850 The Outline of Science 



that multiplication of engines tends to increase safety. This is a 

 debatable point, for very few twin-engine machines are able to 

 fly with only one engine running. Makers frequently claim that 

 the machine will fly with one engine only, but in actual practice 

 with a full load nine twin-engine machines out of ten become 

 unmanageable unless both engines are running or both are cut off. 

 Though aircraft are undoubtedly growing more and more 

 essential as military weapons of defence and offence, there is no 

 doubt that their greatest future lies in civilian spheres. At the 

 present time we are gradually feeling our way, until the time 

 when better machines, better knowledge of the air, better organi- 

 sations, and more public support enable us to cover the earth 

 with a network of airways. There is reasonable hope that in the 

 not distant future all mails will be air-borne and much of the 

 long-distance passenger traffic will be by air. The carriage of 

 heavy goods and short-distance passenger traffic is another mat- 

 ter; it is probable that for many years to come the bulk of this 

 traffic will be carried by older methods of transport. 



Finding the Way in the Air 



The first thing to be done before civil flying becomes an 

 everyday matter is the marking out of the aerial routes which 

 will be used. These should be provided with small emergency 

 landing-grounds at intervals of ten to twenty miles, so that air- 

 craft can always have a clear spot in which to land, no matter 

 what the emergency. These aerodromes must be fitted up with 

 ground lights so that pilots in charge of night-flying machines 

 will have the same advantages as pilots flying by day. There 

 is little need to signpost the air by means of kite balloons or 

 searchlight signals, as has sometimes been suggested, for with 

 the development of wireless for direction finding and of efficient 

 maps, any pilot can find his way with ease. 



There are several methods of finding the way in the air. 

 The first of these is for the pilot to compare the ground over 



