In just the same way, the only position in which an aeroplane will remain steady is that in which the centre of gravity is exactly under the point of suspension or, in other words, the centre of pressure. For the centre of pressure in the aeroplane is precisely similar to the point of suspension of a pendulum. Let us, then, picture to ourselves an aeroplane flying along on a horizontal course with this happy state of things prevailing. Something we will suppose occurs to upset it with the result that it begins to dive downwards. It is then in the position of sliding downhill and instantly its speed increases in consequence. That increase of speed causes the air to press a little more strongly than it did before upon the front edge of the planes. In other words, the centre of pressure shifts forward a little, with the result that the centre of gravity is then a little to the rear of the centre of pressure. A moment's reflection will show that with the centre of pressure (or point of suspension) in advance of the centre of gravity there is a tendency for the machine to turn upwards again, or, in other words, to right itself. If, on the other hand, the initial upset causes it to shoot upwards the speed instantly falls off and the centre of pressure retreats, turning the machine downwards once more. And the same principle applies whatever the disturbance may be. Instantly and automatically a turning force comes into play which tends to check and ultimately to correct what has gone wrong. This principle explains the behaviour of the card dropped from an upstairs window and, no doubt, as has been said, it operated also in the early flying machines, but in their case other factors caused disturbing elements with which the self-righting tendency was not strong enough to cope. As time went on, however, experience taught the makers how to avoid these disturbing factors until at last the self-righting tendency was able to act effectively, thus producing the aeroplane which is inherently stable and which will, for short periods at all events, fly safely without attention from its pilot. Each little improvement in this direction was an invention. Of course, there were certain men whose names stand out prominently in the history of the aeroplane, notable among whom are the Wright brothers, but the final result is due to innumerable inventions, many of them by unknown men. But perhaps someone will say, how can you possibly talk about final results in a matter which is still in its infancy? The answer to that is that so far as the safe, "flyable" machine is concerned, it has arrived. Little now remains to be done in that direction. Further improvements there will, of course, be, but the great fundamental problems of flight have been solved. CHAPTER XXV THE AERIAL LIFEBOAT