190 EXPERIMENT STATION RECORD. [Vol.40 



the purpose of which is to enable the engineer to determine quickly and without 

 much calculation the more important data of light farm tractors such as weight, 

 drawbar pull, rolling resistance, speed, size of engine, gear ratio, etc. The data 

 are all based on well-known mechanical formulas. 



Economic size of farm tractor, E. Goldbekgek (Gas Engine. 20 (1918). Xo. 

 6, pp. 273-276, figs. 8). — This is a paper read before the Mid- West Section of 

 the Society of Automotive Engineers, based on service observations. A con- 

 clusion is reached in favor of the four-plow tractor, as being the most economical 

 owing to its production in large quantities and the fact that its price per horse- 

 power will not be 25 per cent higher than that of the two-plow outfit. 



Design of an enduring tractor, F. H. Craven (Gas Engine, 20 (1918), Xo. 2, 

 pp. 98-102). — This is a theoretical design, but based on service observations 

 in which the author gives his views as to the design of what ultimately will be 

 generally accepted as the standard design of farm tractor. 



Gears for tractors, A. W. Scarratt (Gas Engine, 20 (1918), Xo. G, pp. 278- 

 284, flff- !)■ — This is a paper read before the Minneapolis section of the Society 

 of Automotive Engineers, in which the materials for gears are discussed and 

 tabular data showing tensile strength, elastic limit, machining quality, and 

 characteristics of different metals for gears arc given. The particularly note- 

 worthy point as to these data is the fact that the author indicates that close- 

 grain semi-steel having a tensile strength of 31,000 lbs. per square inch is satis- 

 factory for high or low speeds and low pressures, and wears well. This is par- 

 ticularly Important owing to recenl developments in semi-steel practices, on 

 which very materially depends its production. Data are also given on the 

 strength of gear teeth. 



Tractor transmissions, E. R. Greek (Gas Engine, 2') (1918), No. )■ pp. 200- 

 204, fiffs- 5). — This paper was presented at the tractor section m of the 



Society of Automotive Engineers, and gives a general discussion of the subject 

 of particular interest to engineers. 



Magneto ignition for farm tractors. J. G. Zimmkp.man (Gas Engiiu 

 (1918), Xo. 6, pp. 265-271, figs. 7).— This is a paper read before the Mid-W- 

 section of the Society of Automotive Engineers on April 26, 191S. It atteni 

 to bring out the essential factor of the proper installation of magnetos in the 

 tractor engine for constant duty work. It is shown that the spark for a given 

 cliarge, regardless of its voltage, must be sufficient and with a good factor of 

 safety. 



With reference to wiring, the use of metal tubes to carry hif:h-tension wires 

 is shown to be a bad practice The ideal wiring is considered to consist of 

 the shortest possible wires of equal length from the ignition device to the pillars. 

 All wires should be kept at least 1 in. away from grounded parts and should 

 be kept at least 0.5 in. apart. 



With reference to spark requirements it is shown that the spark must occur 

 exactly when wanted, and that the ignition should occur at a definite time 

 relative thereto. 



Further daia are given on the spark-gap setting of plugs, the initial kick 

 voltage, and the energy of the spark. The importance of the starter coupling 

 with the magneto is also brought out. 



Fuels for tractor engines. J. L. Mowry (Gkw Engine, 20 (19 IS). Xo. 5. pp. 

 239-243). — This is a paper read before the Minneapolis section of the So, 

 of Automotive Engineers, and based on both field observations and labora- 

 tory tests. The author's conclusions are that the substitution of lower-grade 

 fuels for higher-grade fuels under present conditions will require better spark 



