If fog be regarded as any condition of the atmosphere near the ground when objects at a distance are hidden by small particles in the air, it is convenient to divide fog into two classes-those in which particles are so small that they would take a long time to fall to the ground, and those in which the particles are large enough to fall fairly quickly, but in which the supply of particles is continually replenished, so that the air does not clear itself. In this second class would be included sandstorms, in which the supply of particles is replenished from below, and fine rain, in which they come from above. Pilots, however, are not much concermd with fogs of this class. In the first place, sandstorms do not occur in England; and in the second Place, on the occasions when a fine rain is indistinguishable from fog or mist, the clouds are very low, and a pilot would be unlikely to be flying in any case. I have therefore limited myself to fogs of the first class.