128 Mayow 



the outer edge of the vortex to its centre. For let 

 a^ a^ h^ 3, in the aforesaid figure be the aerial cylinder 

 extending from the summit of the atmosphere to the 

 subjacent water. While as yet it was not in rotation, 

 the pressure of the air of which it was composed is 

 certainly quite equal to that of the atmosphere, since 

 their weights were in equilibrium ; but now when the 

 aerial cylinder is made to revolve, the force with which 

 each particle in rotation strives to recede from the 

 centre of its motion is added to the original pressure 

 of the cylinder. Hence it is that these forces in union 

 will preponderate over the pressure of the surrounding 

 atmosphere, and therefore the adjoining air will be 

 pushed out by the revolving air, and will necessarily 

 recede somewhat, say from a to 2, and from b to g^ and 

 consequently the rotated air following it will spread 

 out into a larger space than before and constitute the 

 cylinder i^ /, g^ g. Hence the rotated air is not a little 

 rarefied, and consequently the water beneath is less 

 pressed by it than before. 



That the pressure of the rotated air gradually 

 diminishes from the outer edge to the centre of the 

 vortex, I gather from the following. For when all 

 the particles of the aerial whirlpool strive to recede 

 from the centre, it results that they impel and press 

 against the particles of air adjacent to them on the 

 outside ; while, on the contrary, the air between them 

 and the centre of the vortex is subjected to less pres- 

 sure from them now than while as yet they had no 

 <;ircular motion and no tendency to recede from the 

 centre of their motion. But since the rotated air, in 

 proportion to its nearness to the centre of the vortex, 

 suffers less pressure, it follows that the air particles, 

 the further they are within, expand and rarefy the 

 more, by virtue of their elastic force, and conse- 



