IcS Aenjlation. 



jRill wheel, prefent in fucceffion to the air fometimes their 



fiat fide and fometimes their edge. 



To caufe each oar to turn back on itfelf about the fourth 

 part of a circle, M. Danzel has not only left fufficient play 

 at the point where the flick of each oar is infertcd in the 

 e}'linder, but has placed the ftick in fuch a manner that the 

 air itfelf makes the oar fall back, at each turn, with the ne- 

 cdTary velocity and preclfion. It muft here be underflood 

 that the two cylinders, armed with their four oars or fails,, 

 are deftincd to occupy one each fide of the balloon, four oars 

 en each fide. 



It is well known that aeroftatlon has a refemblance to ffy- 

 \w<y or f\\ tmming rather than to failing, fincc every aerofi;at 

 is immcrfcd in the air as a bird, or as a fifii is immerfcd in 

 water; whereas the effort of a veffel is divided between two 

 elements, one of which, the water, gives it a great point of 

 fupport, and the other, the air, is very cafy to be divided. 

 But the aerofiat being totally immerfed in the air, it was ne- 

 ceifary to find out a movement which, though circular, 

 ftould not deftroy at each half turn the effeft of the preced- 

 ing 0i;e ; for, the refiftance being equal in the whole circum- 

 ference, it is evident that the fupport mufi; be diminiflied or 

 even annulled, if pofilbk, in one half of the circle, and be 

 retained full and perfcft in the other. This M. Danzel has 

 fctfeaed, by prefenting to the air fometimes the edge .;nd 

 fometimes the flat part of the fails on each fide, in turns 

 and with great regularity. The refult is, that during a con- 

 tinued rotation each cylinder produces a firong current of 

 air behind, and rarefa<Slion before; and this current, which 

 continues without intcrmiflion, is very fenfible. It is evi- 

 dent then that a body fufpcnded in air at reft, if thcfe two 

 cylinders arc attached to it, mufi: advance towards the rar, fi.'d 

 Ipace occafioned by their rotation, and produce a continual 

 current of air behind it. 



The coesed wheels of water mills being half in the water 

 and half in air, the co^s may be immoveable, becaufe at 



each 



