VII FLIGHT 193 
sault. Then put in the pin with its head near B and’ 
drop it as before, but make B the lower end. It will 
glide by a steep descent to the ground. It is worth 
while improving the cut and balance of these little toys 
till they behave properly, for they admirably illustrate 
gliding flight: when A leads, a bird with wide-spread 
wings is represented ; when B, a bird with wings 
partly flexed. 
Before closing this subject I must refer to the old 
fallacy that the bird, owing to its hollow bones, is 
like a balloon. This has been already dealt with in 
the previous chapter (see p. 105). 
The General Shape of the Bird. 
If a ship were built on absolutely perfect lines, when 
she lay at anchor and swung to the tide, the pressure 
of the stream upon the bows would be exactly balanced 
by the closing of the waters again at the stern, so 
that the only force straining at the cable would be due 
to friction. The case would be far differrent if the 
vessel presented a flat surface to the current at right 
angles to it. The lines on which a bird is built are 
first-rate: its body is not unlike in shape to that of a 
fish, and investigations have shown. that fish are con- 
structed so as to meet the minimum of resistance in 
passing through a fluid. 
