46 
time, at the end of this time, although they would have 
exactly the same motion, their contents would not, for the 
drops of water in the fog would not be moving so fast as the 
vessel. Now the energy expended on each of these vessels 
would be the same, but, inasmuch as the effects are different, 
the energy acquired by the foggy air would be less than that 
acquired by the diy air, the difference having gone to move 
the water through the air : that is to say, it would require 
more pressure to impress in the same time the same velocity 
on foggy air than on dry air of the same density. 
7. This then fully explains the dulness with which foggy 
air acquires motion. In the passages of a steam engine the 
steam is subjected to continual accelerations and retarda- 
tions, each of which requires more force in the manner 
described with misty than with dry steam, and at each of 
which the particles of water moving through the steam 
destroy energy in creating eddies. 
8. Although not so obvious, the same is true in the case 
of sound. The effect of waves of sound traversing a portion 
of air is first to accelerate and then to retard it. And if 
there are any drops of water in the air these will not take 
up the motion of the air so readily as the air itself. They 
will allow the air to move backwards and forwards past 
them, and so cause friction and diminish the effect of 
the wave as it proceeds, just as a loose cargo will diminish 
the rolling of a ship. 
9. It is important to notice that this action of the par- 
ticles of water is not analogous to their action in reflecting 
the waves of light. 
It has been assumed as an explanation of the action of 
fog on sound that the particles of water break up the wave 
of sound by small reflections in the same way as they 
scatter the waves of light. The analogy however is not 
