Meteorological Sketches. 59 
of whirlwinds and water-spouts, was maintained by Franklin, and 
although at a later period this has been called in question, it —_— 
to have been done without sufficient reason. 
From the equal distribution of the atmosphere as the oceanic en- 
velop of our earth, it results, that no movement of great violence 
can take place in any of its parts, except by means of a direct cir- 
cuit of rotation in the form of a vortex or active whirlwind. 
A vortex will not be regularly formed, nor continue itself in ac- 
tion, without the aid of an external propelling force and a constant 
spiral discharge from that extremity of its axis towards which is the 
tendency of motion. Both these conditions, it is believed, are ful- 
filled to the letter in the case of a common whirlwind or water-spout. 
The air at the upper extremity of the whirling column, owing to its 
elevation, is rarer than at the base, and the column itself, particu- 
larly in its central portions, is mechanically rarefied by the centrifugal 
effect of its own whirling motion. We have thus a sort of rarefied 
chimney into which the denser air at the base of the column is con- 
tinually forced, by the pressure of the surrounding atmosphere ; not 
to ascend in a separate current as in the common chimney, but en- 
tering into the organization of the whirling vortex, to supply the 
place of the preceding portions of air which are winding inwards 
and upwards to be again discharged at the upper extremity. The 
condition of force by which the propulsion is maintained, is found in 
the pressure of the surrounding atmosphere upon all sides of the 
whirling and therefore mechanically rarefied column, and if the ex- 
pansive whirling motion be sufficiently active to produce nearly a 
vacuum at the center, the external propelling force will be nearly 
fifteen pounds to the square inch. As the whirling column turns 
within its own compass like a top or any other rotative body, this 
force i is quite sufficient to account for all the violence that is ever 
‘Were there vorticular or whirling action already excited, aban 
discharge from the upper extremity of the vortex, there could then be 
no i ity of pressure to produce rotation; but this movement and 
upward discharge having once commenced, from any cause, the par- 
ticles near the exterior of the column, like those of water in a funnel, 
yield at a little more than a right angle, to the external pressure, in 
their spirally approximating course towards the rarefied center. By 
the slowness of this central approximation as compared with the whirl- 
ing action, the intensity or magnitude of the external pressure becomes 
