44 PHILOSOPHY OF STORMS. 



spout, which is nothing but visible condensed vapor, may 

 sometimes not reach entirely down to the surface of the earth 

 or sea, when the dew point is too low for such an effect ; in 

 this case, it will appear as an inverted cone, reaching down 

 from a cloud already formed. 



It may here be observed, that a spout will always begin 

 to be formed at a considerable elevation above the surface 

 of the earth, because the vapor will always begin to con- 

 dense there, from a law too well understood by meteorolo- 

 gists to need elucidation here. When, however, it begins 

 to condense, it begins, also, by its diminished specific gravity, 

 to rise, and then, if all circumstances are favorable, the 

 cloud will increase as it ascends, and finally become of so 

 great perpendicular depth, that, by its less specific gravity, 

 the air below it, and contiguous to it, in consequence of di- 

 minished pressure, will so expand, and cool by expansion, 

 as to condense the vapor in it ; and then the air below this 

 again, will, in its turn, experience a greater and greater ex- 

 pansion and refrigeration, and, consequently, condensation 

 of vapor j and this process may go on so rapidly, that the 

 visible cone may appear to descend to the surface of the sea, 

 or earth, from the place where it first appears, in about one 

 or two seconds. 



The terms here employed must not be understood to mean 

 that the vapor, or cloud, actually descends; it appears, to 

 the spectator, to descend, but this is an optical deception, 

 arising from new portions of invisible vapor constantly be- 

 coming condensed, while, all the time, the individual par- 

 ticles are in rapid motion upwards. 



74. For the sake of illustrating the principle, without 

 aiming at absolute numerical accuracy, let us suppose a dew 

 point ten degrees below the temperature of the air. Now, 

 a diminished pressure of one pound to the square inch, will 

 cause a fall of temperature of about seven and half degrees, so 

 that, in this case, the visible cone would not reach down to the 



