THE objects to be obtained in testing tractors are listed, but field testing only is dealt with. Regardless of the attempt of the Society of Automotive Engineers to standardize the practice of drawbar horsepower rating of tractors, the practice followed by the industry is anything but uniform. Drawbar horsepower actually developed in tests at the 1918 National Power-Farming Demonstration, varied from 50 to 200 per cent of the rating. The need of more than 25 per cent reserve, as provided by S. A. E. Standard Practice, is argued. Drawbar pull is suggested as an important part of the rating. Rating of tractors by the number of plows pulled will not be satisfactory, owing to wide variation in soil conditions.
To make satisfactory tests an accurate dynamometer is required. The Hyatt and Iowa instruments are described. The importance of care in making tests is emphasized. In the discussion, the Gulley and Giddings recording dynamometers are described.