FORBIDDEN FRUIT, a name fancifully given to the fruit of different species of citrus. In the shops of Britain, it is a small variety of the shaddock (q.v.) which gen erally receives this name. But on the continent of Europe, a different fruit, regarded by some as a variety of the orange, and by some as a distinct species (citrus paradisi), is• known as the F.F., or Adam's apple. Like some other fruits of the same genus, it was recently introduced into the s. of Europe from China. The tree has broad, tapering, and pointed leaves. the leaf-stalks winged; the fruit is large, somewhat pear-shaped, greenish-yellow, of very uneven surface, having around its base a circle of deeper depressions, not unlike the marks of teeth, to which it probably owes its name. It is chiefly the rind which is the edible part; the rind is very thick, tender, melting, and pleasant; there is very little pulp; the pulp is acid.
The name F. F. has also been given to the fruit of tabernamontana dichotoma, a tree of Ceylon, of the natural order apocunacece, The shape of the fruit—which is a follicle, containing pulp—suggests the idea of a piece having been bitten off, and the legend runs that it was good before Eve ate of it, although it hais been poisonous ever since.
Till we know what matter (q.v.) is, if there be matter, in the ordi nary sense of the word, at all, we cannot hope to have any idea of the absolute 'nature of force. Any speculations on the subject could only lead us into a train of hypotheses entirely metaphysical, since utterly beyond the present powers of experimental science. If we content ourselves with a definition of force based on experience, such a definition will say of its nature, but will confine itself to the effects which are said to .be due to force, and in the present state of our knowledge it is almost preposterous to aim at more.
Our first ideas of force are evidently derived from the exertion required to roll, or lift, bend, or compress, etc., some mass of matter; and it is easy to see that in all such cases where muscular contraction is employed, matter is moved, or tends to move. Force, then, we may say generally, is any cause which produces, or tends to produce, a change iita body's state of rest or motion. See MOTION, LAWS OF. The amount or magnitude of a force may be measured in one of two ways: 1. By the pressure it can produce or the weight it can support; 2. By the amount of motion it can produce in a given time. These are called respectively thestatical and dynamical measuresof force. The latker is,as it stands, somewhat ambiguous. What shall we take as the quantity of motion produced? Does it depend merely on the velocity produced? or does it take account of the amount of matter to which that velocity is given? Again, is it proportional to the velocity itself, or to its square? This last question was very fiercely discussed between Leibnitz, Huygliens, Euler, Maclaurin, the Bernouillis, etc.; Leibnitz being, as usual with him in physical
questions, on the wrong side. Newton, to whom we owe the third law of motion, had long before given the true measure of a force in terms of the motion produced. This law is an experimental result—that when pressure produces motion, the momentum produced (see MOMENTUM) is proportional to the pressure, and can be made (numerically) equal to it by employing proper units. Hence momentum is the true dynamical measure of force, which, therefore, is proportional to the first power only of the velocity pro duced. What is properly measured in terms of the square of the velocity, we shall presently see. For various properties of force, statical and dynamical, see the follow lag articles: ComposmoN or. FORCES, COUPLES, CENTER OF 6RAVITY, CENTRAL FORCES, FAILING BODIES, MECHANICAL POWER§, VIRTUAL VELOCITIES. ' It is obvious that in order to produce any effect at all, or to do WORK, as it is techni-.
(tally called, a force must produce motion, i.e., must move its point of application. A weight laid on a table produces no effect whatever unless the table yields to the pres sure, i.e. unless the weight descends, be it ever so little. We do no work, however much we may fatigue ourselves, if we try to lift a ton from the floor; if it be a hundred weight only, we may lift it a few feet, and then we shall have clone work—and it is evi dent that the latter may be measured as so many pounds raised so many feet—introduc ing a new unit, the FOOT-POUND, which is of great importance, as we shall shortly see, in modern physics. See WORK. This is evidently, however.a statical measure of work, since no account is taken of velocity. Have we then for work, as we had for force, a dynamical measure? Let us take a simple case, where the mathematical investigation is comparatively very easy, and we shall find we have. We know (see VELOCITY; MOTION, LAWS OF) that if a particle be moving along a line (straight or not) and the distance moved (in the time t) along the line from the point where its motion com menced be called 8, its velocity is a -4 Also we know that the force acting on it (in the direction of its motion) is to be measured by the increase of momentum in a given time—this gives (just as the last equation was obtained) F = /4 From these two equa tions, we have, immediately, madv = Fds, or, as the rudiments of the differential calcu luS give at once, if the force be uniform.