DR PETTIGKEW ON THE PHYSIOLOGY OF WINGS. 403 



employment of rigid inclined planes driven forward in a straight line, or revolving 

 planes (aerial screws) ; and (b) such as trust for elevation and propulsion 

 to the vertical flapping of wings. 



Balloon. — The balloon, as all are aware, is constructed on the obvious prin- 

 ciple that a machine lighter than the air must necessarily rise through it. The 

 Montgolfier brothers invented such a machine in 1782. Their balloon con- 

 sisted of a paper globe or cylinder, the motor power being super-heated air 

 supplied by the burning of vine twigs under it. The Montgolfier or fire 

 balloons, as they were called, were superseded by the hydrogen gas balloon of 

 MM. Charles and Robert, this being in turn supplanted by the ordinary gas 

 balloon of Mr Green. Since the introduction of coal gas in the place of 

 hydrogen gas, no radical improvement has been effected, all attempts at guiding 

 balloons having signally failed. This arises from the vast extent of surface 

 which they necessarily present, rendering them a fair conquest to every breeze 

 that blows, and because the power which animates them is a mere lifting power 

 which, in the absence of wind, must act in a vertical line, all other motion being 

 extraneous and foreign to it. It consequently rises through the air in opposi- 

 tion to the law of gravity, very much as a dead bird falls in a downward 

 direction in accordance with it. Having no hold upon the air, this cannot be 

 employed as a fulcrum for regulating its movements, and hence the cardinal 

 difficulty of ballooning as an art. 



Finding that no marked improvement has been made in the balloon since 

 its introduction in 1782, the more advanced thinkers have within the last 

 quarter of a century turned their attention in an opposite direction, and have 

 come to regard flying creatures, all of which are much heavier than the air, as 

 the true models for flying machines. An old doctrine is more readily assailed 

 than uprooted, and accordingly we find the followers of the new faith met by 

 the assertion that insects and birds have large air cavities in their interior, that 

 those cavities contain heated air, and this heated air in some mysterious manner 

 contributes to, if it does not actually produce, flight. No argument could be 

 more fallacious. To render a flying creature buoyant by means of air-cells, it 

 would require to have its superficial area increased a thousand fold (would, in 

 fact, require to be converted into a balloon) ; and, besides, many admirable fliers, 

 such as the bats, have no air-cells, while many birds, the apteryx for examj)le, 

 and many animals never intended to fly, such as the orang-outang and a large 

 number of fishes, are provided with them. It may therefore be reasonably 

 concluded that flight is in no way connected with air-cells, and the best proof 

 that can be adduced is to be found in the fact that it can be performed to 

 perfection in their absence. 



The Inclined Plane. — The modern school of flying is in some respects quite 

 as irrational as the ballooning school. 



VOL. XXVI. PART II. 5 M 



