or its parts, whereas in the two-cycle motor every measurement must be within very exact limits in order for the motor to work at all. In adjusting motors the valves again cause the four-cycle to be the simpler, for a considerable difference in the amount of air or fuel, or even in more important matters, makes but slight difference in the apparent working of the motor, whereas in the two-cycle type a very slight difference in the proportions of the fuel causes a marked difference in the results obtained.

It has been claimed by many that the four-cycle is more flexible, or, in other words, can be varied in speed and power to a greater extent than the two-cycle motor, but I am inclined to doubt this. A properly designed and constructed two-cycle motor may be run satisfactorily at from one hundred to several thousand revolutions per minute without trouble, and many of the best four-cycle engines fail to run at all regularly when the speed is varied suddenly to any extent.

A very important point that is all too frequently overlooked by the owner or user of gasolene motors is the "human element." One man will get splendid results from a certain type or make of engine while another person will have continual trouble with it and condemn it outright. This peculiar adaptability, or "knack" as it may be called, of some operators with certain engines seems remarkable at first sight, but if carefully studied will usually prove to be due to the fact that the operator has thoroughly learned his engine. No two gasolene motors are exactly alike, and no two engines will run equally well with exactly the same treatment. This is

more often due to surrounding conditions than to the motor itself, but nevertheless I have yet to see the engine that does not require some "humoring" and for this reason I strongly advise every purchaser or user of a gasolene motor to be sure that he has thoroughly mastered his motor and learned its ways before deciding that there is really anything wrong with the mechanism itself.

Before deciding on the type or make of motor suited to your particular requirements, study the matter carefully and study the advantages and disadvantages of each until thoroughly satisfied that you have selected the best motor for your purpose. There are many hundreds of motors made, and while practically every well-known and reliable firm turns out a good motor nowadays, yet some are better than others and each make and type possesses certain advantages that fit it for particular purposes or conditions.

Do not expect to get a first-class motor at an absurdly low price. It costs money to design and build a good engine, and while the large manufacturer, with special tools and machinery and every facility for turning out motors in large numbers, can make a much lower price than the man who makes each motor by hand and builds only a few at a time, yet if a motor is advertised at a very low figure, steer clear of it. As a rule the motors made by large firms at a reasonable price are far better than those made in small numbers at a high price, for in the former case the parts are interchangeable and new pieces may be purchased at a low figure and will always fit, whereas the small manufacturer seldom turns out parts that do not require considerable hand work before they

can be made to fit an old motor. Many manufacturers depend on overrating their motors to increase sales. They will advertise a motor at almost double its actual power and sell it at the price others charge for the actual power advertised and thus appear to sell a motor at a very low price. Usually such motors are extremely high-speed, short-lived engines, or else the maker is deliberately trying to defraud you. There is no excuse for being swindled, however, for it is a simple matter to determine the horse-power of any motor. Bore and stroke and speed give the power, and while various motors may give more or less power according to design and workmanship, yet within certain limits the power may be readily determined.

The amount that the explosive gas is compressed before ignition also affects the power of the motor, as well as the proper adjustment of fuel, proper lubrication, and correct ignition. The power computed from these established factors is known as the Indicated Horse-power, but the whole of this power is never available. A large part is consumed in overcoming the friction in the motor itself, and the power remaining after this is overcome is called the Delivered Horse-power. The proportion of the Delivered Power to the Indicated. Power is known as the Mechanical Efficiency, and is usually expressed in per cent. Thus if the Indicated Horse-power of an engine is 10 H.P. and the Delivered Power is 8 H.P., the Mechanical Efficiency will be 8/10 or 80 per cent, and the friction load (overcome in operating) will equal 10-8 or 2 H.P. While the pressure of the gas under compression varies considerably, accord

ing to the fuel used and design of engine, yet the pressures are so well known and the proper pressure so well established that an average of about 70 pounds per square inch may be counted on. With these factors known the following formulas may be depended upon to ascertain with reasonable accuracy the Delivered Horse-power of any gasolene motor.

For four-cycle motors: D.H.P.-Diam. of cylinder multiplied by itself, times the length of stroke, times the number of revolutions per minute, divided by 18,000. For example, the D.H.P. of a motor with a 3-in. bore, 3-in. stroke, running 1,000 revolutions per minute 3X3X3X1,000 = 27,000, divided by 18,000 = 11⁄2 D.H.P.

=

For the D.H.P. of a two-cycle motor proceed in the same manner, but divide by 13,500. Thus, a two-cycle motor with 3-in. bore, 3-in. stroke, and operating at 1,000 revolutions, would show 3X3X3X1,000=27,000, divided by 13,500, or 2 D.H.P.

Usually motors are tested by some form of pressure brake and the D.H.P. thus obtained is called Brake Horse-power. Such tests are reliable and accurate, but it must be borne in mind that the motors while undergoing these factory tests are operated under the most favorable conditions and by skilled engineers and mechanics; and the same motor that delivers 10 H.P. under a factory brake-test may not deliver over 6 H.P. when installed and operated by yourself.

It is far better to have too much power than too little, and a few horse-power more or less makes little difference. in price. All other things considered you should choose the motor having the longest stroke and largest diameter

and operating at a medium speed. High speed means short life, and large bore and short stroke mean high speed. Extreme long stroke and small bore, however, mean slow speed and additional weight, and an excess of either is undesirable.

With these remarks on the operation and principle of gasolene engines in general, we will now take up the matter of the various motors and their parts and accessories in detail.