 

 introduced into the absolute electrometer, the results now ob- 

 tained were more accurate. The mean value on this occasion 

 was 293 million metres per second, still, however, being much 

 lower than the velocity of light. It is important, however, to 

 notice that some single values obtained were as high as 300 

 million metres, although these are again balanced by others as 

 low as 290 million metres per second. 



2. Method employed in this Investigation. 



Now the velocities ascertained for light are : — 



M. Fizeau 314T 



Aberration &c. and sun's parallax 308 (million metres 



M. Foucault 298 f per second. 



M. Cornu 300^ 



When, therefore, Professor Perry and myself, in 1877, took 

 up the question experimentally, it could not be said that the 

 ascertained value of v was equal to the velocity of light, 

 although Professor Clerk Maxwell's electromagnetic theory 

 required the identity for its corroboration. 



In fact the best experiments seemed to show that v was, for 

 some reason, less than the accepted velocity of light. The 

 question therefore arose, was Professor Clerk Maxwell's theory 

 incomplete, or was it that the accepted velocity of light was 

 too -high, or was it that the methods previously employed 

 for the determination of v might be improved on, and a more 

 correct value obtained ? This leads to the consideration of what 

 other methods than those employed by MM. Weber and Kohl- 

 rausch, Sir Wm. Thomson, and Professor Clerk Maxwell were 

 available. Now it was possible to determine its value by an 

 accurate comparison of the electrostatic capacity of a condenser 

 with the electromagnetic capacity of self-induction of a coilf ; 

 but it seemed to us very doubtful, from the nature of the ex- 

 periments, whether this method would give results more accu- 

 rate than those previously obtained. And the same remark 

 applied even with greater weight to the measure of a resist- 

 ance electrostatically and electromagnetically, since the same 

 difficulty would here have been met with that is encountered 

 when it is desired to measure the insulation of a cable very 



* Philosophical Transactions of the Koyal Society, 1873, p. 409. 

 t Clerk Maxwell's { Electricity and Magnetism/ p. 379. 



