Another mechanical means of getting up into the air was suggested by the flying of kites, a pastime dating back at least 2,000 years, perhaps longer. Ordinarily, a kite will not fly in a calm, but with even a little breeze it will mount into the air by the upward thrust of the rushing breeze against its inclined surface, being prevented from blowing away (drifting) by the pull of the kite-string. The same effect will be produced in a dead calm if the operator, holding the string, runs at a speed equal to that of the breeze--with this important difference: not only will the kite rise in the air, but it will travel in the direction in which the operator is running, a part of the energy of the runner’s pull upon the string producing a forward motion, provided he holds the string taut. If we suppose the pull on the string to be replaced by an engine and revolving propeller in the kite, exerting the same force, we have exactly the principle of the aeroplane. As it is of the greatest importance to possess a clear understanding of the natural processes we propose to use, let us refer to any text-book on physics, and review briefly some of the natural laws relating to motion and force which apply to the problem of flight: (_a_) Force is that power which changes or tends to change the position of a body, whether it is in motion or at rest. (_b_) A given force will produce the same effect, whether the body on which it acts is acted upon by that force alone, or by other forces at the same time. (_c_) A force may be represented graphically by a straight line--the point at which the force is applied being the beginning of the line; the direction of the force being expressed by the direction of the line; and the magnitude of the force being expressed by the length of the line. (_d_) Two or more forces acting upon a body are called component forces, and the single force which would produce the same effect is called the resultant. (_e_) When two component forces act in different directions the resultant may be found by applying the principle of the parallelogram of forces--the lines (_c_) representing the components being made adjacent sides of a parallelogram, and the diagonal drawn from the included angle representing the resultant in direction and magnitude. (_f_) Conversely, a resultant motion may be resolved into its components by constructing a parallelogram upon it as the diagonal, either one of the components being known. [Illustration: The Deutsch de la Muerthe dirigible balloon _Ville-de-Paris_; an example of the “cigar-shaped” gas envelope.]