308 FLIGHT FLIGHT, FLYING MACHINES. Of the many scien tific problems of modern times, there are few possessing a wider or more enduring interest than that of aerial navi gation. To fly has always been an object of ambition with man; nor will this occasion surprise when we remember the marvellous freedom enjoyed by volant as compared with non-volant animals. The traditions of Diedalus and Icarus illustrate the attempts in the past ; and at the present day societies exist in Britain, France, Austria, and other countries, for the purpose of solving, if possible, the knotty problem. These societies embrace men of the highest scientific attainments, and as they evince great activity, and publish their proceedings at regular intervals, it is not too much to expect that tbe problem of artificial flight will be actually solved, or at least much simplified. For the first time in the history of the world, the subject of aerial navigation has been taken up in earnest by practical men with the necessary degree of preliminary knowledge and training. Investigators no longer dream about flight : they experiment upon and work towards it. It is, they main tain, a physical problem to be solved by mechanical skill and ingenuity. But while writing thus hopefully, it is necessary to state that as yet no one has succeeded in con structing a fully equipped flying machine. Tbe number of successful flying models, however, is so considerable as to inspire the cultivators of aeronautical science with a very confident hope of success. The object of the present article is threefold (1) to demonstrate the laws of natural flight ; (2) to expound the principles on which a flying machine should be constructed ; and (3) to chronicle some of the more important attempts at aerostation in modern times. It is not necessary to enter upon a history of artificial flight. This has already been done in the article AERO NAUTICS, to which the reader is referred. The fact is that aerostation in its modern form dates back only a very few years. It will suffice, then, if the modern discoveries arc recorded in the order of time as we proceed. The subject of aerostation is admitted on all hands to be one of extreme difficulty. To tread upon the air (and this is what is really meant) is, at first sight, in the highest degree Utopian ; and yet there are thousands of living creatures which actually accomplish this feat. These creatures, however varied in form and structure, all fly according to one and the same principle ; and this is a significant fact, as it tends to show that the air must be attacked in a particular way to ensure flight. The flying machine of the future, there can be na doubt, will be constructed on the type of flying animals, the insect, bird, and bat. It behoves us then at the outset to scrutinize very carefully the general configuration of flying animals, and in particular the size, shape, and movements of their flying organs. Flying animals, it may be premised, differ entirely from sailing ships and from balloons, with which they are not unfrequently though erroneously compared ; and a flying machine constructed upon proper principles can have nothing in common with either of those creations. The ship floats upon water and the balloon upon air ; but the ship differs fiom the balloon, and the ship and the balloon differ from the flying creature and flying machine. The water and air, moreover, have characteristics of their own. The analogies which connect the water with the air, the ship with the bal loon, and the ship and the balloon with the flying creature and flying machine are false analogies. A sailing ship is supported by the water and requires merely to be propelled ; a flying creature and a flying machine constructed on the living type require to be both supported and propelled. This arises from the fact that water is much denser than air, and because water supports on its surface substances which fall through air. While water and air are both to be regarded as fluid media, they are to be distinguished from each other in the following particulars. Water is comparatively very heavy, inelastic, and incompressible ; uir, on the other hand, is comparatively very light, elastic, and compressible. If water be struck with violence, the recoil obtained is great when compared with the recoil obtained from air similarly treated. In water we get a maximum recoil with a minimum of displacement ; in air, on the contrary, wo obtain a minimum recoil with a maximum of displacement. FIG. 1. Chillingham Bull (Has Scoticus). Small travelling extremities adapted for land, r, s, t, u, rlgure-of-8 described by the feet in walking. (Pettlgrcw, 1867.) FIG. 2. The Turtle (C/ielonia imbricata). Enlarged travelling extremities (flippers) adapted for water. (Pettigrew, 1867.) FIG. 3 The Bat (Fhyllocina graei/is). Greatly expanded travelling extremities adapted for air. (Pettigrew, 1867.) Water and air when unconfined yield readily to pressure. They thus form movable fulcra to bodies acting upon them. In order to meet these peculiarities the travelling organs of aquatic and flying animals (whether they be feet, fins, flippers, or wings) are made not of rigid but of elastic materials. The travelling organs, moreover, increase in size in proportion to the tenuity of the fluid to be acted upon. The difference in size of the travelling organs of animals becomes very marked when the land animals are contrasted with the aquatic, and the aquatic with the aerial, as in figs. 1, 2, and 3. The peculiarities of water and air as supporting media are well illustrated by a reference to swimming, diving, and flying birds. A bird when swimming extends its feet simultaneously or alternately in a backward direction, and so obtains a forward recoil. The water supports the bird, and the feet simply propel. In this case the bird is lighter than the water, and the long axis of the body is horizontal (a of fig. 4). When the bird dives, or flies under
