The air, though invisible to the eye, has mass and therefore offers resistance to all moving bodies. Therefore air-mass and air resistance are the first principles to be taken into consideration in the construction of an aeroplane. It must be built so that the air-mass will sustain it and the motor, and the motor must be of sufficient power to overcome the air resistance. A ship ploughing through the waves presents the line of least resistance to the water and so is shaped somewhat like a fish, the natural denizen of that element. It is different with the aeroplane. In the intangible domain it essays to overcome, there must be a sufficient surface to compress a certain volume of air to sustain the weight of the machinery. The surfaces in regard to size, shape, curvature, bracing and material, are all important. A great deal depends upon the curve of the surfaces. Two machines may have the same extent of surface and develop the same rate of speed, yet one may have a much greater lifting power than the other, provided it has a more efficient curve to its surface. Many people have a fallacious idea that the surfaces of an aeroplane are planes and this doubt less arises from the word itself. However, the last syllable in _aeroplane_ has nothing whatever to do with a flat surface. It is derived from the Greek _planos_, wandering, therefore the entire word signifies an air wanderer. The surfaces are really aero curves arched in the rear of the front edge, thus allowing the supporting surface of the aeroplane in passing forward with its backward side set at an angle to the direction of its motion, to act upon the air in such a way as to tend to compress it on the under side. After the surfaces come the rudders in importance. It is of vital consequence that the machine be balanced by the operator. In the present method of balancing an aeroplane the idea in mind is to raise the lower side of the machine and make the higher side lower in order that it can be quickly righted when it tips to one side from a gust of wind, or when making angle at a sudden turn. To accomplish this, two methods can be employed. 1. Changing the form of the wing. 2. Using separate surfaces. One side can be made to lift more than the other by giving it a greater curve or extending the extremity. In balancing by means of separate surfaces, which can be turned up or down on each side of the machine, the horizontal balancing rudders are so connected that they will work in an opposite direction--while one is turned to lift one side, the other will act to lower the other side so as to strike an even balance. The motors and propellers next claim attention. It is the motor that makes aviation possible. It was owing in a very large measure to the introduction of the petrol motor that progress became rapid. Hitherto many had laid the blame of everything on the motor. They had said,--"give us a light and powerful engine and we will show you how to fly."