PLOW
1506
PLUTARCH
recent years have been practically exterminated in haunts where once abounding. Plow, an instrument for breaking up and turning over the soil preparatory to the raising of a crop. The plow is mentioned in the Old Testament as having been used at a very early period; and Dr. E. B. Tylor, in his Anthropology, has clearly shown how it arose by gradual improvement from the hoe as that did from the pick or hatchet. The modern plow, with its moldboard for turning over the soil, was invented in the Netherlands a little more than two centuries since; but it has been greatly improved in that time. England took the lead in this improvement, followed by Scotland, in which country the chief inventors and improvers were Small (the inventor of the Scotch swing-plow), Wilkie, Gray and Sellar. In the American sulky-plow the weight of the implement is supported by wheels, thus decreasing friction and enabling the plowman to ride on the plow instead of walking. The double Michigan plow has a small plow on the beam in front of the other. The small plow pares off the surface of the soil and throws it into the previous furrow, and the large one completely buries it under a heavy furrowslice. Reversible or sidehill plows have the share and mold-board so they can be easily changed from one side to the other. Steam-plowing was first introduced in England in 1832. On the great farms of the American and Canadian prairies plows are set in gangs turning six or more furrows, and driven by traction steam-engines. Plowing can also, if the soil be soft, be done witn gangs of rotary plows. These have cupped disks set at an angle and so weighted as to sink into the soil. With the use of these plows a few men accomplish great results. An engine is sometimes made to draw a line of plows, behind which is hitched a line of harrows, while behind these come a seed-drill and rakes.
Pluck'er Tube, a long, slender, glass tube, first made by Geissler of Bonn (i858) on the suggestion of Flicker, for the examination of the spectra of gases. The original form of these tubes is shown in the accompanying figure. At each end a platinum wire is sealed in; and, when these wires are connected to the terminals of an induction coil, the electric discharge brings the gas to incandescence, so that its spectrum is then ready for examination by the spectroscope. An improved form of spectrum tube is one in which the capillary portion is .placed "end on" instead of "side on" before the slit of the spectro-
scope. See Kayser's Handbook of Spectra-scopy (in German).
Plum, certain species of the genus Prunus to which also belong the peaches and cherries. In the plum the flowers occur in umbel-like clusters and appear either before or with leaves. The fruit is a drupe or stone fruit, as in the case of peaches and cherries. The most common plum of cultivation is P. domestica, which probably is of Asiatic origin. Its fruits have been made to vary exceedingly as to shape and flavor. The common wild plum of the United States is P. Americana, with dull yellow fruit, splashed with spots of red, which is also under cultivation. The wild goose-plum is P. hortulana, with many varieties under cultivation. It is found wild in the Mississippi valley.
Plumb'ing, the art of casting and working in lead or other metals and applying them to various purposes connected with buildings, in its broadest sense including pipes and fixtures used to supply water, gas and heat and also the pipes used to remove liquid wastes. A more restricted and now common use of the term includes only water-supply and house-drainage systems, the providing of pipes and fixtures for gas and for heat being called gas-fitting and steam-fitting respectively. The word is derived from the Latin word meaning lead, this being the metal used in Rome and other ancient cities for purposes of water-supply pipes where the pressure was too great for pipes of earthenware. There has been a tremendous improvement in plumbing in recent years, and there is a rapidly growing appreciation of its importance to health as well as to comfort and convenience. Every city of to-day is provided with a system of water-works, and no house is complete if not provided with hot and cold water, with such fixtures as bath, basins, faucets, sinks and other appliances. In America, even in homes in rural districts, such provisions are now quite generally made, often at very heavy extra expense. It is known that some waters attack and decompose lead, giving rise to lead-poisoning, and in highly expensive plumbing copper is commonly employed for hot-water piping.
Plumule (plu'mul) (in plants), the first terminal bud which appears in the embryos of most dicotyledons. Sometimes it is very minute, in other cases it contains a number of the early leaves of the plant, and in any case it develops the shoot. See EMBRYO.
Plu'tarch, a prolific author of the first century, was born at Chaeronea in Bceotia, in 46 A. D. Scarcely anything is known of his personal history, except that he commenced to study at Athens under the academic philosopher Ammonius in 66, the year of Emperor Nero's progress through
PLUCKER TUBE
