x PRACTICAL PURPOSE 285 



in the future, even more than in the past, not only the prosperity, 

 but even the existence of the Empire will be found to depend 

 upon the " improvement of Natural Knowledge " that is, upon 

 the more complete application of scientific knowledge and 

 methods to every department of industrial and national activity. 

 Sir William Hnqgins. 



It is commonly supposed that the marvellous develop- 

 ment of aviation within recent years owes nothing to 

 scientific work ; indeed, the assertion is often made 

 unjustly so that men of science declared the flight 

 of an aeroplane to be a mathematical impossibility. 

 Aviation engineers have certainly had few scientific 

 principles to guide them in the design of their machines, 

 and the improvements which have been effected have 

 been by trial-and-error methods ; but the error has 

 unfortunately involved the sacrifice of many promising 

 lives. Artificial flight has been achieved chiefly by these 

 empirical methods ; and in the absence of exact know- 

 ledge they are the only methods available, though they 

 are expensive and wasteful. 



As in other cases mentioned already, the problem of 

 flight with heavier-than-air machines was approached 

 by two separate roads of invention and science. Early 

 in the nineteenth century, Sir George Cayley designed 

 an aeroplane driven by an . engine which used the 

 explosive force of gunpowder ; and the machine appears 

 to have lifted itself from the earth. Nearly forty years 

 later, W. S. Henson projected, and his friend J. String- 

 fellow constructed, model aeroplanes from Cayley's 

 designs, driven by a light steam-engine of about one- 

 third horse-power ; and the models are said to have been 

 capable of free flight. About 1875 Alphonse Penaud, 

 a French mechanic, constructed the well-known toy 

 aeroplane with a propeller in the rear and driven by a 



