In an aeroplane the upward pressure, or “lift,” is increased by giving the wings a slightly arched shape, or “camber.” The air flows over the arched wings in such a way as to produce a suction above them which helps the push from below. The actual amount of lift for a given speed has been determined by experiments for wings of various shapes and sizes and set at various angles to the line of motion. If, when the machine is in the air, the lift is just sufficient to counterbalance the weight of the aeroplane, the latter flies horizontally. An increase in lift causes the machine to rise; a decrease in lift permits gravity to pull it down. Now suppose the aviator is flying horizontally and wishes to climb. At the rear of the machine and forming part of its tail is a hinged horizontal flap called the “elevator,” under the control of the pilot. By giving this flap an upward tilt he causes the air to exert a downward pressure on the tail of the machine, and hence the nose of the machine is carried upward. While the inertia of the aeroplane tends to carry it along the original path, its wings now present a greater angle to the air, the lift is increased, and the machine rises. The reverse of this operation will cause the machine to descend. [Illustration: THE BED OF POTOMAC RIVER, AT WASHINGTON. From an altitude of a few hundred or a few thousand feet, submarine features are clearly revealed to great depths. Objects have thus been photographed 45 feet under water. The shoals are submerged to a depth of from 2 to 5 feet. In favorable weather, aerial photographs are valuable in making hydrographic surveys. (_Photographed from the air by Dr. W. T. Lee, U. S. Geological Survey._)] A vertically hinged flap in the tail, acting on exactly the same principle as the rudder of a ship, enables the pilot to turn horizontally. Two or more small horizontal flaps, known as “ailerons,” attached to the wings, are used to preserve the lateral balance of the machine, and to give it the proper “bank,” or inclination, when making a turn. [Illustration: DRILLING WITH COMPRESSED AIR IN A COPPER MINE. The drill also forces a stream of water into the hole to lay the dangerous sulphur-bearing dust. (_Courtesy Sullivan Machinery Co._)] [Illustration: LAUNCHING A WEATHER BUREAU KITE FROM THE “SENECA” DURING THE INTERNATIONAL ICE PATROL, TO EXPLORE THE AIR OVER THE OCEAN. (_Photograph, U. S. Weather Bureau._)] With these few details in mind, we shall be prepared to consider, in a general way, how the behavior of an aeroplane is affected by the wind and other atmospheric phenomena.