258 Proceedings of Societies . [March 
wave is seen of length equal to the length of that interval. To 
determine the number of vibrations of the fork in a given time, 
it is only necessary, therefore, to be able to count the number of 
graduations which pass in that period. An electric counter 
gives the number of complete revolutions accomplished by the 
drum in any given period ; and a fine-pointed tube, containing 
magenta, is carried by a saddle above the drum, and, being 
actuated by an electro-magnet, makes a dot on a piece of white 
paper wrapped round the drum at the beginning and end of the 
experiment. The distance apart of these dots gives the addi- 
tional fraction of a revolution accomplished by the drum during 
the period of the experiment. 
In illustration of the method a determination of one of 
Koenig’s forks was given : — 
Duration of experiment, 5 minutes. 
No. of 1st mark, 163. Number of line on circle, 301*0. 
„ 2nd „ 164. „ „ „ 199-5. 
Temperature by thermometer, i6*8°. 
Number of revolutions shown by counter, 158. 
Number of lines on circle, 486. 
During the experiment the wave had risen in the field of the mi- 
croscope one-half a wave-length above the hair. 
158X486=76,788 lines passed in 158 revolutions. 
301 — 199-5 = 101*5 lines between two marks on drum. 
101*5 — 0-5 = 101 lines corrected for movement of wave. 
76,788+101=76,889 vibrations of fork in 5 minutes. 
-^^§9=256-2966 vibrations per second. 
When this determination was made four other measurement s 
were carried out, lasting for different intervals of time. 
5 min 
256*297 at 16-8° C, 
2 „ 
256-298 ,, 16-9 
3 s y 
256*293 3, 17* 0 
I ,3 
256*292 „ 17*0 
4 3, 
256*292 „ 17*2 
Means 
256*294 16*98 
Correction for error 
of therm. — 1*4 
I5*58 
Clock was losing about o*8 second per day. Correction for 
clock rate — 
r . 86 , 399-2 c 
256*294 X-.- ’X^ =256*292. 
86,400 
The maximum difference between the above numbers is 
0*006, or 0*00234 per cent. 
