vil FLIGHT 181 
are spoken of as oars, it must be borne in mind that 
they are oars of a peculiar kind. The French are 
fond of the terms “vol ramé” 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. Ifa 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 
- A —_—_—— B 
Ae 
ec O 
Fic. 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 
