various devices form a cooling system. Fifth, the lubrication system which comprises positive mechanical means for supplying lubricant. All of these are of sufficient import to merit a detailed consideration, and will be discussed fully in proper sequence.

The transmission system comprises a clutch, which is used to connect the engine power to the wheels and to release the engine when desired; a change speed and

FIG. 10A.-Side View of Phoenix Tractor, A Compact Design. reversing gearing, which is necessary because the gas engine as ordinarily used is not reversible and not as flexible as the steam engine; and positive power transmission means, usually chains or gears, to transmit the engine power from the change-speed gearing to the traction members.

A frame of substantial construction is needed to sup

port the parts and this in turn must be carried on rolling members. Some forms of tractors utilize a three-wheel frame, the single front wheel being used for steering, while others employ four wheels, the two smaller front members being provided with some operating means by which the course of the machine can be changed at the will of the operator.

Selection of Power Plant.-The type and size of engine needed depends on many varying conditions.

FIG. 11.-Diagram Showing Method of Calculating Grade

Percentage.

The weight of the tractor and its capacity are the first things to be considered. The purpose for which the tractor is intended and the nature of the country in which it is to be operated are also factors of some moment. Another important point is the proposed selling price. One would not expect to find a highly refined and expensively built motor on a light or cheap tractor. Then

again there would be nothing gained by installing an engine of large capacity in a machine built only for relatively light work. If the tractor is a type that is likely to be used by the farmer of comparatively small means, the engine should be a simple one that will be easily understood and cared for, without too much trouble or expense. Single-cylinder engines are invariably used on low-powered machines. The two and threecylinder motors are used on machines of moderate capacity, while the four-cylinder power plant is installed in the highest types of construction.

Power Needed for Hauling.-A tractor used for hauling purposes does not need to be as powerful as one used in ploughing or breaking because of the difference in draft required by wheeled vehicles and ploughs. The tractive effort on various surfaces, which means the amount of pull or push necessary to move wheeled vehicles on level roads as given by Norris follows:

From the foregoing, it will be seen that the better the surface of the highways over which the load is hauled, the less the amount of power needed to move a definite weight. If it is necessary to haul a wagon train, weighing ten tons over a macadam road, it would require a draft of 306 pounds. To move the same weight through sand will take 4,000 pounds draft. We have seen that a horse-power was equivalent to a draft of approximately 187 pounds at the rate of two miles per hour. In the first case, where the highway surface is good, a drawbar pull equivalent to that exerted by two horses would be

sufficient to pull ten tons. A 5 to 6 horse-power tractor, if properly designed would be adequate to move this load, provided that there were no grades or stretches of unfavorable highway to be encountered. At the other hand where the road conditions were poor a pull equivalent to that exerted by twenty horses would be needed and a very efficient tractor equipped with a 35 or 40 horse-power motor would be called for.

Other factors besides the condition of road surfaces must be given consideration. The factor of wind resistance, which is so important in figuring power required of motor cars, locomotives or other rapidly moving vehicles can be neglected, but the effect of velocity and gradient must be considered in this connection. The relations of velocity and gradient on traction will be considered fully later.

Energy Absorbed by Ploughs.-If the tractor is to be used for breaking and ploughing it will have to be a powerful one if it is to do an amount of work worth while. Soils differ greatly in their cohesive properties and the amount of draft required to operate a single plough share will vary within wide limits. The average draft of ploughs as determined in an old English test for a furrow 5 inches deep by 9 inches wide made in five different soils was as follows: Loamy sand...

Sandy loam..

Moory soil..

227 pounds

250 pounds

280 pounds

440 pounds

Strong loam..

Blue clay...

661 pounds

This represents an extreme range of 194 per cent. One heavy horse would be needed to pull a plough in sandy loam and in doing this he would be exerting a draft, having a value greater than one horse-power. A team of

two heavy horses would be needed to pull the shares through strong loam, while in ploughing the blue clay, three heavy or four light horses would be required to pull the plough continuously for a working day.

Fifty-seven tests in the varying soils of Missouri gave an average draft of 5.26 pounds per square-inch area of the cross section of the furrow slice turned. Seven trials in clover gave an average of 6.47 pounds per square inch, while six in oat stubble averaged 4.68 pounds. Ploughing in virgin gumbo sod requires a draft of 13.75 to 16.3 pounds per inch. The shape of the plough, the weight, and the adjustment must all be taken into consideration as well as the depth and angle of the cut. The situation may be summed by saying that for ordinary widths and depths of ploughing, the draft per square inch of crosssection ranges from a minimum of three pounds in sandy soil to seven or eight in clay, six or seven in tame clover sod, and ten to fifteen pounds turning the sod of the virgin prairie. The draft of a 6x14-inch furrow would thus present an extreme range of from 250 to 900 pounds, while 400 to 500 pounds is given as an average for old land in the middle West.

From the foregoing it will be evident that in ploughing the full capacity of a tractor must be used. A 30 horsepower tractor, which delivered 50 per cent. of its brake horse-power or 15 horse-power at the drawbar would exert a pull equivalent to 2,810 pounds at the rate of two miles per hour. In soil requiring a draft of 200 pounds per plough it could pull a fourteen-bottom gang, while in exceptionally heavy soil, requiring a draft of 500 pounds per ploughshare its capacity would be but a sixbottom gang. It will be seen that the lighter tractors having comparatively low-powered engines cannot be used as economically as the larger ones for extensive