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FLYING SQtriRRBL

688

FOLKLORE

of 16 or 17 inches. Statements differ as to the distance they are able to fly. President Jordan says that the large flying-fish of southern California sometimes flies for a distance of nearly a quarter of a mile, without rising more than three or four feet. See AERONAUTICS.

Flying Squirrel, a small squirrel provided with a fold of skin on each side of the body between the limbs, which can be spread into a kind of parachute. Ordinarily the folds of skin are close against the body and do not interfere with the movements of the animal. It makes flying leaps of 30 to 50 yards between trees. When sailing through "$ie air, the limbs are extended

PLYING SQUIRREL

and the folds of skin put on a stretch. There is one species in America, with a body about six inches long and the tail of like length. Their habits are like those of other squirrels. They engage in sailing or flying mostly at twilight, and therefore are not familiar members of the squirrel family. They are not found in our desert region, but otherwise are generally distributed. The flying squirrel is our only native, tree-dwelling quadruped provided with a parachute, says Hornaday; and this author declares that it is "one of the most ex-puisite little animals in North America. Its legs are very delicately formed; its fur is as fine and soft as silk; and when at rest the edge of its flying membrane looks like the edge of a lace-ruffle. . . . These little creatures are quite sociable, and nest in hollow trees, where from five to seven young are born. They come out to play about sunset, and are as sportive as school boys playing tag." See Hornaday: Ameri-,can Natural History and Stone and Cram: American Animals.

Focus, a term employed in optics. If a luminous point be placed anywhere on the axis of a lens or mirror, there is in general £ second point on this axis at which the

rays leaving the first point will again meet or from which they will appear to diverge. This second point is said to be the conjugate focus of the first point. If, however, the luminous point be practically at infinity, as, for instance, in the case of a star, then the rays which are incident upon the mirror or lens will be parallel to each other. The focus at which the rays meet in this case is called the principal focus. Mirrors can evidently have but one principal focus; but lenses have two, since the parallel rays may be incident upon either face of the lens. If the reflected (or refracted) ray^s actually intersect at a point, that point is called a real focus. If, however, the reflected (or refracted) rays never meet at a point, but merely proceed as if they came from a point (i. e., would meet at a point if prolonged backward), this point is called a virtual focus. A more satisfactory definition of focus than the preceding is the one introduced by Gauss: "that point on the axis at which, as a luminous point moves along, a discontinuity appears in the position of the image"; in other words, the position of the luminous point on the axis at which the image changes from plus infinity to minus infinity.

Fog, a cloud of very small particles of water suspended in the air near the earth. Fogs appear white because the water-particles are small and transparent. The principal source of fog is a moist soil with colder air above it. The vapor from the moist soil arises, and after the air has become saturated condensation begins and fog becomes visible Another frequent source of fog is the passage of hot moist air over cold water. The water-vapor in the air is cooled until the saturation-point is reached, after which condensation begins. Fogs of this type are seen in the early morning over rivers and small lakes. Aitkin has shown that fogs cannot form except in air in which dust-particles are present, a nucleus of some kind being required before condensation can begin.

Fogo, an electoral district of Newfoundland, Population (1901), 7,570; chief towns Orange Island (population 1,067), Queens-town, Fogo, Gander Bay and Tilton Harbor.

Folk, Joseph Wingate, governor of Missouri from 1905 to 1909, was born at Brownsville, Tennessee, in 1869. He is a graduate of Vanderbilt University. Governor Folk, as a lawyer, has been a prominent prosecutor in cases of bribery and corruption. His career as a barrister dates from 1890; and includes a period of four years during which he was circuit-attorney for St. Louis., gaining a national reputation for integrity and boldness.

Folk'lore, the study of the popular customs, traditions and tales of a people. The systematic study and collection of old