R. Kenig—Exactitude of the French Normal Fork. 147 
ol XXI.—WNote on the Exuctitude Ais the French Normal 
Fork ; a Reply to the paper of Mr. A. J. Ellis ; by Rupoura 
Recs. Ph.D.—[In a letter to the Editors s. | 
been hitherto su posed, but 878 ete vibra Mr. 
having established the further fact that the forks "eiiioketiated by 
me are in perfect accord with the French La,, does not hesitate 
has never examin ed, are peep yea inexact. Not having at my 
disposal the instrument used b y Mr. Ellis, I confess that a 
Myself under some embarrassment in sta ting at one 
error of construction this instrument, in the hands of Mr r. Ellis, fins 
' given results so extraordin ary. F ortunately, zi can refer to a letter 
fro elmholtz addressed to Mr. Appunn and published by 
the latter himself in a paper on i die acoustic thenties of M. Helm- 
oltz. is letter speaks of an instrument of exactly the same 
astonished at the constancy of its sadientious 
would aie date believed that reeds could give sounds so constant 
a8 those given by your apparatus, thanks to your method of reg- . 
ulating the current of air. The instrument, it is true, varies a 
“little with the temperature, as do also forks; and hence it can be 
‘used for dete ermining the absolute number of vibrations, only when 
one can work in a room heated by a stove. By the aid of an 
4stronomical chronometer, I have counted the beats, re believe 
that your seconds pendulum must have been slightly inexact, 
because, though the number of beats agree very 
fpemselven, the absolute number obtained is not 240 but 237 to 
the minute. The ove Greg which was rather low during my 
€xperiments, may count for something; but even a influence 
may be eliminated is counting the beats to the en a major 
third, which took me a quarter of an hour. In this way T have 
fo ound f for my Paris fork 43501 vibrations, ise agrees to the 
512. On comparing this note of 505-6 single vibrations with a 
