t 



On Sal Nitrum and Nitro- Aerial Spirit 133 



from their motion and being condensed into water, 

 are carried down and encounter others which are still 

 ascending at the bottom of the tube, and at last fall 

 down along with them. 



But here we must ask why, on the cessation of the 

 circular motion of the air, the elevated vapours presently 

 fall ; because, although the motion of the air by whirl- 

 ing the water underneath was the cause of its fine 

 particles being carried up, yet, after these vapours have 

 been once set in motion, the whirling motion of the air 

 seems to contribute nothing to their ascent, unless 

 perchance the air is reflected upwards by the water 

 underneath. 



In regard to this, it is probable that when the 

 circular motion of the air gradually abates, the 

 aqueous particles do not, as before, leave the aforesaid 

 mass of water with a force intense enough to raise 

 them as high as the summit of the atmosphere, and 

 therefore these vapours must be heaped up in the 

 manner already described, and rush downwards ; while 

 yet, if the whirling motion of the air has suddenly 

 ceased, the vapours, violently agitated, ascend beyond 

 the top of the tube and are dispersed there, so that 

 the tube (as is sometimes the case) will seem to 

 ascend aloft. To this I add further, that so long as 

 the circular motion of the air continues, the pressure 

 of the atmosphere contributes somewhat to drive the 

 vapours upwards into the tube, as was previously said. 



About the said phenomenon we remark in fine that 

 if it should happen that the aforesaid tube rise right 

 above an island or sea-coast, its lower end will be 

 driven back from the island or coast, sea-wards, as is 

 delineated in Plate VL, Fig. 2. But that the reason 

 of this may be understood, let «, «, be a section of the 

 aerial cyHnder in rotation, which is interrupted at its 



