The tail corrects the see-saw motion or pitching of a flying-machine in flight. The further back that it is placed the greater will be the steadying effect. If placed too far back, however, a " dead centre " will be reached. If there is no tail the pilot must manipulate the horizontal rudder to check the see-saw motion. The Wrights have taken out a patent for a mechanical device, which maintains fore-and-aft stability automatically. In this device the human brain is supplanted by the pressure of the air on a plane. Compressed air is substituted for muscular action. Lateral stability is automatically maintained by means of a pendu-lum. The plane and pendulum open valves which admit compressed air to an engine operating the horizontal or elevation rudder and warping mechanism.

To relieve the pilot of the physical strain of more or less constantly warping planes or manipulating ailerons, it was suggested long before the day of the Wrights that the flying-machine be provided with some automatic device which would prevent any capsizing tendency. The more important of such appliances are moving weights, pendulums, and gyrostats. A gyrostat is any rapidly rotating body, which, by virtue of its rotation, resists any force tending to move it from its plane of rotation. The greater the weight and the higher the speed of the gyrostat the greater must be the force expended to shift it from its plane of rotation. Hence if a gyrostat could be mounted on an aeroplane it certainly would tend to resist any unbalancing force, such as a gust of wind. Paul Regnard in France is said to have conducted very successful experiments with gyrostatically controlled aeroplanes. Roberts in England has also made more or less encouraging tests. In his machine the gyrostat is applied as shown in Fig. 37.

The pilots of present machines object to any device that will relieve them entirely of all hand control. They would much prefer an automatic device which is immediately thrown out of operation when the hand-devices are manipulated. It is argued that a machine must be humoured, that with an automatic device such as the gyrostat, it is impossible to accommodate the machine to variations in the wind.

Moreover, there is the objection that the machine must be elevated rapidly in starting, with a fairly large angle of incidence, but must afterwards assume a fairly flat angle for horizontal flight, with all of which a steadily running gyrostat would seriously interfere. Soaring down a steep angle with motors at full speed could hardly be accomplished with a gyrostat running at a fixed rate, for it is the gyrostat's tendency to resist movement. Besides, there is always the possibility that the motor which drives the gyrostat may stop, so that the aviator is helpless if no hand-controlled devices enable him to prevent rocking from side to side and pitching fore and aft.

The pendulum, as we have seen, has been suggested by the Wrights as well as by other inventors to relieve the aviator of his present duties. The underlying idea is that a freely suspended weight will always tend to hang down, and that it would be an easy matter to connect with it elevation rudders and ailerons, in such a manner that pitching fore and aft, or rocking from side to side could be controlled by the effort of the pendulum to assume a perfectly normal position relatively to the earth. The pendulum, however, will hardly be likely to attain the desired end. It cannot control a flying-machine automatically, as Professor Prandtl has pointed out. The very force which causes an aeroplane to change its horizontal position in flight also retards it, accelerates it, or inclines it from side to side. Consequently a pendulum, which has the momentum of the entire machine, will follow the direction of the aeroplane's inclination and, so far from hanging down, will deviate from the vertical. The result will be, curiously enough, that it will always maintain its position relatively to the planes, whatever their inclination fore-and-aft and side-to-side may be. Hence the pendulum is inoperative. Furthermore, when an aeroplane is rounding a curve at the rate of from forty to sixty miles an hour, centrifugal force would completely nullify the action of the pendulum.

If a gyrostat is to be used, it is likely that it will be combined with some system of hand control, so that the aviator can depend upon the one or the other, as circumstances may dictate.

But how this combination would really improve the situation it is difficult to see. Automatic control is necessarily complicated. Hand control is admittedly dependent upon a cool head and an expert hand. Moreover, an automatic device must be made as small and as light as possible, for the aeroplane as it now stands is a machine in which the weight of every part has been reduced to a minimum. Can control mechanism, dependent upon a gyrostat, be made sufficiently light to meet the requirements of present construction? The more one considers the question, the more likely are we to believe that the best automatic machine is a well-trained hand.

Any one who has seen a skilled man steering a small boat in a heavy sea must realise that there is no possibility of making any automatic device which would take his place, and that any attempt to make the steering automatic by such means as a gyrostat would mean the certain swamping of the boat in a sea through which it could be steered quite simply by hand. The aeroplane is very much in the position of this small boat.

The danger of hand control is to be found in the possibility of making a false move. Locomotive engineers, signal men, automobile chauffeurs, are all of them in a position where a false move means a bad accident. Yet for all that, the number of errors which are made is comparatively small. A ship is dependent upon the engines and skill of the men in the pilot house; yet it is but rarely that we hear of shipwrecks due to bad judgment in the wheel house. All things considered, it is very likely that aeroplanes will be hand controlled for years to come.