FLY-WHEEL. ' < scuts the ultimate tensile strength of the ma- terial per square inch, and <c represents the weight of the material per cubic inch. If, instead of ultimate strength, we let s represent the strength, then by solving the equation for v we get the safe rim-speed in feet per second. If the wheel is made of sections bolted together, the ulti- mate and safe values of s in the above formula must be reduced to from one-half to one-fourth of the figure assumed for solid wheels. The safe rim-speed of a solid cast-iron wheel figured ac- cording to this formula is about 100 feet per sec- ond, of a cast-steel wheel about 233 feet per sec- ond, and of a maple-wood wheel about 155 feet per second. The disruption or bursting of a fly-wheel re- volving at high speed occurs with great force, the flying fragments often wrecking the engine and building in which it is housed, and sometimes being thrown several hundred feet away. Fly- wheel accidents are less common than formerly, but are still frequent enough to make the design- ing of strong and safe wheels a problem for seri- ious study by engineers. In early engines turning with a low number of revolutions the fly-wheel required to be of large diameter, and was for this reason nearly always distinct from the wheel from which the power was taken off. In more modern engines the con- venience of having the fly-wheel serve also as an element of the transmission machinery has brought about the use of fly -band wheels, where belts or ropes are used to take off the power from the engine-shaft. It is so much less the practice in recent years to use gearing in transmitting the power from the engine-shaft that the fly-wheel is seldom a toothed wheel. Small fly-wheels are usually made in one piece of cast iron or cast steel. Larger sizes are cast in halves, which are connected by bolted joints to form the complete wheel, and the largest wheels are cast in several segments which are bolted together. Fly-wheels are sometimes made with metal hub and arms and a rim of some tough, hard wood. Other con- structions are steel plates riveted together, and cast wheels with their rims wound with steel wire of great strength, i See Steam-Engine.) Very complete data for use in the calculation and design of fly-wheels will be found in Kent's Mechanical Engineer's Pocket-Book (New York, L902). FO. The Chinese equivalent for Sanskrit Bud- dha. See Buddhism. FOA, f.Va'. Eugenie (1798-1853). A French romance writer, burn al Bordeaux, she was by descent a Spanish Jewess, and her maiden name was Gradis. She married young, but soon left her husband, and despite delicate health and fail ing eyesight, she supported herself by her pen, using at time, the nom-de-plume 'Maria Fitz- clarence.' Madame Foa's latesf and must suc- cessful books were: Les mimoires d'wn polichimelle l 1839) : Le p< tit l: ! I'aris (1840) ; and /.. vieux Paris (1840). She wrote charming tales for young people, with (he moral not unduly prominent. FOCAL LINES. Sec Fool; LIGHT. FOCI (Neo-Lat. nom. id., points, from Lat. focus, health. ML, central point). Points the distances of which ir ny point cm a given curve are connected by a definite relation. In the case of conies, i he foci are points on the 4 F03TTJS. principal axis, such that the corresponding double ordinate is equal to the parameter of the curve. The term focus, used in this sense, is due to Kepler (1604). By varying the values of the constants in the equation y 2 — m 2 xr = 2px, it may be made the equation of an ellipse, parabola, or hyperbola, according as m* is <[ 0, = 0, or ^> 0. The abscissas of the foci will lie found by substi- tuting y = p in the equation of the curve, whence „,_ — f(l±/r>r + l) m - From these values of the abscissas, considering m 2 as positive, zero, and negative successively, we see that the ellipse and hyperbola have two finite foci, while the parabola has one finite focus and one at infinity. The name focus was originally given to these points from the fact that rays of light emanating from either focus are reflected by the curve through the other. The distance from the foci of curves to every point of the curve can be expressed rationally in terms of the abscissas of the points — a unique property. Curves of higher degree may have several foci. (See Cartesians.) The foci of a conic possess the property that, the straight lines which connect them to the circular points at infinity are tan- gents to the curve. In general a point P is said to be a foctrs of a curve when the lines connecting this point with the circular points at infinity are tangents to the curve. A curve of the nth class has n 2 foci, but the curve being real, n and only n of these foci are real. See Curves; Ellipse; Parabola. In optics, the term focus is employed to desig- nate a point at which rays meet and are collected, after being reflected or refracted. A point from which rays appear to come after reflection or re- fraction is termed a 'virtual focus.' The 'prin- cipal focus' is the focus of parallel rays after reflection or refraction. See Lens; Mirror; Light. FOCUS. See Foci. FODDEE. See Feeding Stuff. FODDEK, Green. See Silage. FffiDEKA, fod'er a (Lat., treaties). A col- lection of documents bearing on State transac tions between the Kings of England and others from 1101. Fifteen volumes were compiled by Thomas Rymer (q.v.) from 1693 to 1713. and after his death five volumes were used by Robert Sanderson (1715-35), covering the documents dow-n to 1654. The work as abridged in a French version by Rapin was retranslated into Engli-h by Stephen YVhatlev. under the title, Acid Regia (1731). FCETTJS (Lat., offspring). The term applied to the unborn young of viviparous animals, especially in the more advanced stages. In the human subject (lie term foetus is applied to the embryo, though more often to the unborn child after the end of the fourth month, when human features are distinguishable. The fourth- month foetus weighs about live ounces, and is about six inches long; its brain convolutions ate found to be developing, and its sex is recognizable; its muscles are capable of pro- ducing movements of the limbs, and its extend- ing ossification can be traced in the bones of the skull. The fifth-month foetus weighs about ten ounces, and is about ten inches long;
