It should be capable of operating every machine on the farm that requires power.

Second, it should have correct weight for the work it is to do and the proper arrangement of components. A logical distribution of weight is necessary to insure proper traction. If the machine is too heavy it will pack the soil and much power will be consumed in moving the huge mass over the ground. The factor of weight is

FIG. 8.-The Wolverine Eighteen Horse-power General Purpose Gas Tractor Utilizes Two Cylinder Power Plant.

especially noticeable when climbing grades. If the tractor is too light, it will not have adequate adhesion with the ground and much power will be lost in slipping between the driving wheels and the ground. The weight should be distributed so the greater part of it will come over the rear wheels, as in most constructions these are called upon to do the driving.

Third, there should be a large contact area between the traction members and the ground in order to avoid

loss of power, packing of soil and slipping. It should be provided with change speed gearing that will provide at least two forward speeds and a reverse motion. The drive from the power plant to the rear wheels should be positive and designed so that there will be minimum loss of energy through friction in gearing.

Fourth, it should be of consistent design for the work it is to do, built of the best materials that can be obtained within reasonable limits and incorporate in the construction strength without excessive weight. The entire mechanism, including the power plant should be simple because lack of complication is practical insurance against mechanical trouble.

Fifth, it should be easy to start and the control elements should be designed so the tractor can be directed without undue expenditure of energy. The simpler the control members, the easier the average farm-hand will find it to handle the machine. The construction should be such that an automatic steering attachment can be used in certain classes of work, such as ploughing, so the engineer may devote part of his time to manipulating the plough shares.

Sixth, it is desirable that the engine be capable of operating on any liquid fuel, especially the cheaper and more plentiful distillates of petroleum.

There are a number of tractors offered at the present day that meet all of the above requirements. The construction is good both from an engineering and practical point of view. The machines are economical to maintain and operate and in many cases, especially in power plant and driving mechanism design, useful lessons have been drawn from current automobile practice. It is safe to say that the tractors which more nearly incorporate the good features and ideal requirements are those which

follow in a modified form, some of the rules of practice established by automobile designers. In essential elements the automobile and tractor are similar. It is merely in detail arrangement of parts that they vary.

Practical Prime Movers.-Three forms of prime movers are available for mechanical traction. The electric current, which has been used so successfully in moving the heavy street cars and trucks of our cities is not available for farm-traction engines because most of these are operated at a distance from sources of electric current. The steam engine was formerly very popular, in fact, the earliest, really successful traction engines were propelled by steam power. At the present time, however, this prime mover has been succeeded by the more efficient and easily understood internal combustion engine.

Steam and Gas Power Compared.-One of the great disadvantages of steam, power is that it requires skilled supervision. In some States, the laws require that the operator of a steam-traction engine shall be a licensed engineer. The supervision of the machine must obtain at all times that it is in service and the engineer is too much occupied in the various duties incidental to keeping up steam and directing the conveyance to give his time to any other work. The average gas tractor does not require skilled supervision. Any person, who can pass the relatively simple examinations required for automobile-driving licenses is permitted to operate a tractor, providing that his physical capacity is equal to the task of starting the engine and handling the tractor under average operating conditions.

Besides this important point, the operating principles of the gas engine are much better understood at the present time because of the wide vogue of gasoline power

plants for various farm uses and automobiles. In every community of any importance one can find a number of mechanics skilled in the operation and repair of gas engines. This condition did not obtain four or five years ago, as the gas engine was then considerable of a mystery to our agricultural population. At the present time, however, many converts to gas power have been made by the gas-engine manufacturer and thousands of engines have been sold. There are a number of other advantages of gas power that should be considered before the writer explains why the gas engine is the most popular prime mover.

Efficiency of Steam and Gas Power.-The thermal efficiency of the steam engine is considerably lower than that of the gasoline motor. In the former, the heat units contained in the fuel are made to do work by a rather roundabout process of burning the fuel under a boiler to make steam and then utilizing the expansive force of steam in the cylinders of the engine to produce power. There is a big loss in heat units in converting water to steam vapor and there is a further loss of heat when the steam is led from the boiler to the engine. In a gas tractor the fuel is burned directly in the cylinders and less heat units are wasted than in the steam power plants.

The steam engine has one important advantage and that is that it can be operated at an overload for some time. For example, an engine with a nominal rating of 50 horse-power may be made to develop 60 or 70 horsepower by increasing the steam pressure from the boiler. A gas engine does not have the surplus energy to draw from and will not run under the overload conditions a steam tractor will.

The thermal efficiency of the average steam-power

plant ranges from 18 to 20 per cent, that is to say, only this amount of the heat units contained in the fuel burnt under the boiler is turned into useful work by the steam engine. The thermal efficiency of the gas engine, at the other hand, is often as high as 30 per cent. When compared on a basis of drawbar pull, the relative efficiencies of steam and gas tractors do not vary widely. In tests, as in ploughing, steam tractors have shown an average drawbar pull of 26 per cent. of their total weight and 36 per cent. of the weight on the driving members. Gas tractors as a rule will exert an average drawbar pull of 25 per cent. of their total weight or 35 per cent. of the weight on the drivers.

The average traction rating of gas tractors are about one-half of the brake horse-power rating. In tests with steam tractors over a firm ploughing course they were able to show 56.6 per cent as much power at the drawbar as at the belt in economy tests. Over a hauling course, which presented many different conditions of road surface from the very best to the poorest, the average drawbar horse-power was reduced to 29.3 per cent. of the power given by the engine. In ploughing, some gas tractors have shown a drawbar pull as high as 70 per cent. of the engine power, while in hauling, 50.8 per cent. of the engine power has been exerted at the drawbar.

Why Gas Tractors Are Most Popular.-When one considers the many advantages, ease of operation and economical maintenance of the gas tractor it is not difficult to understand why this form has attained so great popularity. The mechanism is relatively simple and can be started at any time without exasperating delays. Its radius of action is greater than that of a steam tractor because it is more independent as regards a base of supplies. A man can easily carry the amount of water con