THEORY CONFIRMED BY PHENOMENA. 69 



This last supposition, however, cannot be true, for the 

 moment it began to swell up by expansion, it would begin 

 also to flow off, and the depression of the barometer would 

 be in proportion to the quantity rolling off above, greater 

 than that which ran in below towards the point of least 

 pressure. This difference would be considerable for two 

 reasons ; first, the air below would not begin to run in until 

 the air above had rolled out; for a mere expansion and 

 swelling up of the air would not diminish its gravitation, 

 and second, its resistance would be less from friction than 

 the lower air would experience rubbing along the surface 

 of the earth. Besides, its outward motion from the centre 

 of the vortex, would not so much be a rolling down an 

 inclined plane in consequence of its being swelled into a 

 greater perpendicular height, as a shoving out of the sur- 

 rounding air at an elevation of about three and a half miles 

 and upwards, where the air in the vortex would overbalance 

 the surrounding air, as will easily be conceived by any one 

 who will consider the effect of an upheaving of the at- 

 mosphere by expansion. From all these causes facilitating 

 the outward motion of the upper air in the vortex, it is 

 probable that at least one half of the quantity of air ele- 

 vated in the vortex above the surrounding air, by expan- 

 sion, would flow off, arid if so, it would cause a depression 

 of the barometer, within the region of the rain, of more than 

 one inch and a half. And this corresponds with the de- 

 pressions given in many places. 



This depression would cause a velocity of the air at the 

 surface of the earth, on the outside of the vortex, towards 

 the centre of rarefication, of one hundred and fourteen miles 

 per hour, if there was no friction ; but as the friction at the 

 surface of the earth is very great, the velocity would proba- 

 bly not be more than one half this quantity, or fifty- seven 

 miles per hour. This velocity would not be sufficient to 

 produce the overflowing of the sea at Genoa, Leghorn and 



