PLOWS FOR MECHANICAL POWER 179 



are not well adapted to uneven ground. The disk gang in 

 itself is necessarily rigid, since any vibration of the frame 

 under load gives the plows a jumping motion which results in 

 uneven plowing. The larger gangs, in addition to lacking in 

 flexibility, fail also in the other extreme, it being impossible 

 to produce a wide frame of the necessary rigidity without undue 

 weight. Furthermore, the absence of the landside makes it 

 necessary for the carrying wheels of the disk plow to counter- 

 act all the side pressure of the soil. This is excessive where a 

 large number of disks is carried upon one frame, and the rear 

 wheel must be weighted heavily to keep the plow in the ground. 

 The medium sizes of from four to seven disks are now more 

 popular than either extreme, presenting a compromise between 

 compactness, flexibility, and ease of steering. They are 

 usually capable of variation in the number of disks and the 

 width of cut per disk. For difficult soils one or two more disks 

 can be added without changing the total width of cut. Each 

 disk then cuts a narrower furrow, and has better penetration. 

 The twenty-four-inch disk is most frequently used, though for 

 sandy soil a size two inches larger is popular on account of its 

 longer life. 



In construction the disk gang is very much like the solid 

 moldboard gang. In fact, one fairly successful combination 

 has been offered, either disks or moldboards, or both, being 

 supplied with the frame. The solid frame is open to the same 

 objections as the solid moldboard type, and presents the same 

 problems as to hitching and the securing of uniform depth and 

 quality of plowing. The frame is necessarily heavier and 

 stronger than on horse plows and the wheels are usually cast 

 very heavy, a single wheel weighing as high as 225 pounds for 

 extremely hard ground. The greater number of makers use 

 inclined furrow wheels to take part of the pressure off the land 

 wheels. Castered wheels on most makes permit the gang to 

 follow the engine closely in either direction, provided the method 

 of hitch will permit. 



