gasoline tractor was placed on the market in 1893 but could not compare with the steam tractor because of the crudeness of the gasoline engine of that period. It was not until 1903 that the gas tractor became a commercial success. The development of the practical internal combustion tractor was even more important than the invention of the mechanical reaper and binder or the threshing machine. It gave the agricultural industry the power that was needed, and its advent marks the greatest jump of progress in the history of agriculture. From its advent, barely a decade ago, the story has been one of steady progress. Difficulties have been gradually eliminated and to-day one may say that many of the tractors sold have reached a high efficiency, while all made by reputable manufacturers are practical and capable of wide application. It is estimated that over seventy-five firms are offering gas tractors of all varieties and the sales at the present time run well up into the thousands yearly. They have been shipped to all parts of the civilized world, yet the industry is but in its infancy.

Power traction is superior to animal energy because it will do more work in a given time at less expense than possible with any other traction means. It can be adapted to tasks that cannot be accomplished by any other power and it is always ready for work. When intelligently managed its operating cost is so much less than that of horses necessary to do the same work or the amount of work done in a given time is so much greater than possible with any other form of power of the same cost that its merits are apparent to any one able to analyze its performance.

Comparing Work of Horse and Tractor.-A brief summing up of the comparative merits of horse-power

and mechanical energy show clearly the reason why the animal is doomed to give way to the gas tractor on everything except, perhaps, very small farms, and even in this field of application, some of the lighter tractors are available and will do anything the animal can and much work that the horse cannot be made to perform. In the prime advantage of economy, both in time and money, the tractor is cheaper by a wide margin. A tractor works in any kind of weather without fatigue. The horse is affected by extremes of temperature, either hot or cold. The horse is subject to all ills that flesh is heir to, if the animal breaks down, nature is the only possible repairman and it works slowly.

The tractor is machinery, any one of average intelligence can replace worn or broken parts with but slight delay. The tractor feels neither heat or cold. It will work equally well in the torrid heat of summer or the icy blasts of winter. A tractor can be housed cheaper than the number of horses needed to do the same work can, it requires no care and does not eat when not in use as horses do. Its days work is limited only by the number of hours in the day and the endurance of its operators. Its working period is not measured by periods of 6, 8, or 10 hours, with frequent stops for rest. The tractor will work 24 hours per day, and pull as strong the last minute of the period as when first started.

To be convinced of the relative value of horse and mechanical power, one can compare the original cost of any tractor with the first cost of a sufficient number of horses to do an equal amount of work. The comparison favors the tractor. If one compares the maintenance cost of the two outfits, the machine is overwhelmingly superior to the animal. Compare the amount of work

accomplished by them and its approximate cost, you will soon appreciate the value of the tractor. A farmer cannot afford to feed and house the number of horses necessary to do the work a 50 or 60 horse-power tractor

Fig. 3.-A Man Easily Carries Fuel and Water Enough for an Extended Period of Operation When Gas Engine is Used for Power.

Thermal Efficiency of the Horse.-The horse may be considered as a motor in the sense that it utilizes the heat units contained in its food, which represents the fuel that is burnt in the cylinder of the gasoline engine or under the steam boiler. The ratio between the energy given out by the animal and the amount of fuel consumed respresents the thermal efficiency, or utilization of heat relative to work done. There are different methods of com

paring the efficiency of the horse and the inanimate engine.

The engineer considers the ratio of the heat units delivered by the motor as useful work to the total heat units in the fuel supply. The student of animal physiology first determines the number of digestible elements contained in a given food and considers this the fuel value of the feed. It is said that 30 per cent. of the fuel value of food is lost in the energy expended by the animal in chewing and digesting it. In addition to a food having a fuel value it also has a maintenance value. The amount of external work that can be obtained when the animal is given a certain definite amount of food is termed the "production value." Even when the horse is working a certain proportion of energy is being consumed in moving its body and as it is able to deliver work only when in motion the thermal efficiency of the horse is very low, being given as not more than 6 to 10 per cent. This is exceeded by most internal combustion tractors. Experts of the U. S. Department of Agriculture have obtained an efficiency of 20 per cent. under laboratory conditions. While this may be true of scientifically fed animals under careful supervision, the horses that work under actual farm conditions have a much lower efficiency.

Many experiments have been made to determine the actual power of a horse. James Watt, the inventor of the steam engine, placed the working power of a 1,500pound animal as the ability to lift 33,000 pounds to a height of one foot each minute or 550 pounds to the same height each second. This originated the term "horse-power" which has been used in the mechanical world ever since these classical experiments were made as a unit for the measurement of power delivered from, or consumed by mechanical devices.

Recent experiments seem to indicate that the work performed by the average animal approximates 22,000 foot-pounds per minute, or two-thirds of a horse-power. Other investigators declare that the average horse will deliver three-quarters of an actual horse-power. The working ability of a horse is measured by its pulling power which is called "draft" and it is on this basis that traction engines are compared to horse flesh. The pulling power of the animal is given at one-tenth of its weight when working continuously ten hours each day pulling some object at the rate of 21⁄2 miles per hour. Under these conditions a 1,200-pound horse will develop but eight-tenths of a horse-power and a 1,500-pound animal one horse-power.

For short periods the horse may exert a maximum draft of about one-half its weight but obviously this work cannot be done continuously and the amount of energy can be exerted for but a short time without injury. The pulling power of the average plough horse is given as 150 pounds. One horse-power is equal to a draft of 187.5 pounds at the rate of two miles per hour. It is evident that no matter whether one considers the horse from a practical or scientific point of view that mechanical power shows a marked advantage over animal energy.

Tractor Furnishes Power for Various Farm Machines. We have seen that the horse can do its best

work only when pulling a load. If used for power through the medium of a tread-mill, for operating various forms of machines, the efficiency is still lower and the ratio between useful work obtained and amount of food consumed is such that it is not economical to utilize the animal in this way. The usefulness of a tractor is more varied than that of any other farm machine. It