ploughing, though they are practical for hauling purposes, or for ploughing on small farms.

Power Delivery Under Belt.-Most traction engines are provided with a pulley by which the motor may be coupled to any form of machinery that can be driven by a belt. In this work practically the entire brake horsepower of the motor is available, as there is but little loss in transmission. When it is used for traction purposes part of the power is being used in moving the machine over the ground and part is being lost in friction in change speed and driving gearing. An engine that may deliver 30 horse-power under belt would not deliver more than 15 horse-power at the drawbar under ordinary conditions.

Why Proper Distribution of Weight is Essential. To get the largest proportion of power at the drawbar the gas tractor designer is forced to consider a number of important conditions. First, the engine must be strong and durable and the friction losses through gears and shafting must be reduced to a minimum. Second, the design of the machine and the distribution of weight must allow the engine to travel over a great variety of soils.

The weight must be so distributed and carried that it will consume the least possible amount of power to drive the engine. The successful traction engine must be capable of running over wet, and sometimes muddy ground, over soft-ploughed ground, or over rough field surfaces. Every horse-power that is used to move the engine is lost and only that available at the drawbar can be counted in traction work. The engine designer should consider concentrating the weight of the engine where it can be carried to best advantage. If the bulk of the weight is placed at the center of the frame so the load is

carried on both front and rear axles, the amount of power delivered at the drawbar is greatly reduced unless the machine is a four-wheel drive or other special design.

The front wheels should not carry any more load than will hold them in contact with the ground with sufficient pressure to insure positive steering. The smaller the front wheels are in diameter and the more weight carried by them the greater the amount of power it requires to force them ahead and the more difficult it is to control the tractor.

In order to secure the greatest tractive efficiency the greater part of the weight should be carried over the rear axle. The engine should be placed at such a point that the front end will not be overloaded or the frame stressed unduly. When the motor is placed parallel with the frame and when the bulk of the gearing is carried by the rear axle the greater portion of the strain due to weight and vibration will pass directly to the ground. The degree of adhesion between the rear-driving members and the road or field surface depends upon the amount of weight that keeps them in contact with the ground. It should not be inferred that tractors having the engine placed in front or in the middle of the frame are not practical, as many of these have given exceptionally good results in practice.

Influence of Weight on Traction.-When a motorpropelled vehicle travels on a level course its tendency to forward motion is resisted by three main items. The most important of these is the rolling resistance at the point of contact of the wheels with the ground. Of the other two, the friction in the rear axle and driving mechanism and the air resistance; the influence of the latter may be neglected in such slow-moving vehicles as tractors. If the vehicle is travelling up a grade the power

required to lift the weight up the incline should also be considered. The road resistance, as we have seen, depends to some extent on the character of the road surface, but another factor is the diameter of the wheels and the speed at which the vehicle travels. In the table previously given, we have seen that on a good macadam road a pull of 30.6 pounds would move a ton, therefore the heavier the tractor the greater the amount of power needed to overcome the resistance of the road. The horizontal effort required to pull a tractor up a grade is equal to about 1 per cent. of its weight for each per cent. of the grade. If we have a tractor weighing 10,000 pounds and we wish to pull it up a 10 per cent. grade it will require a pull equal to 10 per cent. of the weight or 1,000 pounds. This added to the traction resistance of 30.6 pounds per ton means that an added pull of 153 pounds must be considered in connection with the amount of force needed to climb the grade. Obviously a lighter tractor would not require so much power on grades and could be operated by an engine of less power. At the same time as the amount of adhesion between the wheels and the ground would be less in case of the light machine, its traction capacity would be reduced in direct proportion.

It will be apparent that there are extremes which the careful designer will avoid. A heavy machine with an inadequate power plant or with an inefficient system of transmission will not be practical because it could not surmount grades and would not have the range of work that its size would indicate. At the other hand the mistake can be made of using too large a power plant in a light machine. This would mean that while it would prove to be a good hill climber or have a capacity for hauling, it would not have weight enough to insure the

delivery of its full drawbar horse-power on account of the limits in traction imposed by the light weight on the rear wheels. This is overcome in some constructions, notably in the combined self-contained ploughing and traction machines where the ploughs are carried under the frame of the tractor in such a way that the resistance offered by the earth to the cutting action of the plough will hold the wheels firmly against the ground and thus increase the available tractive effect. In this way a light machine that would not be adequate to pull a trailer plough will handle the ploughs provided as a part of its structure with ease.

Influence of Road or Field Surface on Traction.We have seen by the table, previously considered that the draft required to haul wagons varied according to the nature of the road surface. The harder the surface over which the wheels rolled, the less the amount of pull needed to keep the vehicle in motion. The following

Draft H.P. Draft H.P. Draft H.P. Draft H.P.

2,135 10.82 2,500 12.67 2,650 13.43 3,040 15.40 2,405 12.19 2,770 14.04 2,920 14.80 3,310 16.77 2,675 13.55 3,040 15.40 3,190 16.16 3,580 18.14 2,945 14.92 3,310 16.77 3,460 17.53 3,850 19.51 3,215 16.29 3,580 18. 14 3,730 18.90 4,120 20.88 3,485 17.66 3,850 19.51 4,000 20.27 4,390 22.44 4,295 21.76 4,660 23.61 2,810 24.37 5,200 26.35 4,835 24.50 5,200 26.35 5,350 27.11 5,740 29.08 5,375 27.24 5,740 29.08 5,890 29.84 6,284 31.82 6,185 31.34 6,550 33.19 6,700 33.95 7,090 35.93

table gives the horse-power and draft required to pull a 132-tons tractor over various grades and road surfaces at a net speed of 1.9 miles per hour and shows clearly the effect of varying road and field surfaces on traction.

Brief study of the table will show that the variation in draft for different road surfaces is not as great as for wagons. The amount of pull needed ranges from 160 to 225 pounds per gross ton. These figures are higher than are usual with wagons and may be ascribed to the greater internal friction existing between the various parts of the mechanism. In a test a ton of engine weight required a draft 20 per cent. greater than the same weight of wagon and load. If a constant figure or allowance for the friction of gearing could be made and substracted from the draft per gross ton in each case the difference in draft due to road surface would be more strongly emphasized. As it is, it required from 40 per cent. to 50 per cent. more power to pull the tractor over soft-field surface than it did over macadam road.

Effect of Grades on Traction.-On grades the internal friction and ground resistance are constant factors, providing that the road surface on the gradient is the same as obtains on the level highway. In the above table the variation between the power needed to move a tractor on the level and on grade is due to the effort expended in lifting the tractor. Under actual service conditions, the internal friction to be allowed for would depend entirely upon the design of the transmission mechanism and the care taken to keep the various bearings, gears, etc., well lubricated and in line.

An authority has stated that allowing 250 pounds of resistance per ton of weight would not be too much in estimating the power required to move the average tractor over an ordinary road. Each per cent. of grade