THE APPLICATION OF POWER.--The thing now to consider is not form, or shape, or the distribution of the supporting surfaces, but HOW to apply the power so that it will rapidly transfer a machine at rest to one in motion, and thereby get the proper support on the atmosphere to hold it in flight. THE SUPPORTING SURFACES.--This brings us to the consideration of one of the first great problems in flying machines, namely, the supporting surfaces,--not its form, shape or arrangement, (which will be taken up in their proper places), but the area, the dimensions, and the angle necessary for flight. AREA NOT THE ESSENTIAL THING.--The history of flying machines, short as it is, furnishes many examples of one striking fact: That area has but little to do with sustaining an aeroplane when once in flight. The first Wright flyer weighed 741 pounds, had about 400 square feet of plane surface, and was maintained in the air with a 12 horse power engine. True, that machine was shot into the air by a catapult. Motion having once been imparted to it, the only thing necessary for the motor was to maintain the speed. There are many instances to show that when once in flight, one horse power will sustain over 100 pounds, and each square foot of supporting surface will maintain 90 pounds in flight. THE LAW OF GRAVITY.--As the effort to fly may be considered in the light of a struggle to avoid the laws of nature with respect to matter, it may be well to consider this great force as a fitting prelude to the study of our subject. Proper understanding, and use of terms is very desirable, so that we must not confuse them. Thus, weight and mass are not the same. Weight varies with the latitude, and it is different at various altitudes; but mass is always the same. If projected through space, a certain mass would move so as to produce momentum, which would be equal at all places on the earth's surface, or at any altitude. Gravity has been called weight, and weight gravity. The real difference is plain if gravity is considered as the attraction of mass for mass. Gravity is generally known and considered as a force which seeks to draw things to the earth. This is too narrow. Gravity acts in all directions. Two balls suspended from strings and hung in close proximity to each other will mutually attract each other. If one has double the mass it will have twice the attractive power. If one is doubled and the other tripled, the attraction would be increased six times. But if the distance should be doubled the attraction would be reduced to one-fourth; and if the distance should be tripled then the pull would be only one-ninth. The foregoing is the substance of the law, namely, that all bodies attract all other bodies with a force directly in proportion to their mass, and inversely as the square of their distance from one another.