Dr. Moll's account of experiments , &c. 425 
1738, the most accurate on this subject of that time, the 
value of k was found equal to 0,4254. It is plain that this 
correction of the original formula is merely empirical, and 
dependant on the accuracy of experiments, which in 1738, 
had certainly not attained the perfection which is required at 
present. 
In consequence, this formula was thus altered by Laplace, 
v/fV?; 
in which c * is the specific heat of the air under a constant 
pressure, and c is the specific heat of the air under a constant 
volume.* 
My friend Dr. Van Rees, Professor in the University of 
Liege, gave a demonstration of this correction which 
will be subjoined to the present paper, -f and which may be 
compared with that of Mr. Poisson. J The value of — was 
determined by Laplace from experiments of Messrs. La- 
roche and Berard,§ and found equal to 1,4954; but later 
and more accurate experiments of Messrs. Gay Lussac and 
Welter brought it to 1,3748. 
Another cause of the difference between actual experi- 
ments on the velocity of sound and its theory, exists in the 
variable force of the wind, which either accelerates or re- 
tards the velocity of sound, according to the direction from 
which it is blowing. It appears that this cause of error may 
be annihilated in the following manner. Let sounds be ex- 
* Laplace in Ann. de Phys. et Chim. Tom, iii. p. 238. 
f Dissertatio de celeritate soni, Trajet. 1818. 
X Annales de Phys. et de Chim. Mai 1823, d. 5. 
§ Ibid. Annales de Chimie, Tom. 85, p. 72. 
MDCCCXXI V. 3 I 
