_Helicopter_ is used to designate machines which are lifted vertically and sustained in the air by propellers revolving in a horizontal plane, as distinguished from the propellers of the aeroplane, which revolve in vertical planes. A _parachute_ is an umbrella-like contrivance by which an aeronaut may descend gently from a balloon in mid-air, buoyed up by the compression of the air under the umbrella. For the definition of other and more technical terms the reader is referred to the carefully prepared Glossary toward the end of the book. Chapter II. THE AIR. Intangibility of air--Its substance--Weight--Extent--Density--Expansion by heat--Alcohol fire--Turbulence of the air--Inertia--Elasticity--Viscosity--Velocity of winds--Aircurrents--Cloud levels--Aerological stations--High altitudes--Practical suggestions--The ideal highway. The air about us seems the nearest approach to nothingness that we know of. A pail is commonly said to be empty--to have nothing in it--when it is filled only with air. This is because our senses do not give us any information about air. We cannot see it, hear it, touch it. When air is in motion (wind) we hear the noises it makes as it passes among other objects more substantial; and we feel it as it blows by us, or when we move rapidly through it. We get some idea that it exists as a substance when we see dead leaves caught up in it and whirled about; and, more impressively, when in the violence of the hurricane it seizes upon a body of great size and weight, like the roof of a house, and whisks it away as though it were a feather, at a speed exceeding that of the fastest railroad train. In a milder form, this invisible and intangible air does some of our work for us in at least two ways that are conspicuous: it moves ships upon the ocean, and it turns a multitude of windmills, supplying the cheapest power known. That this atmosphere is really a fluid ocean, having a definite substance, and in some respects resembling the liquid ocean upon which our ships sail, and that we are only crawling around on the bottom of it, as it were, is a conception we do not readily grasp. Yet this conception must be the foundation of every effort to sail, to fly, in this aerial ocean, if such efforts are to be crowned with success. As a material substance the air has certain physical properties, and it is the part of wisdom for the man who would fly to acquaint himself with these properties. If they are helpful to his flight, he wants to use them; if they hinder, he must contrive to overcome them. In general, it may be said that the air, being in a gaseous form, partakes of the properties of all gases--and these may be studied in any text-book on physics, Here we are concerned only with those qualities which affect conditions under which we strive to fly.