Velocity of Transmission of Disturbances through Sea-ivater. 511 



Comparison of the Fork with a Clock. 



The fork employed was a very excellent and massive one, belong- 

 ing to a ribbon chronograph apparatus and made by Koenig. The 

 fork was marked 100 YD. We felt confidence in this fork from the 

 first, as it was supplied with a style and platinum cord contact 

 apparatus for electric maintenance. A comparison was made with 

 the laboratory clock (a very fine one, by Cooke, of York) by one of us 

 and Mr. Arthur Pollock. Lord Rayleigh's method was employed. 

 The fork compared directly with the clock was also by Koenig, and 

 was reduced from about 35 complete vibrations per second to 33'194 

 approximately, by very carefully equalised weights. The approxi- 

 mate period was found by counting whole and estimating partial 

 vibrations, over a period of 27 seconds; the chronograph above 

 alluded to was used for this purpose — one scriber marked periods of 

 2 seconds direct from the clock, and the other was included in the 

 driving circuit of the fork. The length of ribbon used was 56 feet. 

 We insist on the accuracy of this approximate measurement, because 

 the exact comparison requires it. The clock, with its contact 

 arrangement, had been carefully rated for some time and adjusted so 

 that for this purpose the rate is negligible — it was about a second 

 a day. The observation of the light spot was undertaken by Mr. 

 Pollock; it was found most convenient to interpose the adjustable 

 slit of a spectroscope between the object-glass and the eye-piece of 

 the observing telescope. The pendulum of the clock made a complete 

 vibration in 2 seconds, and the slits of the fork opened twice in each 

 complete vibration. The period of recurrence of the flash system 

 was 432 seconds, and the fork was losing on the clock. 



The result is that the actual frequency of the fork is 33*1933 com- 

 plete vibrations per second. The second harmonic of this was the 

 frequency of the driven fork with which the standard was compared. 



Hence frequency of driven fork is — 



99-5799. 



The beats were counted over a period of five minutes, the driven fork 

 being at 20° C. by a Kew standard (and re-examined) thermometer. 

 This temperature was then changed to 30° C, and the beats again 

 counted. Two sets of experiments were made, which agreed very 

 well together. During the observations at 30° C. the driving-fork 

 period of flash recurrence changed to about 270 seconds. The final 

 result, the calculation of which presents no new features, is that 



» = 100-2622-0-01472 t, 



n being frequency at f C, on the assumption that the temperature 

 coefficient is constant. In practice, the lowest temperature on any 

 vol. xlvi. 2 m 



