On Sal Nitrum and JViiro- Aerial Spirit 127 



whether as the result of winds rushing from opposite 

 directions, and at length dashed against each other 

 and bent into a circular path, or as the result of their 

 being suddenly thrown back by the resistance of lofty 

 mountains or of dense clouds, or in the manner 

 indicated in the previous chapter. 



2. It is to be observed that aerial particles when 

 revolving in a circle, try to recede from the centre of 

 their motion, as was observed long ago by Descartes. 

 Hence if we suppose that a cylindrical column of air,, 

 reaching from the summit of the atmosphere to the 

 water beneath, is made to rotate, and if that aerial 

 vortex is large enough and is carried round with a 

 very rapid rotation, the total force by which all the 

 particles of that aerial vortex strive to depart from the 

 centre of their motion will be great enough to resist 

 the pressure of the surrounding air and even to over- 

 come it. But if this happens, and the aerial particles 

 recede from the centre of the vortex, the middle of it 

 will be like an empty tube : for the case will be 

 exactly like that in which a large number of globules 

 are placed in a concave circular vessel and made to 

 rotate rapidly with it, when you will see these globules 

 go away from the centre of the cavity and describe 

 their circular orbits at its side. 



But now when a vacuum is made in this way in 

 the middle of the air-vortex, the water which lies 

 beneath will be forced to ascend by atmospheric 

 pressure, just as it would be into a vacuous tube. 

 Nay, although the aerial cylinder may not rotate with 

 so swift a motion as is needed for completely over- 

 coming the pressure of the surrounding air and pro- 

 ducing a vacuum in the middle of the vortex, still, with 

 however feeble force it rotates, the weight and pres- 

 sure of the air will certainly diminish gradually from 



