Book IV. 



SCARIFIERS AND GRUBBERS. 



297 



S8l 



without any attention from the driver. Weir has also simplified and strengthened 



298 



this machine in other respects, so that his modification of it {Jig. 298.) is, 

 at present, by far the best. Still we think it an implement that very seldom can 

 be profitably used; that this may be the case, the surface of the field must have a natural 

 drainage, by lying in one even slope or slopes : it must be in pasture, and the soil must 

 be of uniform consistency, and free of stones. But even where these favorable circum-, 

 stances combine, we think two swing ploughs, with finless shares, following in the same 

 tract, would effect the same object sufficiently well for all agricultural purposes ; and for 

 drains in ornamental grounds, no machine will ever equal manual labor. 



2525. Various draining ploughs have been invented and tried by Arbuthnot, Makic, 

 M' Dougal, and others, but they have never found their way among practical men. 



252G. IVie only essential plough to be selected from these three sections, is the Scotch 

 swing plough, with or without one or two-wheels, according to circumstances, and with 

 the share and coulter set to suit different soils, as flinty, chalky, gravelly, &c. , or soils in 

 diflferent states of culture, as old turf, ley, fallow, &c. 



SuBSECT. 3. Of the Tillage ImplemeTits, known as Scarifiers, Scvfflers, CiUtivators, and 



Grubbers. 

 2527. The use of pronged implements as substitutes for the plough, is comparatively. of 

 recent invention. They differ from the plough, in stirring the soil without reversing its 

 surface or altering its form, unless, indeed, they in some cases tend to even or level ine- 

 qualities; they act both as the plough and harrow at the same time, and on suitable 

 soils, and at proper seasons, much more labor is eflPected with less expense of men and 

 cattle. Wherever therefore lands require to be stirred for any purpose, excepting that of 

 reversing the surface, or laying them into beds or ridges, recourse may be had to pronged 

 tillage implements, such as we are about to describe. 



2528. In estimating the value of pronged tillage implements. General Beatson {New System of Cultivation^ 



says, if we apply the principle of 

 can easily be ploughed with two 



1820.) applies the principle of lessening power and employing time. He says, if we apply the principle of 

 petty operations to any stiff land, by taking that depth of furrow which can easily be ploughed with two 

 horses, and repeat the operation, (or plough the land a second time,) we shall arrive at the end proposed. 



that is, the same depth of ploughing, with absolutely less exertion of animal strength, than if we were to 

 plough the same depth with four horses at one operation. 



2529. This may be illustrated by supposing the resistances to the plough to be in proportion to the 

 squares of the depth of the land. If so, and we are to plough at once with four horses, six inches deep, 

 the resistance at that depth would be 6 jt 6 = 36 ; but if with the same four horses, using two at a time, 

 we plough the same depth of six inches at two operations, taking only three inches at each, then the square 

 of the^first depth is nine, and the square of the second, nine making 18 for the total resistance, or the 

 power expended by the two horses, in ploughing six inches deep, at two operations. 



2530. A farther illustration may be made by supposing the same four horses, which had ploughed at once 

 ftix inches deep, and had overcome the resistance of (> it 6 = 36, applied, separately, to four light ploughs, 

 or other implements, and to plough only 1| inch deep at a time, and to go over the same land four times. 

 In this case the sum of all the resistances to be overcome, or the animal force expended, in these repeated 

 ploughings, would be no more than 9 instead of 36 ; because the square of 1|: = 2^, which multiplied by 

 the four ploughings, gives 9, or only one fourth of the power expended in ploughing at once six inches deep. 

 Hence, it appears, that in ploughing six inches deep, with four horses, each horse exerts a force = 9 ; 

 whereas, in taking only 1^ inch deep, the force he exerts is not more than 2|. 



2531. Farther^ supposing that a horse exerts, in drawing a plough, a force of 160 pounds, it is evident 

 if four horses are ploughing six inches deep, the total force exerted will be &10 pounds, or 160 pounds by 



