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THE HUSBANDRY OF THE CAKE.single mould board plough. This plough throws a slice of earth on either sideof the share, and finds an extended use in the sugar industry in forming thefurrows in which the cane is planted, in opening irrigation channels, in ' burstingout' the middle of the cane rows, and in turning over weeds between the rowsin young canes.Disc Ploughs.—The essential part of the disc plough {Fig. 30) is therevolving discs; these are of concave shape and revolve about their centre;the slice of soil is turned over by the action of the concavity of the disc.The disc principle was originally devised to lessen the draft on the ploughand these ploughs find use in tenacious soils, where the mould board ploughwill not scour properly, and in very hard lands where it is not possible to usethe latter plough. In open loose soils the disc ploughs are inferior to the othertype.Eta. 30.By the use of two discs inclined towards each other, they may be used forfurrowing, and they also find one of their most extended uses in turning overand burying the pea vines grown as a green manure.Steam Ploughs.—This term applies only to the power by which theplough is operated, the latter remaining essentially the same in principle asalready described; instead, however, of there being only one plough, a numberare combined into one implement, the whole forming a gang; as many as sevenmay be united in one gang, the ploughs being either mould board or discs.T wo methods of applying power may be distinguished; in one the plough ishitched directly to the engine which draws the plough across the field. Inthe other two engines are required; these are located on opposite sides of thefield; each engine is furnished with a winding drum, and through the agency105