44 
Irom 3 to 5 times as much back pressure to expel misty 
steam from a cylinder as when the steam was dry. 
3. My object in this paper is to give and to investigate 
what appears to me to be an explanation of these pheno- 
mena; from which it appears that they are intimately 
connected; that, in fact, they are both due to the same 
cause. This explanation will be the clearer for a few pre- 
liminary remarks. 
4. The nature of a fog and the manner in which the small 
spherical drops are suspended against their weight is well 
understood. So long as the fog is at rest or moving uni- 
formly, the drops being heavier than the air tend to sink 
like a stone in water, and consequently they are not at 
rest in the air but are moving through it with greater or 
less velocities according as they are large like rain or small 
like haze. This motion is caused entirely by the difference 
in the specific gravity of the air and water, if the drops were 
merely little hard portions of air they would have no ten- 
dency to descend. 
In some fogs the drops are so fine that they appear to 
be absolutely at rest, and will remain for a long time with- 
out any appreciable motion. The force which retards the 
downward motion of the drops is the friction of the air, and 
this is proportional to the surface of the drop and the square 
of the velocity. As the drops get smaller their weight 
diminishes faster than their surface, and consequently the 
friction will balance the weight with a less velocity. The 
exact law is that the velocity caused by the weight of a 
drop is proportional to the square root of its diameter. 
This is the general explanation of what goes on under the 
action of gravity when the fog is at rest or moving uni- 
formly, and we may make use of it to illustrate what goes on 
when the fog is subjected to accelerating or retarding forces. 
