Velocity of Sound. 135 
velocity of sound; when the direction of the wind opposes that of 
the sound, the difference of the separate velocities mtist be taken. 
6th, That in the case of echoes, the velocity of the reflected 
sound, is the same as that of the direct sound. 
7th, That, therefore, distances may frequently be measured by 
means of echoes. 
8th, That an augmentation of temperature occasions an aug- 
mentation of the velocity of sound ; and vice versa. 
(See Newton, Principia, Lib. 2. Prop. 50. Parkinson’s Me- 
chanics, Vol. IT. p. 148.) 
The enquiries with regard to the transmission of sound in the 
atmosphere,* which notwithstanding the curious investigations of 
Newton, Laplace, Poisson, and others, require the farther aid of 
experiment for satisfactory determination, are, I thik, the follow- 
ing: viz. 
Ist, Whether hygrometric changes in the atmosphere have much 
or little influence on the velocity of sound? 
2d, Whether barometric changes in the atmosphere have much 
or little mfluence? 
3d, Whether, as Muschenbroek conjectured, sound have not 
different degrees of velocity, at the same temperature, in different 
regions of the earth? And whether high barometric pressures would 
not be found (even independently of temperature) to produce greater 
velocities ? ; 
4th, Whether, therefore, sound would not pass more slowly be- 
tween the summits of two mountains, than between their bases? 
5th, Whether sound, independently of the changes in the air’s 
elasticity, move quicker or slower near the earth’s surface, than at 
some distance from it ? 
* Tsay nothing in this Paper, of the transmission of sound through the gases, along metallic 
conductors, &c. These furnish a most interesting department of separate enquiry. 
