242 Remarks on the Influence of the 



of the blast diminished;" from this experiment he derives 

 four corollaries, the first two of which are, 1. " That we have 

 here a clear and natural account of the descent and vibrations 

 of the mercury during a 510™!." And, 2, " That not only the 

 different forces, but also the different directions of the wind, 

 are capable of producing a difference of subsidence of the 

 mercury.'' 1 Upon this Professor Leslie* remarks: " This ex- 

 periment has a specious appearance, and might seem to war- 

 rant the conclusions drawn from it, but a closer examination 

 dispels the illusion ; since the air had been condensed four 

 times, it must issue from the vessel with the velocity of 2700 

 feet in a second ; this is a rapidity, however, twenty times 

 greater than the most tremendous hurricane; the very small 

 change of the 400dth part of an atmosphere would hence have 

 been sufficient to produce the strongest wind ever known, and, 

 therefore, its influence in passing over the mercurial column 

 must have been quite insignificant. But the experiment itself 

 is absolutely fallacious; the peculiar result proceeded from a 

 casual circumstance, the exit-pipe being larger than the pipe 

 which introduced the air; for the air being previously con- 

 densed, and still restrained in its passage through the induc- 

 tion pipe, on entering the cavity of the box, immediately ex- 

 pands beyond the limit of equilibrium, and finding an easy 

 escape through the exit- pipe, allows that state of dilatation over 

 the mercury during the time of the horizontal flow, but the 

 air contained in the other cistern must, from its communica- 

 tion by the pipe, suffer a like expansion, and the columns will 

 subside equally. 11 



That this reasoning is also fallacious may, I think, be thus 

 shown : That the air, even after its ' ; dilatation" in its pas- 

 sage through the cistern, is still considerably denser than the 

 surrounding air, (otherwise the blast would cease,) is beyond 

 dispute; whence then \\\ejall of the mercury? it should ra- 

 ther rise ; this explanation is evidently inadequate. That the 

 difference of size in the induction and exit pipes will effect 

 the result is admitted, indeed, it is evident ; and I am inclin- 

 ed to think, that, if in the above case, the blast had been 

 equally swift and less confined, the result would have been 



* Vitlc Suppl. Encycl. Brit. Art. Meteorology. 



