After the experience with the 1902 machine, and not before, did the brothers feel encouraged to build a craft with a motor. They decided to construct the engine themselves, so that they might have it ready for use in 1903. They were capable engineers, they had their own workshops, and above all they knew just what they wanted. So they made a petrol engine with four cylinders, developing about 25 horse-power. It was water-cooled, and followed upon the lines of a motor-car engine--save that it was lightened where they considered weight could be spared. As a matter of fact, when compared with the light and ingenious motors afterwards made in France, this engine was a heavy and clumsy piece of work, weighing as it did about 200 lbs. But they had calculated what load their machine would bear; and they wanted to ensure that the motor should run reliably. They attempted, therefore, no drastic cutting down of weight. [Illustration: FIG. 31.--Man lifting a 100 horse-power aeroplane motor.] An explanation may well be interposed as to the working of the petrol motor, seeing that it plays so large a part in aviation. First should be remembered the steam engine and its disadvantages--its boiler, weight of water, and need for a heating agent to make this water boil. In the petrol-motor none of these are required--none, at least, save a tank with liquid fuel and another, a smaller tank, containing lubricating oil. Beyond these tanks and their contents, the weight of the engine is no more than the weight of metal which composes it; and so it is possible, with a specially-lightened motor, to deliver one horse-power of energy for a weight of less than 3 lbs. How lightly a petrol engine can be made was demonstrated by the firm constructing the Antoinette motor, with which many of the pioneers fitted their craft. A 16-cylinder engine was made so that a man could raise it upon his shoulders--as shown in Fig. 31--and carry it without much difficulty; and yet this same motor, which one man could lift from the ground, developed 100 horse-power. The principle of the petrol engine is simple. From the tank containing petrol runs a pipe, and the liquid passes through this into the carburettor,--a small chamber in which the petrol is vapourised and made to mix with air and so become explosive. Petrol is a liquid which, when in contact with air, evaporates in the form of vapour, and this forms a powerful explosive, only needing a spark to ignite it. In the carburettor, therefore, the petrol and air are mingled until they form an explosive mixture, then they pass through another pipe to the engine itself. [Illustration: FIG. 32.--The working of a petrol motor.]