60 Meteorological Sketches. 
gradually merged in the velocity of the rotative action. As the area 
of the spiral circuit decreases rapidly as we approach the center, it 
follows that the velocity of the whirling movement must be propor- 
"tionally increased, as we perceive it to be in the funnel and in all reg- 
ular formed vortices. Thus, if the rotative velocity near the exterior 
of the whirling column be at the rate of but ten miles an hour, at one 
third nearer the center the velocity must be more than doubled, and 
at two thirds of the distance from the first named point to the center, 
the absolute whirling velocity must be increased nine fold, which in 
this case is equal to ninety miles an hour; and in consequence of 
the reduced diameter of the circuit of gyration at the last point, the 
number of revolutions must here be as four hundred, to one at the 
point first mentioned. ‘The increased ascending velocity, however, 
is not here taken into account, which may perbaps reduce the num- 
ber of comparative revolutions in the central portions of the col- 
umn. The extraordinary condensing and electric effects which 
often attend or follow these active whirlwinds, have been cursorily 
noticed under the head of thunder storms and hail. 
_ It is not intended to dwell here upon the causes by which whirl- 
winds and spouts are excited or first set in motion, but local disturb- 
ances in a heated stratum, at points where the same is beginning to 
be penetrated by the colder air of a higher stratum, are probably the 
chief exciting cause as in thunder storms. The agency of heat may 
also be effective in continuing the upward discharge and \vorticular 
organization, in cases where there is great disparity in the tempera- 
ture of the air at the upper and lower extremities of the whirling 
mass or column, but it is to the mechanical expansion caused by the 
centrifugal action and the powerful impulse of the external atmos- 
pheric pressure, that the increased and ahaa activity of the whirl- 
wind is chiefly to be referred. 
The term water-spout is undoubtedly a misnomer, as there is no 
effect produced of which this term is properly descriptive, although 
the term air-spout would not be greatly inappropriate. ‘The visible 
¢olumn of condensed vapor which often appears in the rarefied cen- 
ter of the vortex when the latter is not enveloped in cloud, has 
probably given name to this meteor. But the water of the sea is 
not taken up by the spout or whirlwind, except in a slight degree 
and inthe form of fine spray, like other light matter which is swept 
from the surface. This soeiy stem or column frequently nas 
2 the action of the whirlwind continues without 
Pa a ae 
. 
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