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
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