Fog, a body of aqueous vapor in the atmosphere, like the clouds seen in the sky above, but distinguished from them both by its position and by the manner of its formation. A large class of clouds result from the diminution of temperature produced by the elevation and expansion of moist air, and a small class is due rather to the cooling of the air by radiation in a horizontal direction to bodies of cooler air; while again a very important class arise from the radiation of heat vertically into the cold interstellar regions. To these simple causes also must be attributed the formation of a large class of fogs. On the other hand, the peculiar feature in the origin of a number, and especially of the heaviest fogs that occur, consists in this, that the moist air radiates its heat downward to a comparatively cold body of either water, earth, or air. The former case occurs when the earth, after a period of low temperature, becomes quite cold, and the winds then waft warm moist air over the cold regions, while the small conducting power of the earth, ice, or snow does not allow its surface at once to follow the change in temperature.

Fogs of the second class occur only during very clear nights; the radiation from the earth then takes place with great freedom, and the moist air by this means coming into contact with the cold earth becomes greatly reduced in temperature, and after depositing a heavy dew lies still in the valleys over the whole surface of the ground. To this body of cold air the superincumbent atmosphere radiates heat as freely as to the outer regions of the air, and even more rapidly because of its nearness. When by this process the temperature is reduced to the dew point, the aqueous vapor begins to condense as fog, the particles of which attach themselves to neighboring solid bodies, such as leaves and branches of trees, but in a manner slightly different from the formation of dew. The third class of fogs, that produced by the radiation of atmospheric heat to a body of cold water, may occur in two ways: either warm air may be wafted over bodies of cold water, or currents 6f cold water may under-run bodies of warm moist air. The fogs on the coasts of New' England, Nova Scotia, and Newfoundland, and those of the Gulf stream, belong in great measure to the former class, as the motion of the cold ocean currents must be considered quite slight in comparison with the rapidly moving winds.

To the latter class belong fogs over rivers whose cold waters are flowing rapidly toward warmer regions. These are important features of the lower portions of the Mississippi, and have been well studied by Dr. W. M. Carpenter. (See "American Journal of Science" for 1843.)-Concerning the exact condition of the vapor when in the state that constitutes a fog, much has been written since the first announcement by Kratz-enstein of his theory of the existence of fog vesicles as distinct from rain or dew drops. Notwithstanding the labors of Kamtz (1836), Meissner (1867), Muhry (1873), and others, it must be admitted that this theory is still supported by too few minutely accurate observations to allow its unqualified adoption; and it is safer to presume that the particles of a fog are maintained floating in the air simply by the resistance offered by the atmosphere to the fall of minute spherical bodies.-The dry fog that constitutes a characteristic feature of the North American Indian summer is not essentially different from the moor smoke {Moor-rauch) of Europe, and has been satisfactorily traced to the burning of extensive tracts of forest and prairie land. From such fires the diluted smoke spreads with the winds over immense areas.

The progress of these masses of smoke in the United States and Canada has of late years been very closely followed by the officers of the weather bureau of the army signal office, who have frequently been able to predict this phenomenon. The great fires of 1871 in the northwest, and indeed throughout the whole country, gave rise to remarkable exhibitions of this haze. An extraordinary dry fog is recorded to have covered the whole of Europe in 1783 for nearly two months.