Children born in the seventh month of gesta tion are eapable of living, although they usually require much care; and children may be horn alive at any period after the beginning of the sixth month, or even in some instances earlier than the sixth, but this is rare, and if horn liv ing, they eommonly die soon after birth. See :ESTA•ION EAIBRY0 ; EMBRYOLOGY, lir MAN.
FOG, or MIST Dan. fog, Icel. fok, spray, from //Oka, to be driven by the wind). .1ziy thing that obscures the clearness of the at 1110S phone, but specifieally in meteorology the obscura tion caused by the very minute globules of water floating in the air. Because Of their minuteness, the fog particles have lint sufficient weight to descend rapidly in the presence of the atmos phere; they are not hollow vesicles or spheres having an inherent buoyant gas. as was once supposed, but are upheld by the slightest cur rents of air; the slowness of their progress down ward is due mostly to the viscous resistance or internal friction of the air. These particles of water represent the eondensation of vapor that was invisible a short time before. This has been con densed into fog particles not by coming in con tact with or by passing over colder bodies. but either by radiation of heat or by expansion and dynamic cooling. The latter is more frequent in the case of high fogs, but the former in the case of the ordinary fogs at sea-level. In general, when a mass of warm moist air rolls over cold land or cold water it radiates its warmth down ward to the colder layers. On the other hand,
when cold air .flows over warm land or 'water the vapor rising from the latter is also quickly condensed, as it cools both by contact and by radiation. Thus fogs are formed over lakes, rivers, and marshes in the evening when the warm vapor from these penetrates the colder air above. Whenever cold dry air from the American continent flows southeastward over the Gulf Stream, fogs are formed. Or whenever cold and warm streams of water lie adjacent to each other, the air from the warmer, moister region is carried over to the cold water, and cools by radia tion until fog is formed. The densest fogs off the eastern coast of New England and Newfound land come with an east or southeast or south west wind. Those on the Pacific Coast of Cali fornia come with the west and northwest winds. The light morning fogs over grassy plains and swamps are due to cooling by radiation, and occur in still, clear weather within areas of high pressure.
The fog known as London fog, or dry fog, is due principally to the condensation of aqueous vapor upon the immense number of nuclei float ing in the atmosphere as smoke from the soft coal fires. In general, every particle of fog has a minute nucleus of dust or germs, or other solid particles, so that it is often injurious to health to inhale it. (See DusT.) In London the deaths during foggy weeks always exceed those during pleasant weather. See FOG-SIGNALS.