388 



EVENING DISCOURSES. 



turn indicator, which marks zero when the aeroplane is travelling straight. 

 In the gyro-turn indicator, developed at Farnborough, the principle of which is 

 explained by the diagram Fig. 6, the gyro-wheel is placed outside the machine 



Fig. 5. R. A. E. SEXTANT MARK Ha. 



■^.Mirror 



Lexjnp. 

 BubbleLerxs 



SilverecLFace 



leseo.£j 



-^ 



^Lene 



so as to rotate in a vertical plane at right angles to the direction of flight, 

 the rotation being maintained by the wind playing upon suitable holes in the 

 wheel. If the machine when moving straight horizontally begins to turn about 

 a vertical axis, a pointer attached to the gyrostat indicates tliat motion or, if 

 controlled by a spring, shows the force tending to make the gyrostat move. 



There remains the dangers from mists and fogs, which are more formidable 

 to the aviator than to the sailor. The sailor has fog signals, leader cables. 



»sea^ 



buoys, and harbour lights to guide him; the aviator frequently has little warning 

 that he is within a few feet of the ground. Captive balloons, if feasible, are 

 themselves sources of danger ; sound signals would not be appreciated by the 

 neighbours of aerodromes ; signal beams do not penetrate far through mists ; 

 radio-telegraphy and radio-telephony promise best. But the landing troubles 

 and means of control at low speeds, as well as the construction of reliable aero- 

 engines, remain urgent problem^^s of modern aircraft. 



