45 
5. If we imagine a vessel, full of such a compound as the 
fog is made of, to be set in motion or stopped, the acceler- 
ating or retarding force will have to be transmitted from 
the sides of the vessel to the fluid within it by means of 
pressure. These pressures will act equally throughout the 
fluid, and if the fluid were homogeneous they would produce 
the same effect throughout it, and it would all move together 
but the pressures will obviously produce less effect on the 
drops of water than they do on the corresponding volumes 
of air, and the result will be that the drops of water will 
move with a different velocity to the air — that the drops of 
water will in fact move through the air just as they do under 
the action of gravity. In fact, if the air is subject to an 
acceleration of 32 feet per second the effect on the drops 
(their motion through the air) will be the same as that 
due to their weight. It is easy to conceive the action 
between the air and the drops of water. If a mass of 
air and water is retarded it is obvious that the water, by 
virtue of its greater density, will move on through the air. 
This property has, in fact, been made use of to dry the steam 
used in steam engines. The steam is made to take a sharp 
turn, when the water, moving straight on through it, is 
deposited on the side of the vessel. 
6. Owing to this motion of the water through the air it 
would clearly take longer with the same force to impress 
the same momentum on foggy air than on the same when 
dry. This is obvious, for at the end of a certain time the 
particles of water would not be moving as fast as the air;, 
and consequently the air and water would have less momen- 
tum than the same weight of dry air all moving together : 
that is to say, if we had two light vessels containing the 
same weight of fluid, the one full of dry air and the other 
full of fog, and both subjected to the same force for the same 
