An internal expanding clutch connects the motor with the driving gear by operating on the inner run of the fly-wheel. The motion of the engine is transmitted to an intermediate reversing device through bevel gears, this being necessary for the reason that the crank-shaft runs “fore and aft,” or parallel to the length of the tractor. A double acting jaw clutch engages with either one or the other of a pair of bevel pinions that run in opposite directions. Motion from the reverse gear is transmitted directly to the different shaft, and from there it is transmitted to the master gears on the bull wheels. The differential shaft is in one piece. [Illustration: “Big Four,” Four Cylinder Tractor Motor. ] [Illustration: Showing the Position of Engine on “Big Four” Tractor. ] The main driving wheels are very large when compared with the wheels of an ordinary tractor, for they are eight feet in diameter and are proportionately broad. This no doubt gives splendid tractive effect in soft and uneven fields and must save the machine from “stalling” under adverse conditions. Another unusual feature is the automatic steering device used in plowing. This device consists of a long tubular boom that is fastened to the swiveled front axle of the tractor and a small wheel fastened to the outer end of the boom. The small wheel rolls in the next furrow and compels the tractor to plow in a line parallel to it. This steers the tractor more accurately than would be possible by hand and at the same time enables one man to operate both the engine and the plows. [Illustration: The “Case” Gas Tractor. ] Cost of Gas Engine Operation (American).