PRISMOID

, is a solid, or body, somewhat resembling a prism, but that its ends are any dissimilar parallel plane figures of the same number of sides; the upright sides being trapezoids.—If the ends of the Prismoid be bounded by dissimilar curves, it is sometimes called a cylindroid.

PROBABILITY of an Event, in the Doctrine of Chances, is the ratio of the number of chances by which the event may happen, to the number by which it may both happen and fail. So that, if there be constituted a fraction, of which the numerator is the number of chances for the events happening, and the denominator the number for both happening and failing, that fraction will properly express the value of the Probability of the event's happening. Thus, if an event have 3 chances for happening, and 2 for failing, the sum of which being 5, the fraction 3/5 will fitly represent the Probability of its happening, and may be taken to be the measure of it. The same thing may be said of the Probability of failing, which will likewise be measured by a fraction, whose numerator is the number of chances by which it may fail, and its denominator the whole number of chances both for its happening and failing: so the Probability of the failing of the above event, which has 2 chances to fail, and 3 to happen, will be expressed or measured by the fraction 2/5.

Hence, if there be added together the fractions which express the Probability for both happening and failing, their sum will always be equal to unity or 1; since the sum of their numerators will be equal to their common denominator. And since it is a certainty that an event will either happen or fail, it follows that a certainty, which may be considered as an infinitely great degree of Probability, is fitly represented by unity. See Simpson's or Demoivre's Doctrine of Chances; also Bernoulli's Ars Conjectandi; Monmort's Analyse des Jeux de Hasard; or M. De Parcieu's Essais sur les Probabilites de la Vie humaine. See also Expectation, and Gaming.

Probability of Life. See Expectation of Life, and Life<*> Annuities.

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Entry taken from A Mathematical and Philosophical Dictionary, by Charles Hutton, 1796.

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