Some monoplanes are built so they have a support on wheels placed fore and aft. In others the tail is supported by curved skids, as shown at A, Fig. 59, in which case the forward supporting wheels are located directly beneath the planes. As the planes are at about eighteen degrees angle, relative to the frame, and the tail plane B is at a slight negative angle of incidence, as shown at the time when the engine is started, the air rushing back from the propeller, elevates the tail, and as the machine moves forwardly over the ground, the tail raises still higher, so as to give a less angle of incidence to the planes while skimming along the surface of the ground. _Fig. 59. Side Elevation, Monoplane._ In order to mount, the tail is suddenly turned to assume a sharp negative angle, thus swinging the tail downwardly, and this increases the angle of planes to such an extent that the machine leaves the ground, after which the tail is brought to the proper angle to assure horizontal flight. The drawing shows a skid at the forward end, attached to the frame which carries the wheels. The wheels are mounted beneath springs so that when the machine alights the springs yield sufficiently to permit the skids to strike the ground, and they, therefore, act as brakes, to prevent the machine from traveling too far. CHAPTER X POWER AND ITS APPLICATION THIS is a phase of the flying machine which has the greatest interest to the boy. He instinctively sees the direction in which the machine has its life,--its moving principle. Planes have their fascination, and propellers their mysterious elements, but power is the great and absorbing question with him. We shall try to make its application plain in the following pages. We have nothing to do here with the construction and operation of the motor itself, as, to do that justice, would require pages. FEATURES IN POWER APPLICATION.--It will be more directly to the point to consider the following features of the power and its application: 1. The amount of power necessary. 2. How to calculate the power applied. 3. Its mounting. WHAT AMOUNT OF POWER IS NECESSARY.--In the consideration of any power plant certain calculations must be made to determine what is required. A horse power means the lifting of a certain weight, a definite distance, within a specified time. If the weight of the vehicle, with its load, are known, and its resistance, or the character of the roadway is understood, it is a comparatively easy matter to calculate just how much power must be exerted to overcome that resistance, and move the vehicle a certain speed. In a flying machine the same thing is true, but while these problems may be known in a general way, the aviator has several unknown elements ever present, which make estimates difficult to solve.