That is one of the most exasperating and human things about a gasolene-engine. It is efficient, but not thoroughly dependable. The best of them are liable to break down at the most needed moment, due to a hundred causes outside of the control of a mechanic or pilot. Care and rigid inspection will reduce the possibilities, but engine failure cannot yet be eliminated. That is one of the principal reasons why the roofs of buildings around big cities are so dangerous. The sides of a building drop away from the roof. An error in judgment and the machine is over the edge. It is even more dangerous to take-off. An airplane motor is ten times as likely to develop a weakness while it is cold. A motor starting a flight is never well warmed up, and fifty feet from the edge of the roof it may give out, with awful consequences. As a practicable thing, roofs are at present impossible. There is not a flying-officer in the world who will not agree. An interesting series of experiments has been carried out in England on what has been known as the helicopter machine. This machine is not dependent upon speed to fly, but merely on engine power applied through a propeller of great pitch. The idea is not new, but is along the lines specified by Orville Wright when he said that a kitchen table could fly if it had a good enough engine. The effort is being made to make a machine which can hover, can hold itself in the air by brute force of its propeller blades beating the air. The thing sounds impossible to adapt, say some aeronautical engineers. Those who have seen the experiments, however, express great optimism. A machine of this sort would land and take-off in a very small space, and might be adapted to use around cities. It might even make flying over cities safe but for the human equation of the engine again. This machine is dependent on engine power. Apparently there would be two engines, or two driving mechanisms, one operating the lifting propeller and the other the pulling propeller. For the present the great need is for landing-fields as near the heart of most American cities as possible. There should be quick transportation to the business section provided, as well as hangars and mechanics. When that is done we may very well say that aerial transportation for passengers and freight is an accomplished fact. VIII THE AIRPLANE'S BROTHER