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The first petrol tractors seem to have been those built by the Huber Mfg. Co., of Marion, O., in 1898. These were not successful and only 30 were completed. In 1901 the Hart-Parr Mfg. Co., of Charles City, la., engaged in the manufacture of petrol tractors, and, although this company met with many difficulties, it was successful and, after 19 years of continuous manufacture, produced in 1920 5,000 tractors. There are two general types of agricultural tractor, the wheeled and the crawler, or caterpillar, types. The crawler type, in which the weight is spread over a large supporting surface on the ground, can pass over marshy land where the wheeled tractor would be mired, and when used for pulling barrows, seeders and other implements over ploughed ground, does not pack the soil as a wheeled tractor would. Early technical development was chiefly to- ward large tractors, designed to operate from 6 to 12 plough bottoms, because the only farmers who then took an interest in tractors were the owners of large ranches in western America. The largest producers of petrol tractors in the United States between 1905 and 1913 were manufacturers whose main line was general agricultural machinery and steam traction engines, such as the International Harvester Co., the J. I. Case T. M. Co., and Emerson-Brantingham Co. The large petrol tractors of that period were crude in design and very heavy; their frames were built up of heavy rolled channels, and they had driving wheels of enormous diameter. Their great weight naturally rendered them inefficient, for a large proportion of the engine power was consumed in moving the tractor itself. The oil-pull tractor shown in fig. i is characteristic of tractors of that period.

FIG. i.

Agricultural Tractors. Tractors are usually rated in terms of the

number of 14-in. plough bottoms which they will pull. In 1920, of

loo models on the American market capable of pulling two or more

' bottoms, 20 were two-plough tractors; 42 three-plough; 17 four-

' plough and the rest were capable of pulling five or more ploughs.

Table I shows the production of tractors in the United States each

year from 1909 to 1920.

Table I. Tractor Production in the United States.

1909

. . . 2,270

1915 .

21,900

1910

4,500

1916

29,670

1911

7,400

1917 .

62,742

1912

11,400

1918 .

. 132,697

1913

7,450

1919 .

136,162

1914

10,400

1920

. 203,300

According to a statement issued by the Ministry of Transport, there 'were 10,161 agricultural tractors in the United Kingdom in 1921. Of the tractors produced in the United States 14,854 were exported in 1917, 36,351 in 1918, 19,693 in 1919, and 29,143 (valued at 830,850,000) in 1920. It will be seen from the above table that a slump in tractor production occurred in 1913. It was in 1912 that the business in the large tractors of 6-to 12-plough capacities reached its zenith, and thereafter it declined rapidly. The first popular small tractor, the Bull, was brought out the following year, and in 1914, no fewer than 3,000 tractors of this make were sold. Although smaller, it was still of the same crude design as the larger machines, with exposed gears, plain bearings and similar features. About 1916 a number of engineers with motor-car experience entered the tractor field, and as a result of their efforts tractor design was greatly improved. Before 1920 many farm tractors were designed to use paraffin as fuel, because paraffin was considerably cheaper than petrol, and the conditions of engine operation on a tractor

(constant speed and steady, heavy load) made it comparatively easy to burn paraffin with a moderate degree of success. Petrol, however, proved much more satisfactory in the average engine, and when in 1919 the price of paraffin advanced sharply, tractor users generally turned to petrol. The consumption of petrol in gal- lons per acre is less, there is less difficulty in cold weather and the oil in the crank case does not lose its lubricating value so quickly. For the sake of economical manufacture many of the earlier small tractors were made with only three wheels, but the buying public did not approve of this feature of construction, and the typical 1920 wheeled tractor had four wheels, two in front steering and two in the rear for driving. The Twin City 12-20 H.P. tractor shown

FIG. 2.

in fig. 2 is a good example of the lighter tractors based on motor- car practice. The following is a composite description of a 1920 model three-plough tractor, the features mentioned being those found on the greatest number of models of this capacity; there was a four-cylinder, four-stroke vertical engine at the front, of about 4f-in. bore by 6-in. stroke, adapted to burn either petrol or paraffin. Ignition was by a high-tension magneto with impulse starter. (The latter device consisted of a spring attachment for magnetos specially developed for use on tractors, which made it unnecessary to " spin " the heavy engines in order to generate a spark in the magneto.) The carburetor was fitted with an air cleaner, which prevented gritty dust from getting into the engine and rapidly wearing out the cylinder barrels. These air cleaners usually acted on the centrifugal principle, but in the lava-ash districts on the Pacific coast it was necessary to employ air washers, owing to the lightness of the dust. Behind the engine there was a friction clutch, and then came the change-speed gear, which gave two forward speeds and one reverse. The higher forward speed was for regular use in ploughing and the lower for emergencies, for ploughing up steep inclines, in heavy soil, etc. Then there was a further reduc- tion by gears to the rear axle. The engine speed was limited by a governor to about 800 revolutions per minute, and as the ploughing speed was about 2f m.p.h.; with 48-in. driving wheels, a reduction of roughly 40 to I had to be provided between engine crankshaft and rear axle or rear wheels. In the early tractors the final drive was through exposed bull gears directly on the driving wheels, but the typical 1920 tractor had all its gearing enclosed and protected from dust and mud, which is an important feature in a machine

FIG. 3.

operating in the fields during the wet season as well as when the soil is dry and dusty. The wheelbase of the tractor was 96 in. and its weight about 5,000 pounds. The wheels were built up of rolled rims and flat spokes riveted to the hubs and rims. The driving wheels were provided with angle-iron lugs to increase the traction, and the front wheels with central skid rings to make the steering more positive. The front axle was swivelled to the frame at the middle, to permit the wheels to accommodate themselves to uneven ground. In addition to the pivot joint a spring could be used between the axle and the frame at the front to relieve shocks. These 1920 tractors were provided with a belt pulley for operating threshing