552 



Popular Science Monthly 



operate; probably much less to sell. It 

 displaces a smaller percentage of the 

 farm's total animal power, which cannot 

 be wholly dispensed with. 



The smaller units of machinery which 

 it operates are apt to present a higher 

 cost per unit of work. Its earning 

 capacity in custom work off the farm is 

 less. It appeals more as a convenience, 

 but the ability to rush work at the proper 

 time on the farm is often really justified 

 by a greater net return, regardless of 

 cost. 



From a mechanical standpoint, the 

 difficulties are perhaps even greater. 

 The small tractor is called upon for a 

 greater variety of work than its large 

 counterpart^light hauling, cultivation, 

 and other jobs formerly done only by 

 horses. It must, of course, run 

 stationary machines and thus take 

 the place of some of the larger sta- 

 tionary and medium-sized 

 portable engines. It must do 

 its field work over unfavor- 

 able grades and surfaces such 

 as do not usually 

 confront the auto- 

 mobile and motor- 

 truck. 



In plowing, the 

 possible width of 

 furrow cut is much 

 less in proportion 

 to the width of 

 a small round- 

 wheel tractor than 

 in the larger out- 

 fits, and this has 

 presented extraor- 

 dinary difficulties 

 in the way of side draft, hard steering 

 and unequal wear. The plow must 

 travel at or near the right-hand side of 

 the tractor, or else the tractor must 

 move partially upon the plowed ground, 

 with a loss of tractive power and the un- 

 doing of part of the plow's work. 



This problem of hitching, probably 

 more than any other, is responsible for 

 the failure of the small-wheel tractor to 

 follow at once the lines of the large units, 

 which are now practically all of the four- 

 wheel type, with the two driving-wheels 

 at the rear. The small tractors which do 

 follow this conservative type* are proba- 

 bly further advanced at present than the 



A small tractor loading a silo 



many radical variations from it, though 

 this may prove to be due less to the 

 principle than to greater experience on 

 the part of the manufacturers. 



Some of these variations are meeting 

 with considerable success, especially that 

 group which employs but one driving- 

 wheel, mounted at the right-hand side so 

 as to place the power directly ahead of 

 the plows. An idler wheel on the left 

 merely serves to distribute the weight 

 of the machine and give the necessary 

 stability. 



Several small tractors dispense with 

 the third and fourth wheels, carrying the 

 entire weight upon two drivers. The 

 hitch is made directly to the plow, culti- 

 vator or wagon, which completes what 

 is virtually a self-contained outfit. 



Other tractors, both three and four- 

 wheel, are made self-contained by hang- 

 ing the plows from the frame, usually 

 ^underneath. The plows 

 may be removed and the 

 tractor used for pulling 

 other implements. This 

 type is at a disad- 

 vantage in soft 

 ground, however, in 

 that in case of mir- 

 ing down, the plows 

 form an anchor 

 from which it is 

 difficult to cut 

 loose. 



Soil conditions 

 are far from uni- 

 form, and the plow- 

 ing tractor cannot 

 depend upon mo- 

 mentum to help it 

 through the hard spots and up short 

 grades. For this reason, very largely, 

 the tendency seems to be toward the use 

 of the more flexible four-cylinder motor. 

 For the same reason, and the low co- 

 efficient of friction between a wheel and 

 the soil, such extreme lightness of weight 

 has never been found practical in the 

 tractor as it has in the motor-car. 



The average soil resistance to the plow 

 in well-tilled loam, is close to five pounds 

 per square inch of cross-section of the 

 furrow slice. A furrow fourteen inches 

 wide and seven inches deep will there- 

 fore require a pull of five hundred pounds, 

 varying of course, with the type of 



