Comparison of flying machines and ships suggests many points of difference. Water is a fluid of great density, with a definite upper surface, on which marine structures naturally rest. A vessel in the air may be at any elevation in the surrounding rarefied fluid, and great attention is necessary to keep it at the elevation desired. The air has no surface. The air ship is like a submarine--the dirigible balloon of the sea--and perhaps rather more safe. An ordinary ship is only partially immersed; the resistance of the fluid medium is exerted over a portion only of its head end: but the submarine or the flying machine is wholly exposed to this resistance. The submarine is subjected to ocean currents of a very few miles per hour, at most; the currents to which the flying machine may be exposed exceed a mile a minute. Put a submarine in the Whirlpool Rapids at Niagara and you will have possible air ship conditions. A marine vessel may _tack_, _i.e._, may sail partially against the wind that propels it, by skillful utilization of the resistance to sidewise movement of the ship through the water: but the flying machine is wholly immersed in a single fluid, and a head wind is nothing else than a head wind, producing an absolute subtraction from the proper speed of the vessel. [Illustration: HOW A BOAT TACKS The wind always exerts a pressure, perpendicular to the sail, which tends to drift the boat sidewise (R) and also to propel it forward (L). Sidewise movement is resisted by the hull. An air ship cannot tack because there is no such resistance to drift.] Aerial navigation is thus a new art, particularly when heavier-than-air machines are used. We have no heavier-than-water _ships_. The flying machine must work out its own salvation. SOARING FLIGHT BY MAN Flying machines have been classified as follows:-- Lighter than Air Fixed balloon, Drifting balloon, Sailing balloon, Dirigible balloon rigid (Zeppelin), ballonetted. Heavier than Air Orthopter, Helicopter, Aeroplane monoplane, multiplane. We will fall in with the present current of popular interest and consider the aeroplane--that mechanical grasshopper--first. What Holds It Up? [Illustration: OCTAVE CHANUTE (DIED 1910)] To the researches of Chanute and Langley must be ascribed much of American progress in aviation.