DIFFERENT SPEEDS WITH SAME POWER.--Now, we may be able to do two things with an airship under those conditions. It may be propelled through the air thirty miles an hour, or sixty miles, with the expenditure of the same power. An automobile, if propelled at sixty, instead of thirty miles an hour, would require an additional power in doing so, but an airship acts differently, within certain limitations. When it is first set in motion its effective pull may not be equal to four pounds for each horse power, due to the slow speed of the propeller, and also owing to the great angle of incidence which resists the forward movement of the ship. INCREASE OF SPEED ADDS TO RESISTANCE.--Finally, as speed increases, the angle of the planes decrease, resistance is less, and up to a certain point the pull of the propeller increases; but beyond that the vacuum behind the blades becomes so great as to bring down the pull, and there is thus a balance,--a sort of mutual governing motion which, together, determine the ultimate speed of the aeroplane. HOW POWER DECREASES WITH SPEED.--If now, with the same propeller, the speed should be doubled, the ship would go no faster, because the bite of the propeller on the air would be ineffective, hence it will be seen that it is not the amount of power in itself, that determines the speed, but the shape of the propeller, which must be so made that it will be most effective at the speed required for the ship. While that is true when speed is the matter of greatest importance, it is not the case where it is desired to effect a launching. In that case the propeller must be made so that its greatest pull will be at a slow speed. This means a wider blade, and a greater pitch, and a comparatively greater pull at a slow speed. No such consideration need be given to an automobile. The constant accretion of power adds to its speed. In flying machines the aviator must always consider some companion factor which must be consulted. HOW TO CALCULATE THE POWER APPLIED.--In a previous chapter reference was made to a plane at an angle of forty-five degrees, to which two scales were attached, one to get its horizontal pull, or drift, and the other its vertical pull, or lift. PULLING AGAINST AN ANGLE.--Let us take the same example in our aeroplane. Assuming that it weighs 900 pounds, and that the angle of the planes is forty-five degrees. If we suppose that the air beneath the plane is a solid, and frictionless, and a pair of scales should draw it up the incline, the pull in doing so would be one-half of its weight, or 450 pounds. It must be obvious, therefore, that its force, in moving downwardly, along the surface A, Fig. 60, would be 450 pounds. The incline thus shown has thereon a weight B, mounted on wheels a, and the forwardly-projecting cord represents the power, or propeller pull, which must, therefore, exert a force of 450 pounds to keep it in a stationary position against the surface A.