FLYING AND FLYING-MACHINES. 327 



true, however, that the flight of the wet sponge exem- 

 plifies the kind of flight which the aeronaut requires. 

 The sponge, in fact, was neither more nor less than a 

 projectile ; and most assuredly, the problem of flight 

 is not to be solved by making projectiles of our flying- 

 machines or of our own bodies. It may be, and 

 indeed we shall presently see that it probably will be 

 necessary, that some form of propulsion from a fixed 

 stand should have to be applied to the flying-machines 

 of the future. But after such propulsion has been 

 applied, the flying-machine must be supported in 

 some way, not left as an ordinary projectile is left 

 to the action of unresisted gravity. M. Nadar's wet 

 sponge is no analogue, then, of the flying-machines we 

 require. 



Before leaving the subject of buoyancy, however, it 

 will be desirable to inquire whether buoyancy is, in 

 any marked degree, an attribute of the flying creatures 

 we are acquainted with birds, bats, and insects. The 

 structure of such creatures has been supposed by some 

 to be such as to secure actual buoyancy, to a greater 

 or less degree ; and many would be disposed, at a first 

 view of the matter, to regard the hollow bones and the 

 quill-feathers of birds as evidences that buoyancy is 

 essential to flight. I have even seen the strange 

 theory put forward, that during life, the quills of birds, 

 as well as their hollow bones, are filled with hydrogen. 

 ' Flying animals,' says a writer in All the Year Round 

 for March 7, 1868, 'are built to hold gases every- 

 where in their bones, their bodies, their skins ; and 



