PROGRESSION IN OR THROUGH THE AIR. 



115 



ylien hunting. In both these cases the body is exchisively 

 sustained by the action of the wings, the weight of the trunk 

 taking no part in it ; in other words, the weight of the body 

 does not contribute to flight by adding its momentum and ^ 

 the impulse which momentum begets. In the flight of the 

 albatross, on the other hand, the momentum acquired by the 

 moving mass does the principal portion of the work, the wings 

 for the most part being simply rotated on and off" the wind to 

 supply the proper angles necessary for the inertia or mass to 

 operate upon. It appears to me that in this blending of 

 active and passive power the mystery of flight is concealed, 

 and that no arrangement will succeed in producing flight 

 artificially which does not recognise and apply the principle 

 here pointed out. 



Air-cells in Insects and Birds not necessary to Flight. — The 

 boasted levity of insects, bats, and birds, concerning which so 

 much has been written by authors in their attempts to explain 

 flight, is delusive in the highest degree. 



: Insects, bats, and birds are as heavy, bulk for bulk, as most 

 other living creatures, and flight can be performed perfectly 

 by animals which have neither air- sacs nor hollow bones ; air- 

 sacs being found in animals never designed to fly. Those 

 who subscribe to the heated-air theory are of opinion that the 

 air contained in the cavities of insects and birds is so much 

 lighter than the surrounding atmosphere, that it must of 

 necessity contribute materially to flight. I may mention, 

 however, that the quantity of air imprisoned is, to begin 

 with, so infinitesimally small, and the difierence in weight 

 which it experiences by increase of temperature »o inappre- 

 ciable, that it ought not to be taken into account by any one 

 endeavouring- to solve the difficult and important problem of 

 flight. The Montgolfier or fire-balloons were constructed on 

 the heated-air principle ; but as these have no analogue in 

 nature, and are apparently incapable of improvement, they 

 are mentioned here rather to expose what I regard a false 

 theory than as tending to elucidate the true principles of 

 flight. 



When we have said that cylinders and hollow chambers 

 increase the area of the insect and bird, and that an insect 



