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the same quantity of water in the air, we find in the first place 
that if the air is subjected to a uniform acceleration, which 
acts for a sufficient time for the drops to acquire their maxi- 
mum velocity through the air, the effect of the drops in a 
given time — that is to say, the energy dissipated in a given 
time — is proportional to the square root of the diameters of 
the drops. This appears from the action of gravity. As 
previously stated, the maximum downward motion of the 
drops ; and hence the distance they will have fallen in a 
given time and the energy destroyed is proportional to the 
square root of their diameters. Hence where the accelera- 
tion acts continuously for some time, as would be the case 
in a steam pipe, the effect will increase with the size of the 
drops. 
This effect may be represented by a parabolic curve in 
which distances measured from the vertex along the axis 
represent the size of the drops and the corresponding ordi- 
nates represent their effect in destroying energy. 
If on the other hand the acceleration alternates very rapidly 
then there will not be time for the drop to acquire its maxi- 
mum velocity, and if the time be very short the drop will 
practically stand still, in which case the effect of the drops 
will be proportional to the aggregate surface which they 
expose. And this will increase as the diameter diminishes, 
always supposing the same quantity of water to be present. 
This latter is somewhat the condition when a fog is tra- 
versed by waves of sound, so long as the drops are above a 
certain size ; when, however, they are very small, compared 
with the length of the waves, there will be time for them to 
acquire their maximum velocity. So that starting from 
drops the size of rain, their effect will increase as their size 
diminishes, at first in the direct proportion, then more and 
more slowly until a certain minuteness is reached, after 
