Oil Sal Nitrum and Nitro- Aerial Spirit 131 



follow them, it comes to pass that vapours are 

 <:arried in a continuous stream towards the centre of 

 the vortex. 



About the said tube, it is noteworthy that at first, 

 when, namely, the whirl of vapours is not yet observed 

 in it, it looks like a rather thin smoke and also gradu- 

 ally descends, while yet it would seem that the vapours 

 entering its lower end would cause it to be seen there 

 first. I think the reason of these things is that when 

 the air first began its rotating movement, only a few 

 rather thin vapours — those, namely, which, already in 

 the atmosphere, were carried upwards — had entered 

 the said tube. And these are first seen at the top of 

 the tube because they are assembled there in greater 

 abundance,and are alsosomewhat condensed on account 

 of their diminished velocity. However, I do not know 

 whether or not these things depend also on another 

 cause, for, since the air receding from the middle of the 

 vortex leaves the tube there nearly vacuous, the aerial 

 particles, and there are but few remaining in it, will 

 as they expand, appear under the form of smoke, not 

 otherwise than it happens in a glass vessel when the air 

 is being exhausted by Boyle's pump, as will be explained 

 more fully elsewhere. And it seems to be for this reason 

 that the smoky tube is first seen high in the air, where 

 the force by which the rotated air strives to recede 

 from the centre of its motion first takes effect and 

 produces a vacuum, as has already been shown. 



It is besides to be noted that the nearer to the 

 water the said tube descends, the higher does the water 

 underneath rise. And the reason of this seems to be 

 that a long descending tube cannot be formed unless 

 there is a very swift whirl of the air, and the ascent 

 of the water depends on this. 



When the vapours driven up in the said tube have 



