l~Sd On Aerial-Navigation. 



that the velocity of any machinery used should be consider- 

 able, a small difference in that producing a considerable dif- 

 ference in the resulting force. The principle is applied in 

 swimming. 



4. One writer recommends a series of explosions with gun- 

 powder. This is very crude. It would indeed be possible 

 to gain some power by creating a vacuum in front of the ma- 

 chine, and by impelling the air drawn from the front tlu-ough 

 a pipe, to rush out behind. This might be effected by con- 

 necting the centre of the pipe with a tube and piston. In 

 raising the piston, the air will rush through the pipe from the 

 front, while it is prevented by a valve from passing in through 

 the other part; on depressing the piston, the air is impelled 

 through the pipe behind, while another valve prevents its re- 

 gress to the front. The principle differs but little from that 

 of the blowing engine and common bellows. 



5. Some have thought it possible to tame the eagle, or 

 imitate his wings ; while others, as Blanchard, matle an ineflec- 

 tual attempt with valved oars. (See vol. xlix. p. 197, and Ixi. 

 p. 6, of this Journal.) 



6. But let us look philosophically on the matter. In what 

 cases is the air used as an agent ; and can we by reveiiing the 

 operation use sunilar means to re-act on the air itself? It is 

 used to fill the sails of ships, and it has convei'sely been pro- 

 posed to push out canvass agaftist the air : some power so 

 gained might be retained, by withdrawing the canvass in a 

 iblded form, or by valves, or by protruding it with consider- 

 able velocity, and withdrawing it slowly. 



7. Wind is used to turn the sails of mills, and in so act- 

 ing swells them into curves investigated by mathematicians. 

 Would it be possible, by turning sails, injiexiblij maintaining 

 the form so produced.) in an opposite direction, to re-act upon 

 the air, as it had acted upon the sails ? These sails woulcl be 

 fixed behind and turned by machinery. 



8. We might combine the princijile of the steam-boat pad- 

 dles with that of the horizontal windmill. The paddles gain 

 their power by being only partly immersed in the water, so 

 that in returning they do not act at all. In the horizontal 

 windmill this difficulty has been and may be obviated by a 

 great number of ingenious contrivances ; for some of which see 

 Dr. Brewster's Encijcloptsdia, " Mechanics," and its references, 

 p. 570. Those paddles may be fixed on the edges of the 

 platform connecting our two balloons, the sails of the paddles 

 being so constructed as to furl themselves in returning. There 

 )s great advantage ni makmg oin- power operate by a rotar)' 



motion, 



