The pilot has various rudders by which he steers parts of his machine independently of the other parts. If he finds the left-hand side of his machine dipping, he can steer the side up, or _vice versa_. In this way he has at his command the means of correcting any tendency to ‘heel over’ to one side or the other, or to ‘pitch skyward.’ But without a natural tendency on the part of the machine to keep a safe angle, such precautions would, of course, be futile. It has been said that the bicycle affords a good illustration. ‘The rider of a bicycle instinctively balances himself on his machine, but it would be exceedingly difficult for him to do so were it not for the fact that a rolling wheel tends of itself to keep upright.’ As regards air pressure, a little thought will show that when a machine is moving along horizontally in the air the upward pressure must be equal to the downward pull of gravity. Consequently, a machine travelling steadily through the air has been likened to a pendulum. ‘It is just as if the machine,’ writes Mr. Thomas Corbin, author of _Aircraft_, ‘were suspended upon a point at the centre of pressure. And just as a pendulum always hangs, when it is steady, with its centre of gravity exactly under the point of support, so the flying machine hangs with its centre of gravity exactly under the centre of pressure.’ The designer and user of an aeroplane have, therefore, so to arrange surfaces and weights that when the machine is in the right position of horizontal flight the centre of gravity and the centre of pressure will be in the same vertical line. Suppose, for instance, that the machine tips forward and tends to dive downward; the centre of pressure is thrown forward, though of course the centre of gravity remains still. In such a case the natural righting tendencies of the machine come into operation, causing it to steer upward and so right itself. On the other hand, if the machine tries to deflect upward the very opposite happens. The only pose in which the machine is stable is when it is moving horizontally. As we turn from the horizontal to the vertical, the effective surface of the plane diminishes, but when turning from the vertical towards the horizontal it increases. When the machine tips to the left the effective area of its right hand half diminishes, whilst that of the left hand half increases. Similar action will take place if the machine tips over to the other side, but whichever way it tips the self-righting tendency brings it back. And so we see that an aeroplane is far safer than is thought by many persons. But a great deal, as we have seen, rests with the pilot; in his hands is the general system of control.