In aviation, as we know it to-day, aeroplane builders believe in giving their planes a slight arch upward and backward from the entering edge, letting it reach its highest point about one third of the way back and then letting it slope down to the level of the rear edge gradually. This curve, which is called the camber, is mathematically figured out with the most painstaking care, and was one of the things the Wright brothers worked out very carefully in their early models. Also, planes are driven through the air at an angle--that is, with the entering edge higher than the rear edge--because the upward tilt gives the air current a chance to get under the plane and support it. This angle is called by the scientists the angle of incidence and is very important because of its relation to the lifting powers of the planes. [Illustration: MODEL AEROPLANE FLIERS Every fair Saturday the model makers and fliers spend in the parks either practising for or holding flight tournaments.] [Illustration: A MODERN COLLEGE MAN'S GLIDER] [Illustration: OTTO LILIENTHAL MAKING A FLIGHT IN HIS GLIDER] Another one of the difficult problems the inventors had to struggle with was the balance of their fliers. Before the Wright brothers flew, it was thought that one of the best ways was to incline the planes upward from the centre--that is--make them in the shape of a gigantic and very broad V. This is known in science as a dihedral angle. The idea was that the centre of gravity, or the point of the machine which is heaviest and which seeks to fall to earth first through the attraction of gravitation, should be placed immediately under the apex of the V. The scientists thought that the V then would keep the machine balanced as the hull of a ship is balanced in the water by the heavy keel at the bottom. The Wrights decided that this might be true from a scientific point of view, but that the dihedral angle kept the machine wobbling, first to one side and then righting itself, and then to the other side and righting itself. This was a practical fault and they built their flier without any attempt to have it right itself, but rather arched the planes from tip to tip as well as from front to rear.