So long as a balloon sails only with the breeze it offers no resistance to the force of the wind, and can be made of the lightest and thinnest material. But directly it has to face the wind, and fight its way against it as an airship must do, then it has to be made of sufficient strength and rigidity to withstand the wind’s power, or it will be blown to pieces. To make so large a thing as an airship withstand a rough wind, it must be built of very strong and rigid materials. To do this means to add to the weight of the machine. To lift the increased weight, a larger machine which can hold more gas is needed. The larger the machine the more surface it offers to the wind, and the stronger therefore must be its construction. It will now be seen that we are arguing in a circle, and we can understand that a point must be reached in the making of airships when, with our present materials, the advantage gained by increase of strength will be more than counterbalanced by increased weight. On this point Sir Hiram Maxim says: “It is not possible to make a balloon, strong enough to be driven through the air at any considerable speed, at the same time light enough to rise in the air; therefore balloons must always be at the mercy of a wind no greater than that which prevails at least 300 days in the year;” adding, “Those who seek to navigate the air by machines lighter than air have, I think, come practically to the end of their tether.” With the flying machine, on the contrary, the same difficulty does not arise. Since it is at all times heavier than air, and is kept aloft simply by its motive power and mechanism, its weight is of no consequence, provided only its engine is sufficiently powerful. It may, therefore, be built as rigidly as need be, while, from its size--which is much smaller in proportion to its lifting power than in the case of the airship--and also from its construction, it is much less liable to be affected by the wind. In constructing a flying machine which is heavier than air the inventor has before him two examples of bodies which, though heavier than the atmosphere, yet contrive to rise upwards into the sky; these are, firstly, birds, and secondly, the familiar schoolboy toys, kites. To imitate the flying powers of birds and kites, he must first understand the means by which their flight is accomplished; and he will find, on examination, that to a large extent the same principle underlies each--the principle of what is termed the “aeroplane.” [Illustration: KESTREL.]