180 THE STRUCTURE AND LIFE OF BIRDS cunap, 
perhaps, would be the only advantage. But it is quite 
possible to propel a boat in this way, and such a 
system of rowing would illustrate what takes place in 
flight. True, the blades of a bird’s “oars” face 
differently, so that, while they propel him, they at 
the same time raise or maintain him in the air. But 
the system of leverage is the same. This diagram is 
a further illustration. 
Y’ 
Fic. 47. 
X and Z are rigid rods representing the bird’s wings hinged at J” to Y Y’ the bird's 
body ; a4 and c é are the muscles which lower the wings. The shortening of a 
and ¢ 4 will cause Y Y’ to rise, since the air resists the descent of XY and Z. After 
Alix, Appareil Locomotenr des Oiseaux. 
Florizontal Flight. 
Why does a bird advance horizontally when it 
works its wings up and down? The common metaphor 
which makes them oars is picturesque, but may be, as 
I have shown, misleading. Vergil, who, in describing 
Dedalus’ wings, uses the expression “Remigium 
alarum,” doubtless never intended to commit himself 
to any theory of flight. If in a scientific work, wings 
