328 THE BORDERLAND OF SCIENCE. 



as their blood is several degrees warmer than the blood 

 of walking or running animals, their gases are probably 

 several degrees lighter. Azote, or hydrogen, or what- 

 ever the gas held in the gaseous structures may be, it 

 is proportionately warmer, and therefore proportion- 

 ately lighter than air.' 



But it appears to me that on a careful consideration 

 of the structure of flying creatures, the hollow portions 

 of their bodies will be found to fulfil a purpose quite 

 distinct from that of imparting buoyancy. If we 

 examine a quill we find that the most remarkable 

 feature which it presents to us, is the proportion 

 which its strength, especially as respects resistance to 

 flexure, bears to its weight. It would be difficult, 

 indeed, to construct any bar, or rod, or tube, of the 

 same length and weight as a portion of a bird's quill, 

 which would bear the same pressure without per- 

 ceptible flexure ; and it is scarcely conceivable that 

 any structure appertaining to a living creature, could 

 possess greater strength with an equal degree of light- 

 ness. In the hollow bones, again, we see the same 

 association of strength and lightness. Precisely as a 

 tubular bridge, like that which spans the Menai 

 Straits, is capable of bearing far greater strain than a 

 eolid metal bar of equal weight and length, so the 

 hollow bones of birds are far stronger than solid bones 

 of equal weight would be. We see then, that lightness 

 is secured in these parts of a bird's structure. But 

 lightness and buoyancy are different matters. We can 

 understand that it is absolutely essential, that the 



