The type B has a rounded under body, the forward end being also skiff-shaped to decrease as much as possible the resistance of the water impact. _Fig. 75. Hydroplane Floats._ The third type C is made in the form of a closed boat, with both ends pointed, and the bottom rounded, or provided with a keel. Or, as in some cases the body may be made triangular in cross section so that as it is submerged its sustaining weight will increase at a greater degree as it is pressed down than its vertical measurement indicates. All this, however, is a matter left to the judgment of the designer, and is, in a great degree, dependent on the character of the craft to which it is to be applied. CHAPTER XII EXPERIMENTAL WORK IN FLYING THE novice about to take his first trial trip in an automobile will soon learn that the great task in his mind is to properly start the machine. He is conscious of one thing, that it will be an easy matter to stop it by cutting off the fuel supply and applying the brakes. CERTAIN CONDITIONS IN FLYING.--In an aeroplane conditions are reversed. Shutting off the fuel supply and applying the brakes only bring on the main difficulty. He must learn to stop the machine after all this is done, and this is the great test of flying. It is not the launching,-- the ability to get into the air, but the landing, that gives the pupil his first shock. Man is so accustomed to the little swirls of air all about him, that he does not appreciate what they mean to a machine which is once free to glide along in the little currents which are so unnoticeable to him as a pedestrian. The contour of the earth, the fences, trees, little elevations and other natural surroundings, all have their effect on a slight moving air current, and these inequalities affect the air and disturb it to a still greater extent as the wind increases. Even in a still air, with the sun shining, there are air eddies, caused by the uneven heating of the air in space. HEAT IN AIR.--Heat is transmitted through the air by what is called convection, that is, the particles of the air transmit it from one point to the next. If a room is closed up tight, and a little aperture provided so as to let in a streak of sunlight, it will give some idea of the unrest of the atmosphere. This may be exhibited by smoke along the line of the sun's rays, which indicates that the particles of air are constantly in motion, although there may be absolutely nothing in the room to disturb it. MOTION WHEN IN FLIGHT.--If you can imagine a small airship floating in that space, you can readily conceive that it will be hurled hither and thither by the motion which is thus apparent to the eye.