By speed in this connection is not meant high velocity, but that a movement, produced by power expressed in some form, is the sole and most necessary requisite to movement through the air with all heavier-than-air machines. If sufficient power can be applied to an aeroplane, surface is of no consequence; shape need not be considered, and any sort of contrivance will move through the air horizontally. CONTROL OF THE DIRECTION OF FLIGHT.--But the control of such a body, when propelled through space by force alone, is a different matter. To change the machine from a straight path to a curved one, means that it must be acted upon by some external force. We have explained that power is something which is inherent in the thing itself. Now, in order that there may be a change imparted to a moving mass, advantage must be taken of the medium through which it moves,--the atmosphere. VERTICAL CONTROL PLANES.--If vertically-arranged planes are provided, either fore or aft of the machine, or at both ends, the angles of incidence may be such as to cause the machine to turn from its straight course. In practice, therefore, since it is difficult to supply sufficient power to a machine to keep it in motion horizontally, at all times, aeroplanes are provided with supporting surfaces, and this aid in holding it up grows less and less as its speed increases. But, however strong the power, or great the speed, its control from side to side is not dependent on the power of the engine, or the speed at which it travels through the air. Here the size of the vertical planes, and their angles, are the only factors to be considered, and these questions will be considered in their proper places. CHAPTER III THE FORM OR SHAPE OF FLYING MACHINES EVERY investigator, experimenter, and scientist, who has given the subject of flight study, proceeds on the theory that in order to fly man must copy nature, and make the machine similar to the type so provided. THE THEORY OF COPYING NATURE.--If such is the case then it is pertinent to inquire which bird is the proper example to use for mechanical flight. We have shown that they differ so radically in every essential, that what would be correct in one thing would be entirely wrong in another. The bi-plane is certainly not a true copy. The only thing in the Wright machine which in any way resembles the bird's wing, is the rounded end of the planes, and judging from other machines, which have square ends, this slight similarity does not contribute to its stability or otherwise help the structure. The monoplane, which is much nearer the bird type, has also sounded wing ends, made not so much for the purpose of imitating the wing of the bird, as for structural reasons.