112 Lord Rayleigh and Dr. A. Schuster. [May 5, 



but at night, when the actual observations were made, the image of an 

 Argand gas flame was thrown upon the pointer and the part of the 

 card near it. On account of the necessity of removing the electric 

 fork and its appliances to a distance, the card, if looked at directly, 

 would appear too much f ore-shortened, and a looking-glass was there- 

 fore introduced. The eyepiece of the telescope, close in front of the 

 slits, was adjusted to the exact height, and the eye was placed imme- 

 diately behind the slits. By cutting off stray light as completely as 

 possible, the observation may be made without fatigue and with slits 

 narrow enough to give good definition when the speed is correct. 



As governor I had originally intended to employ an electro-mag- 

 netic contrivance, invented a few years ago by La Cour and myself,* 

 in which a revolving wheel is made to take its time from a vibrating 

 fork, and it was partly for this reason that the water engine was 

 placed at a considerable distance from the revolving coil. I was, 

 however, not without hopes that a governor would be found unneces- 

 sary, and a few trials with the stroboscopic apparatus were very en- 

 couraging. It appeared that by having the water power a little in 

 excess, the observer looking through the vibrating slits could easily 

 control the speed by applying a slight friction to the cord connecting the 

 engine and coil. For this purpose the cord was allowed to run lightly 

 through the fingers, and after a little practice there was no difficulty 

 in so regulating the speed that a tooth was never allowed finally to 

 pass the pointer, however long the observation was continued. If, 

 from a momentary inadvertence or from some slight disturbance, a 

 tooth passed it could readily be brought back again. The power of 

 control thus obtained will be appreciated when it is remembered that 

 the passage of a tooth per second would correspond to less than one 

 per cent, on the speed. In many of the observations the pointer 

 covered the same tooth all therwhile, so that the introduction of a 

 governor could only have done harm. 



Another, and perhaps still more important, improvement on the 

 original method related to the manner of making correction for the 

 changes of declination which usually occur during the progress of 

 the experiments. The Committee relied for this purpose upon com- 

 parisons with the photographic records made at Kew, and they recog- 

 nise that considerable disturbances arose from the passage of steamers, 

 &c. All difficulty of this kind is removed by the plan which we 

 adopted of taking simultaneous readings of a second magnetometer, 

 called the auxiliary magnetometer, placed at a sufficient distance from 

 the revolving coil to be sensibly unaffected by it, but near enough to 

 be similarly influenced by changes in the earth's magnetism, and by 

 other disturbances having their origin at a moderate distance. The 



* " Nature," May 23, 1878. 



