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Rh Under Ammonius Plotinus became imbued with the eclectic spirit of the Alexandrian school. Having accepted the Platonic metaphysical doctrine, he applied to it the Neo-Pythagorean principles and the Oriental doctrine of (q.v.). The results of this introspective mysticism were collected by him in a series of fifty-four (originally forty-eight) treatises, arranged in six “Enneads,” which constitute the most authoritative exposition of Neoplatonism. This arrangement is probably due to Porphyry, to whose editorial care they were consigned. There was also another ancient edition by Eustochius, but all the existing MSS. are based on Porphyry's edition.

The Enneades of Plotinus were first made known in the Latin translation of Marsilio Ficino (Florence, 1492) which was reprinted at Basel in 1580, with the Greek text of Petrus Perna. Later editions by Creuzer and Moser (“Didot Series,” 1855), A. Kirchhoff (1856), H. F. Muller (1878–1880), R. Volkmann (1883–1884). There is an English translation of selected portions by Thomas Taylor, re-edited in Bohn’s Philosophical Library (1895, with introduction and bibliography by G. R. S. Mead).

On Plotinus generally see article in Suidas; Eunapius vitae sophistarum, and above all the Vita Plotini by his pupil Porphyry. Among modern works, see the treatises on the school of Alexandria by J. F. Simon, i. (1845), and É. Vacherot (1846); A. Richter, Ueber Leben und Geistesentwicklung des Plotin (Halle, 1864–1867); T. Whittaker, The Neoplatonists (1901); A. Drews, Plotin und der Untergang der antiken Weltanschauung (1907); E. Caird, Evolution of Theology in the Greek Philosophers (1904), ii. 210–257; Rufus M. Jones, Studies in Mystical Religion (1909). A detailed account of Plotinus’s philosophical system and an estimate of its importance will be found in the article, the works above referred to, and the histories of philosophy. For his list of categories, see
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PLOUGH AND PLOUGHING. To enable the soil to grow good crops the upper layer must be pulverized and weathered. This operation, performed in the garden by means of the spade, is carried on in the field on a larger scale by the plough, which breaks the soil and by inverting the furrow-slice, exposes fresh surfaces to the disintegrating influence of air, rain and frost.

The first recorded form of plough is found on the monuments of Egypt, where it consists simply of a wooden wedge tipped with iron and fastened to a handle projecting backwards and a beam, pulled by men or oxen, projecting forwards. Many references to the plough are found in the Old Testament, notably that in 1 Sam. xiii. 20: “All the Israelites went down to the Philistines to sharpen every man his share and his coulter.” Descriptions of ploughs found in Hesiod’s Works and Days and in Virgil’s Georgics i. 169–175, show little development in the implement. The same may be said of the Anglo-Saxon ploughs. These are shown with coulter and share and also with wheels, which had in earlier times been fitted to ploughs by the Greeks and also by the natives of Cis-Alpine Gaul (Pliny, Hist. nat. 18, 18). A mattock with which to break the clods is often found represented in Anglo-Saxon drawings as subsidiary to the plough. All these types of plough are virtually hoes pulled through the ground, breaking but not inverting the soil. In the first half of the 18th century a plough with a short convex mould-board of wood was introduced from the Netherlands into England and, as improved at Rotherham in Yorkshire, became known as the Rotherham plough and enjoyed considerable vogue. At this period ploughs were made almost wholly of wood, the mould-board being cased with plates of iron. Small, of Berwickshire, brought out a plough in which beam and handle were of wrought iron, the mould-board of cast iron. The shares, when made of the same material, required constant sharpening, this necessity was removed by the device, patented by Robert Ransome in 1803, of chilling and so hardening the under-surface of the share; the upper surface, which is soft, then wears away more quickly than the chilled part, whereby a sharp edge is always assured. Nowadays the mould-board is of steel with a chilled and polished surface to give greater wearing qualities and to reduce friction. In the latter part of the 19th century there were numerous improvements but no fundamental alterations in the construction of the ordinary plough.

The working parts of the plough are the coulter, the share, and the breast or mould-board. These are carried on the beam, to which are attached the handles or tilts at the back, and the hake or clevis and draught-chain at the front. The hake is notched so that, by moving the draught-chain higher or lower thereon, the plough is caused to go more or less deeply into the ground. It may also be adjusted to suit the height of the horses used. The hake moves laterally on a quadrant and it is thus possible to give the plough a tendency to left or right by moving the hake in the reverse direction. A frame is bolted to the beam and this carries the breast or mould-board to the fore-end of which the share is fitted. The side-cap, a plate of iron fixed to the land-side of the frame, is intended to keep the edge of the unploughed soil vertical and prevent it from falling into the furrow. A piece of iron called the slade is bolted to the bottom of the frame, and this, running along the sole of the furrow, acts as a base to the whole implement. The coulter (either knife or disk) and sometimes a skim-coulter (or jointer) are attached adjust ably to the beam, so as to act in the front of the share.

The coulter is a knife or revolving disk which is fixed so that its point clears the point of the share. The skim-coulter is shaped like a miniature plough, substituted for or fixed in front of the coulter; it is used chiefly on lea land, to pare off the surface of the soil together with the vegetation thereon, and turn it into the previous furrow, where it is immediately buried by the furrow slice. Two wheels of unequal height are commonly fitted to the front of the beam. By means of them the depth and width of the furrow are regulated, whereas in the case of “swing” or wheelless ploughs these points depend chiefly on the skill of the ploughman. In the wheeled plough some of the weight and downward pull due to its action on the ground is taken by the wheels; the sliding friction is thus to some extent converted into a rolling friction, and the draught is correspondingly diminished.

In operation the coulter makes a perpendicular cut separating the furrow-slice which is divided from the “sole” of the furrow by the share and then inverted by the curve of the breast as the plough moves forward. The process is indicated in the illustration of different types of furrow. The form of a furrow is