The heart of an airplane is its engine. We know a great deal about gasolene-engines, especially automobile engines; but an airplane engine is a very different thing. It must be tremendously powerful, and at the same time extremely light. Every ounce of unnecessary weight must be shaved off. It must be built with the precision of a watch; its vital parts must be true to a ten-thousandth part of an inch. It takes a very powerful horse to develop one horse-power for a considerable length of time. It would take a hundred horses to supply the power for even a small airplane, and they would weigh a hundred and twenty thousand pounds. An airplane motor of the same power would weigh less than three hundred pounds, which is a quarter of the weight of a single horse. It was this powerful, yet most delicate, machine that we were called upon to turn out by the thousand. There was no time to waste; a motor must be designed that could be built in the American way, without any tinkering or fussy hand-work. Two of our best engineers met in a hotel in Washington on June 3, 1917, and worked for five days without once leaving their rooms. They had before them all the airplane knowledge of our allies. American engine-builders offered up their trade secrets. Everything was done to make this motor worthy of America's reputation. There was a race to have the motor finished by the Fourth of July. Sure enough, on Independence Day the finished motor was there in Washington--the "Liberty motor," a birthday present to the nation. Of course that did not mean that we were ready at once to turn out Liberty motors by the thousand. The engine had to undergo many tests and a large number of alterations before it was perfectly satisfactory and then special machinery had to be constructed before it could be manufactured in quantity. It was Thanksgiving Day before the first manufactured Liberty was turned out and even after that change upon change was made in this little detail and that. It was not until a year after we went to war that the engine began to be turned out in quantity. There was nothing startlingly new about the engine. It was a composite of a number of other engines, but it was designed to be turned out in enormous quantities, and it was remarkably efficient. It weighed only 825 pounds and it developed over 420 horse-power. Some machines went up as high as 485 horse-power. An airplane engine weighing less than 2 pounds per horse-power is wonderfully efficient. Of course the Liberty was too heavy for a light battle-plane (a heavy machine, no matter how powerful, cannot make sharp turns), but it was excellent for other types of airplanes and large orders for Liberty engines were made by our allies. Of course we made other engines as well, and the planes to carry them. We built large Caproni and Handley-Page machines, and we were developing some remarkably swift and powerful planes of our own when the Germans thought it about time to stop fighting. FLYING BOATS