Page:Collier's New Encyclopedia v. 03.djvu/414

DIFFERENTIAL CALCUBUa 360 DIGBY the quantity rather than the quality of food is the main consideration. Stewed and boiled meats are more difficult to digest than meat cooked by fire alone. The flesh of young animals seems to be more difficult of digestion than that of old; and the flesh of tame than that of wild animals. All sorts of fat m^at must be taken in smaller quantities. Hence, also, ham, bacon, and salted meats cannot be eaten in such quantities as the tender flesh of poultry. Fish has the advantage of being easily soluble. All boiled vegetables are in general easy of digestion; raw vegetables and salads are more difficult. Fruit should be taken in the forenoon rather than after a hearty meal. In all diseases attended with fever the stomach loathes animal food, and there is generally an increase of thirst, to quench which cool water, or tepid, or rendered acid, may be freely indulged. Infusions, too, of barley, sage, balm, etc., may be taken. In chronic diseases at- tended with hectic fever, milk is the prop- er diet. The best food for infants is, of course, their mother's milk; but when they begin to cut teeth a little animal food, such as soft-boiled eggs, or chicken minced very fine, may be given. DIFFERENTIAL CALCULUS, that branch of mathematics which has for its object the explanation of the method of deriving one determinate function from another by the process of differentiation. If in any determinate function of one variable we give to the variable a con- stant increment, and find the correspond- ing increment of the function, and then divide the increment of the function by the increment of the variable, we shall find a ratio which will in general be de- pendent upon the increment of the vari- able. If now we pass to the limit of this ratio, by making the increment of the variable equal to 0, we shall in general obtain a function of the original vari- able, which is called the differential co- efficient of the function. If this be multiplied by the differential of the vari- able, the result is called the differential of the function. Any function of a single variable will have one, and only one, differential coefficient, and conse- quently it will have but one differential of the same order. The differential cal- culus consists of two parts. The first embraces the science of the differential calculus, and explains the methods of finding the differentials and successive differentials of all determinate functions. The second treats of the application of the differential calculus to the other branches of mathematics, as algebra, analytical geometry, etc. See Calculus. DIFFERENTIAL THERMOMETER, an instrument for determining very mi- nute differences of temperature. Leslie's differential thermometer consists of two glass bulbs containing air connected by a bent tube containing some sulphuric acid, the movement of which (as the air expands and contracts) serves to indicate any slight difference of temperature be- tween the two bulbs. DIFFRACTION, a term applied to certain phenomena connected with the modification that rays of light undergo in passing close to the edge of an opaque body. Thus when a beam of direct sun- light is admitted into a dark room through a narrow slit, and falls upon a screen placed to receive it, there appears a line of white light bordered by colored fringes; these fringes are produced by diffraction. DIFFUSION, a word having several applications. (1) The act of diffusing or spreading about of a liquid, fluid, etc.; (2) a spreading or diffusing abroad of a matter; (3) the state of being spread or dispersed widely; (4) the act of spread- ing, extending or propagating widely, as the diffusion of knowledge; (5) copious- ness, exuberance of style, prolixity, ver- bosity. Diffusion of gases is the passing of one gas into the space occupied by an- other. The name given to that phenom- enon by which the composition of the at- mosphere is kept uniform, or nearly so. Gases diffuse into one another according to a fixed law, that is, inversely as the square root of their densities. Diffusion of heat is a term applied to those modes by which the equilibrium of heat is effected — viz., conduction, radia- tion, and convention. Diffusion of liquids: When two liquids that are capable of mixing are put in contact they gradually diffuse one into the other, notwithstanding the action of gravity. DIGAMMA, the name given to a letter in the oldest Greek alphabet, which early fell into disuse, being retained longest in the ^olian dialect. It is considered to have had the power of the English w or V, and is frequently represented in Latin by u (v) : thus, Gr. oikos = hat. vi<nis, Eng. wick; Gr. omos = Lat. vinum, Eng. wine. See Alphabet. DIGBY, a small seaport of Nova Scotia, on St. Mary's Bay, reputed for its curing of a variety of small herrings or pilchards ("Nova Scotia sprats"). DIGBY, SIR KENELM. an English natural philospher; bom in Gayhurst, near Newport Pagnell, July 11, 1603.