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III. On Aerial Navigation. 



^"^HIS art, which in its origin appeared so promising, stands 

 -*■ in a peculiar manner distinct from other departments of 

 mechanics. They gradually improve by experience, and the 

 theory often but slowly follows })ractice. Here the theory must 

 precede, and should be well digested befbie considerable ex- 

 penses are incurred, which may be fiuitless and even ridi- 

 culous. Hence the benefit of a frequent agitation of the sub- 

 ject, which may strike out new views, and make others more 

 distinct, and will at least have the negative advantage of show- 

 ing what system cannnt be successful. In looking upon the 

 history of the art, we are naturally led to divide it into stages, 

 and, from analogies to ordinary navigation, may endeavour to 

 surmise its future progress. 



1. The first step was purely theoretic, and I believe due to 

 Bishop Wilkins. He saw that a a body lighter than air viust 

 rise in it, but offered no practicable means for the purpose. 

 He proposed to exhaust a copper vessel, not seeing that the 

 pressure of the air nuist crush it inwards, unless it be of such 

 thickness as will make it too heavy. All j^revious attempts 

 were merely mechanical, generally imitations of wings; we 

 are now however satisfied that any apparatus wliich could 

 support a human body nnist be lar beyond its own muscular 

 power, and only to be worked by the steam-engine. 



2. The second step was made by Dr. Black and Mr. Ca- 

 vendish. They thought of applying hydrogen and other gases, 

 which, though lighter than the atynospheric air, have equal elas- 

 ticity, and eari countervail its pressure. This important prin- 

 ciple was practically applied little further than to raise bubbles 

 of soap, other substances sufficiently light permitting the gases 

 to pass through them. 



3. The third great step is due to the Montgolfiers. They 

 found that, by enlarging the balloon, an air even of less levity 

 will raise a covering of much thicker texture. They observed 

 that the weight of the covering increases onhj as the square of 

 the diameter, -jchile the effect of the gas increases as the cube. 

 On this principle gases of moderate levity maj' be used, as was 

 lately proved at the Coronation, in tlie very happy experiment 

 of using coal gas. It is however easy to foresee that balloons 

 will ultimately be directed by the steam-engine; and in such 

 case, the elevating power will probably be heated air, which also 

 possesses the invaluable property of easy increase and decrease. 



4. For the next step we are indebted to Sir George Cayley. 

 He distinctly saw, that every attempt to direct the machine 

 must be fruitless, as long as its small size subjects it to every 



fuctualion 



