VII 



FLIGHT 



arc spoken of as oars, it must be borne in mind that 

 they arc oars of a peculiar kind. The French arc 

 fond of the terms "vol rame " and "vol a voiles," 

 which have the merit of neatly distinguishing ordinary 

 from sailing flight. 



Wings work by movement up and down. If they 

 faced as the blades of an oar face, they would be 

 useless for flight at the rate of fifty miles an hour, since 

 the stroke would be over before any force could be 

 put into it. If a boat is moving rapidly— at the rate, 

 say, of a mile in five minutes— its pace alone brings the 

 oar in quick to the oarsman's chest, even if he puts 



Fig. 48. 



little force into it. Hence the importance of " getting 

 on at the beginning," to use the language of rowing 

 "coaches," otherwise the happy moment is missed. 

 Rowing, then, is out of the question for a bird which 

 is to move at a speed far greater than that of any 



boat. 



How, then, is horizontal movement gained ? To 

 understand this a knowledge of the parallelogram of 

 forces is necessary. The proof of the principle involved 

 will be found in any book on elementary mechanics. 

 I shall merely try to make clear what is meant. If 

 two forces act upon a body at one point, they combine 

 to make one force. If B A and C A are two forces 



