JESUITS, in church history, or the so ciety of Jesus, a celebrated religious or der in the Romish church, founded by Ig natius Loyola, a Spaniard, who, in the year 1738, assembled ten of his companions, at Rome, and proposed to form a new or der, when it was agreed to add to the three ordinary vows of chastity, poverty, and obedience, a fourth, which was to go wherever the Pope should command, to make converts. They were admitted on their own terms ; but the order was abolished, on account of the enormities committed by them, in 1773.
JET, a black, inflammable, bituminous substance, harder than asphaltum, and susceptible of a good polish ; it becomes electrical by rubbing, attracting light bodies like yellow amber ; it resembles cannel-coal in some particulars, as in hardness, receiving a polish, and not soiling the fingers by the touch. It has sometimes been confounded with this substance, but the distinction between them is not difficult : cannel-coal wants the electrical properties of jet, and is much heavier. Magellan supposed that jet was true amber, differing from the yellow kind only in the circumstance of colour, and being lighter, on account of the greater quantity of bituminous matter which enters into its composition. It emits, in combustion, a bituminous smell ; it is never found in strata or con tinued masses, like fossil-stones, but al ways in separate unconnected heaps, like true amber. It is found in abundance in the Pyrennean mountains ; also in some parts of Portugal and Spain, in Sweden, Prussia, Germany, Italy, and Ireland.
JET Wean, a French term, frequently also used, with us, for a fountain that casts up water to a considerable height in the air. A jet of water is thrown up
by the weight of the column of water above its ajutage, or orifice, up to its source or reservoir ; and therefore it would rise to the same height as the head or reservoir, if certain causes did not prevent it from rising quite so high. For, first, the velocity of the lower parti cles of the jet being greater than that of the upper, the lower water strikes that which is next above it ; and as the fluids press every way, by its impulse it widens, and consequently shortens the column. Secondly, the water at the top of the jet does not immediately fall off, but forms a kind of ball or head, the weight of which depresses the jet; but if the jet be a little inclined, or not quite upright, it will play higher, though it will not be quite so beautiful. Thirdly, the friction against the sides of the pipe and hole of the adju tage will prevent the jet from rising quite so high, and a small one will be more im peded than a large one. And the fourth cause is the resistance of the air, which is proportional to the square of the velo city of the water nearly ; and therefore the defect in the height will be nearly in the same proportion, which is also the same as the proportion of the heights of the reservoirs above the ajutage. Hence, and from experience, it is found that a jet properly constructed, will rise to dif ferent heights, according to the height of the reservoir, as in the following table of the heights of reservoirs and the heights of their corresponding jets ; the former in feet, and the latter in feet and tenths of a foot