One such form is illustrated in Fig. 74, which represents a cylindrical shell A, which has at each end a head of concentrically formed corrugations. These heads are securely fixed to the ends of the shell A. Within, one of the disk heads has a short stem C, which is attached to the short end of a lever D, this lever being pivoted at E. The outer end of this lever is hinged to the short end of another lever F, and so by compounding the levers, it will be seen that a very slight movement of the head B will cause a considerable movement in the long end of the lever F. This end of the lever F connects with one limb of a bell-crank lever G, and its other limb has a toothed rack connection with a gear H, which turns the shaft to which the pointer I is attached. Air is withdrawn from the interior of the shell, so that any change in the pressure, or weight of the atmosphere, is at once felt by the disk heads, and the finger turns to indicate the amount of pressure. HYDROPLANES.--Hydro means water, hence the term hydroplane has been given to machines which have suitable pontoons or boats, so they may alight or initiate flight from water. There is no particular form which has been adopted to attach to aeroplanes, the object generally being to so make them that they will sustain the greatest amount of weight with the least submergence, and also offer the least resistance while the motor is drawing the machine along the surface of the water, preparatory to launching it. SUSTAINING WEIGHT OF PONTOONS.--A pontoon having within nothing but air, is merely a measuring device which determines the difference between the weight of water and the amount placed on the pontoon. Water weighs 62 1/2 pounds per cubic foot. Ordinary wood, an average of 32 pounds, and steel 500 pounds. It is, therefore, an easy matter to determine how much of solid matter will be sustained by a pontoon of a given size, or what the dimensions of a pontoon should be to hold up an aeroplane which weighs, with the pilot, say, 1100 pounds. As we must calculate for a sufficient excess to prevent the pontoons from being too much immersed, and also allow a sufficient difference in weight so that they will keep on the surface when the aeroplane strikes the surface in alighting, we will take the figure of 1500 pounds to make the calculations from. If this figure is divided by 62 1/2 we shall find the cubical contents of the pontoons, not considering, of course, the weight of the material of which they are composed. This calculation shows that we must have 24 cubic feet in the pontoons. As there should be two main pontoons, and a smaller one for the rear, each of the main ones might have ten cubic feet, and the smaller one four cubic feet. SHAPES OF THE PONTOONS.--We are now ready to design the shapes. Fig. 75 shows three general types, A being made rectangular in form, with a tapering forward end, so constructed as to ride up on the water.