When in motion, therefore, while the lift, so far as its mass is concerned, does not change, the drift does decrease, or the forward pull is less than when at 45 degrees, and the decrease is less and less until the plane assumes a horizontal position, where it is absolutely nil, if we do not consider head resistance. TABLES OF LIFT AND DRIFT.--All tables of Lift and Drift consider only the air pressures. They do not take into account the fact that momentum takes an important part in the translation of an object, like a flying machine. A mass of material, weighing 1000 pounds while at rest, sets up an enormous energy when moving through the air at fifty, seventy-five, or one hundred miles an hour. At the latter speed the movement is about 160 feet per second, a motion which is nearly sufficient to maintain it in horizontal flight, independently of any plane surface. Such being the case, why take into account only the angle of the plane? It is no wonder that aviators have not been able to make the theoretical considerations and the practical demonstrations agree. WHY TABLES OF LIFT AND DRIFT ARE WRONG.-- A little reflection will show why such tables are wrong. They were prepared by using a plane surface at rest, and forcing a blast of air against the plane placed at different angles; and for determining air pressures, this is, no doubt, correct. But it does not represent actual flying conditions. It does not show the conditions existing in an aeroplane while in flight. To determine this, short of actual experiments with a machine in horizontal translation, is impossible, unless it is done by taking into account the factor due to momentum and the element attributable to the lift of the plane itself due to its impact against the atmosphere. LANGLEY'S LAW.--The law enunciated by Langley is, that the greater the speed the less the power required to propel it. Water as a propelling medium has over seven hundred times more force than air. A vessel having, for instance, twenty horse power, and a speed of ten miles per hour, would require four times that power to drive it through the water at double the speed. The power is as the square of the speed. With air the conditions are entirely different. The boat submergence in the water is practically the same, whether going ten or twenty miles an hour. The head resistance is the same, substantially, at all times in the case of the boat; with the flying machine the resistance of its sustaining surfaces decreases. Without going into a too technical description of the reasoning which led to the discovery of the law of air pressures, let us try and understand it by examining the diagram, Fig. 7. A represents a plane at an angle of 45 degrees, moving forwardly into the atmosphere in the direction of the arrows B. The measurement across the plane vertically, along the line B, which is called the sine of the angle, represents the surface impact of air against the plane.