FCETUS, fetus, an unborn child, or unborn young of an animal. In the human foetus the growth is most pronounced after the fourth month. The convolutions of the brain, dis tinguishable organs of sex, ossification and muscular movement advance in the fifth month, during which nails and hair appear. During the sixth month the pubic bones ossify, eyelids and eyelashes form, fat develops under the skin. In the seventh month, the fat increases, the eyelids are open. During the eighth month the nails are fully developed, and the normal foetus at tains a weight of from five to nine pounds. See also EMBRYO.
FOG, a very thick mist • small hollow vesi cles of water suspended in the air, but so low as to be but a short distance from the earth, in place of rising high above it and becoming so illuminated by the sun as to constitute clouds of varied hue. Fogs often arise when the air above warm, moist soil is colder than the soil itself. The hot vapors from the ground are then condensed by coming in contact with the colder air above, as the warm steam of a kettle is by the comparatively cold air of a room. But no fog arises till the cold air has absorbed vapor enough to bring it to the point of satura tion. Fogs often hang over rivers. Their cause is the condensation by contact with the cold water, of the vapor in a hot and moist air current passing over the river. The "pea-soup)) fogs of London life are produced by the carbon of the smoky atmosphere coloring the fog vesi cles; a fog which is brown in London's busi ness district is generally white a few miles off, and wanting altogether at the further extrem ities of the city. On hills and mountains of any size it is easy to rise above a fog, and see it like an ocean beneath one's feet. Ocean fogs are dangerous to vessels traveling at high speed, as in the case of the Titanic disaster in 1912. Where cold-water currents underlie warm air, a common condition off the Newfoundland banks, the formation of fog is often rapid and dense. Such fogs persist after the temperature has altered, and also the fog drifts with the wind; it is therefore a most uncertain quantity.
It has been noted that electric potential is very high during a fog. Consult McAdie, "Fog (in Inventor,) Vol. IX, p. 209, 1902).
signals given by means of sound or light to warn vessels of danger during fogs. Various kinds of fog-signals are used, among which may be mentioned bells, drums, gongs, guns, compressed-air whistles, steam-whistles and fog trumpets or horns, and latterly powerful electric flashlights. Gongs are not very powerful as signals, often failing to be heard at more than the distance of a quarter of a mile. Bells may be heard during fogs at a distance of from one to three miles. Guns have been heard as far as 10 miles, with a light breeze blowing across the sound. One of the most powerful signals is the siren fog-horn, the sound of which is produced by means of a disc perfo rated by radial slits made to rotate in front of a fixed disc exactly similar, while a long iron trumpet forms part of the apparatus. The disc is made to revolve rapidly, and when the slits are opposite each other openings are formed through which electricity, steam or compressed air is forced. This causes a sound of very great power, which the trumpet collects and com presses, and which under favorable circum stances is heard from 20 to 30 miles out at sea. Fog-signals are also used on railways during foggy weather. They consist of cases filled with detonating powder, which are laid on the rails and exploded by the engine when it comes up to them. Bell-buoys, common to the coast of the United States, are operated by the cur rent, by the ebbing and flowing tide, by the swaying of the waves, by the wind and by clockwork impelled by weight or spring. As to construction, they are adapted for anchorage on spits, sand-bars, or shoals. The use of elec tricity in fog-signals is well illustrated at Sandy Hook, N. J., where a powerful searchlight flashes every few moments in varying directions. In clear weather this flashlight can be observed a distance of 50 miles at sea. There are in the United States under government control nearly 10,000 fog-signals of all kinds, 3,000 of which are lighted.