Structurally, the modern seaplane has two small pontoons on the end of each wing and a small boat in the centre, or sometimes only two pontoons in all which are side by side near the fuselage. The flying-boat has one large boat instead of a fuselage, with a small pontoon on the end of each wing. The former is used for fast flying, but owing to the air resistance to the pontoons, and especially to the boats, the speed cannot be compared to that of the scout aeroplanes. Moreover, they are much harder to do stunts with and few are known to have looped the loop. Like the big land bombers the flying-boats may be equipped with as many as three motors. One of these has carried as many as fifty passengers at one time. Contrary to the accepted notion, these flying-boats are very hard to land on the sea because it is so difficult to calculate the position of the wave when you strike—both are moving so rapidly. As we have already seen that due to the fact that a heavier-than-air machine must be moving at least 35 miles an hour to get off the ground or water, a strong and powerful motor is absolutely essential to make aeroplane flying possible. We have already discovered that the Wrights had to construct their own motor because none was light enough for an aeroplane. Their 16 horse-power single-cylinder engine weighed over 200 pounds. To-day the Liberty is rated at from 400 to 450 horse-power, and it weighs less than two pounds per horse-power. An Italian aeronautical engine develops 700 horse-power, and one sixteen-cylinder American motor generates 900 horse-power. This shows the tremendous development of the motor for modern flying. [Illustration: A Shortt “pusher” seaplane equipped with a one-and-a-half-pounder gun.] [Illustration: _From a photograph by Bain News Service._ British-built Curtiss flying-boat, at Brighton, England.] But, aside from the matter of weight and horse-power, the aeromotor has been called upon to perform at altitudes of as high as 30,000 feet as efficiently as on the ground. Since the atmospheric pressure at that height weighs a great deal less than at sea-level the flow of gasoline and lubricants is very much decreased, so that the efficiency of the motor may fall off proportionately. To meet these requirements the aviation motor must be especially designed, and since the vibration of the propeller shakes the frail frame on which the engine is mounted, the materials must have the greatest strength and resistance. Nevertheless, in both types of motor, the rotary air-cooled and the stationary V type, the engineers have succeeded in making engines that would climb still higher than the 30,500 ceiling already made, if the aviators could stand the cold or have enough hydrogen to keep them from fainting. The motor then is the heart of the heavier-than-air machine, and when it stops the aeroplane must volplane or fall to the earth, a slave to the laws of gravity. CHAPTER IV LEARNING TO FLY