1911 Encyclopædia Britannica/Harrow
HARROW,[1] an agricultural implement used for (1) levelling ridges left by the plough and preparing a smooth surface for the reception of seeds; (2) covering in seeds after sowing; (3) tearing up and gathering weeds; (4) disintegrating and levelling the soil of meadows and pastures; (5) forming a surface tilth by pulverizing the top soil and so conserving moisture.
The harrow rivals the plough in antiquity. In its simplest form it consists of the boughs of trees interlaced into a wooden frame, and this form survives in the “bush-harrow.” Another old type, found in the middle ages and still in use, consists of a wooden framework in which iron pegs or “tines” are set. This is now generally superseded by the “zig-zag” harrow patented by Armstrong in 1839, built of iron bars in which the tines are so arranged that each follows its own track and has a separate line of action. This harrow is usually made in two or three sections which fold over one another and are thus easily portable, the arrangement at the same time giving a flexibility on uneven ground. Additional flexibility may be imparted to the implement by jointing the stays of the frame which are in the line of draught. The liability that the tines may snap off is the chief weakness of this type, and improvements have consisted chiefly in alterations in their shape and the method of fixing them to the frame.
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| Fig. 1.—Jointed Zig-zag Harrow. (Ransomes, Sims & Jefferies, Ltd.) |
The other type of harrow most used is the chain harrow, consisting of a number of square-link chains connected by cross links and attached to a draught-bar, the whole being kept expanded by stretchers and trailing weights. It is used for levelling and spreading manure over grass-land, from which it at the same time tears up moss and coarse herbage. Mention may also be made of the drag-harrow, a heavy implement with long tines, approximating closely to the cultivator, and of the Norwegian harrow with its revolving rows of spikes.
A few variations and developments of the ordinary harrow require notice. In the adjustable harrow (fig. 2) the teeth are secured to bars pivoted at their ends in the side bars of the frame, and provided with crank arms connected to a common link bar, which may be moved horizontally by means of a lever for the purpose of adjusting the angle which the teeth make with the ground, and thus convert the machine from a pulverizer to a smoothing harrow. The small figure illustrates a spring connexion between the adjusting lever and its locking bar, which allows the teeth to yield upon striking an obstruction. As the briskness of the operation adds to its effectiveness, the harrow is often made with a seat from which the operator can hasten the team without fatiguing himself.
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| Fig. 2.—Adjustable Harrow. |
Fig. 3 illustrates a spring-tooth harrow. In this harrow the independent frames are carried upon wheels, and a seat for the operator is mounted upon standards supported by the two frames. The teeth consist of flat steel springs of scroll form, which yield to rigid obstructions and are mounted on rock shafts in the same manner as in the walking harrow before described. The levers enable the operator to raise the teeth more or less, and thus free them from rubbish and also regulate the depth of action.
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| Fig. 3.—Spring-tooth Harrow. |
Another variation of the harrow with great pulverizing and loosening capabilities consists of a main frame, having a pole and whipple-trees attached: to this frame are pivoted two supplemental frames, each of which has mounted on it a shaft carrying a series of concavo-convex disks. The supplemental frames may be swung by the adjusting levers to any angle with relation to the line of draught, and the disks then act like that of the disk plough (see Plough), throwing the soil outward with more or less force, according to the angle at which they are set, and thus thoroughly breaking up and pulverizing the clods. Above the disks is a bar to which are pivoted a series of scrapers, one for each disk, which are held to their work with a yielding action, being thrown out of operation when desired by the levers shown in connexion with the operating bar. Pans on the main frame are used to carry weights to hold the disks down to their work. The cut away disk harrow differs from the ordinary disk harrow in that its disks are notched and so have greater penetrating power. The curved knife-tooth harrow consists of a frame to which a row of curved blades is attached. Other forms of the implement are illustrated and discussed in Farm Machinery and Farm Motors by J. B. Davidson and L. W. Chase (New York, 1908).