= 
Kk Onig. } 80 {July 20, 
Fic. 2.—BRayYTOn’s PETROLEUM MorTor. 
This remarkable and admirable engine acts like an instrument of pre- 
cision. _ It can be started with a match and comes to its regular speed in 
less than a minute; it preserves its rate entirely unchanged for hours to- 
gether. Moreover, it is economical, cleanly, and not more noisy than a 
steam engine. The one of two horse power I have, ran for six months, 
day and night, supplying water and air to the aquaria in the Centennial 
Exhibition at Philadelphia. At any time on going into the laboratory it 
can be started ina few seconds even though it has not been running for 
days. 
Henry Draper's Observatory, 
Hastings-on-Hudson, New York. 
Note on the Exactitude of the French Normal Fork ; a Reply to the paper 
of Mr. A. J. Ellis ; by Rupour# Konte, Ph. D. 
(Communicated by Professor Barker to the American Philosophical Society, 
July 20, 1877. 
An attack, as strange as it was unexpected, has just been made in Eng- 
land upon the exactitude of the official French fork. Mr. Alexander J. 
Ellis, having found that the notes of a tonometer, composed of sixty-five 
harmonium reeds, constructed by Mr. Appunn, do not agree with this 
fork, has considered himself entitled to assert, in a paper published in the 
Journal of the Society of Arts (May 25, 1877), and in Nature (May 31, 
1877) that the normal French fork La, gives, not 870 single vibrations, 
as has been hitherto supposed, but 878 single vibrations. Mr. Ellis, having 
established the further fact that the forks constructed by me are in perfect 
