l;-J PNEUMATICS. 



these casks tin tubes proceed, which unite into one 

 that is connected with the silk tube of the balloon. 

 Balloons cannot be made smaller than five or six 

 feet in diameter, of oiled silk, as the weight of the 

 material is too great for the air to buoy it up. They 

 may be made smaller, of thin strips of bladder, or 

 other membrane, glued together. The best for 

 this purpose is the allantois of a calf, which is the 

 membrane which encloses the fcetus in the womb. 

 With this they maybe made 18 inches in diameter. 



OF THE WINDS. 



Wind is a stream or current of air. As air is a 

 fluid, its natural state is that of rest, which it en- 

 deavours always to keep or retrieve by an universal 

 equilibrium of all its parts. When, therefore, this 

 natural equilibrium of the atmosphere happens, by 

 any means, to be destroyed in any part, there ne- 

 cessarily follows a motion of all the circumjacent 

 air towards that part, to restore it; and this motion 

 of the air is what we call wind. 



If the air were uniformly of the same density at 

 the same height, and the lighter parts always re- 

 posed upon the heavier, it is evident that the 

 lateral pressure being equal in every horizontal 

 direction, it would remain at rest; but if, on the 

 contrary, any portion or part of the air were heavier 

 than the rest, it would descend, or, if lighter, 

 ascend, until the equilibrium was restored; so that 

 either the displaced air would occasion a wind, 

 diverging from a central space, in consequence of 

 the descent or pouring down of the heavier air; or 

 else the air, rushing in, would occasion a wind 

 converging to a central space,, in consequence of 

 the descent or pouring down of the heavier air; or 



