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Rh convex disk, pivoted on the plough beam, for the mould-board and share of the ordinary plough. This disk is carried on an axle inclined to the line of draught, and also to a vertical plane. As the machine is drawn forward the disk revolves and cuts deeply into the ground, and by reason of its inclination crowds the earth outwards and thus turns a furrow. A scraper is provided to keep the disk clean and prevent sticking. The controlling levers and draught arrangements are similar to those in the “sulky” plough. The advantage of this plough over the ordinary form is in the absence of sliding friction, and in the mellow and porous condition in which it leaves the bottom of the furrow.



Disk ploughs are unsuitable for heavy sticky soils and for stony land, but may be used with effect on stubbles and on land in a dry hard state. Perhaps their most common use is in ploughing on a large scale in conjunction with steam power.

Steam is employed as motive power when it is necessary to plough large areas in a short time. In the United Kingdom steam ploughing is generally carried on on the double-engine system (introduced by Messrs John Fowler about 1865), in which case two sets of ploughs are arranged on the one-way balance principle, so that while one set is at work the other is carried clear of the ground. In this arrangement, a pair of locomotive engines, each having a plain winding drum fixed underneath the boiler, are placed opposite to each other at the ends of the field to be operated upon; the rope of each of the engines is attached to the plough, or other tillage implement, which is drawn to and fro betwixt them by each working in turn. While the engine in gear is coiling in its rope and drawing the plough towards itself, the rope of the other engine is paid out with merely so much drag on it as to keep it from kinking or getting ravelled on the drum.



In the United States and elsewhere engines drawing behind them a number of ploughs, arranged in echelon and taking perhaps 30 ft. at a time, are frequently seen. On smaller areas petrol motors with one or more ploughs attached are sometimes used.

There is a large variety of ploughs which differ in their purpose from the ordinary plough.

The ridging plough is an implement with a mould-board on each side, terminating in front in a flat point, and used for moulding up potatoes, and for throwing up the ridge on which to plant roots.

The sub-soil plough has the beam and body but not the mould-board of an ordinary plough. Following in the furrow of an ordinary plough it breaks through the sub-soil to a depth of several inches, making it porous and penetrable by plant roots.

Gripping and draining ploughs are employed in opening the grips and trenches necessary both in surface and underground drainage.

See Davidson and Chase, Farm Motors and Farm Machinery; articles in L. H. Bailey's Cyclopedia of American Agriculture (New York, 1907) and Standard Encyclopaedia (London, 1908), &c.  PLOVER, a bird whose name (Fr. pluvier, O. Fr. plovier) doubtless has its origin in the Latin pluvia, rain (as witness the German equivalent Regenpfeifer, rain-fifer). P. Belon (1555) says that the name Pluvier is bestowed “pour ce qu'on le prend mieux en temps pluvieux qu'en nulle autre saison,” which is not in accordance with modern observation, for in rainy weather plovers are wilder and harder to approach than in fine. Others have thought it is from the spotted (as though with rain-drops) upper plumage of two of the commonest species of plovers, to which the name especially belongs—the Charadrius pluvialis of Linnaeus, or golden plover, and the Squatarola helvetica of recent ornithologists, or grey plover. Both these birds are very similar in general appearance, but the latter is the larger and has an aborted hind-toe on each foot. Its axillary feathers are also black, while in the golden plover they are pure white. The grey plover is a bird of almost circumpolar range, breeding in the far north of America, Asia and eastern Europe, frequenting in spring and autumn the coasts of the more temperate parts of each continent, and generally retiring farther southward in winter—examples not infrequently reaching Cape Colony, Ceylon, Australia and even Tasmania. Charadrius pluvialis has a much narrower distribution, though where it occurs it is much more numerous. Its breeding quarters do not extend farther than from Iceland to western Siberia, but include the more elevated tracts in the British Islands, whence in autumn it spreads itself, often in immense flocks, over the cultivated districts if the fields be sufficiently open. Here some will remain so long as the absence of frost or snow permits, but the majority make for the Mediterranean basin, or the countries beyond, in which to winter; and stragglers find their way to the southern extremity of Africa. Two other cognate forms, C. virginicus and C. fulvus, respectively represent C. pluvialis in America and eastern Asia, where they are also known by the same English name. The discrimination of these two birds from one another requires a very acute eye, but both are easily distinguished from their European ally by their smaller size, their greyish-brown axillary feathers, and their proportionally longer and more slender legs. All, however—and the same is the case with the grey plover—undergo precisely the same seasonal