47 
admissible ; for in the case of light the wave length is 
shorter than the thickness of the drops, and the surface of 
the drop acts in the same way as if the drop were of large 
extent; but in the case of sound the waves length may be 
thousands of times the thickness of the drop, and instead of 
the whole wave being reflected it will only be a very small 
portion of it. Even this portion can hardly be called a 
reflection ; it is due to the motion of the air past the drops 
like the waves of sound caused by a bullet, or the waves 
thrown off by the bow of a ship. 
10. A certain portion of the resistance which the air offers 
to the motion of the water through it is this — what is called 
in naval science wave resistance; but it can be shown 
that the proportion of this resistance to the resistance in 
causing eddies diminishes with the velocity, and conse- 
quently it can have very little to do with the effect of the 
drops of water on the waves of sound, in which the velocity 
of the water through the air must be very small.^' 
11. So far, then, I have shown the manner in which the 
fog diminishes the sound ; it remains to consider the con- 
nection between the size of the drops and their effects. 
I am not aware that any observations have been made 
with respect to this. I do not know whether it has ever 
been noticed whether a fine or a coarse mist produces the 
most effect on sound. It does not appear, however, that 
rain produces the same effect as fog; and considering rain 
as a coarse fog, we must come to the conclusion that a cer- 
tain degree of fineness is necessary. 
If we examine theoretically into the relation between the 
size of the drops and the effect they produce, always assuming 
* This reflection has nothing to do with the reverberation from clouds which 
occurs in a thunderstorm, which is probably due to the different density of the 
clouds, and takes place at their surfaces. 
