208 Mr. W. Brown on the Storms of Tropical Latitudes. 



plete demonstration of the fact in question." And in the same 

 essay (p. 125) he says, in speaking of the storm of August 

 1830, " It occupied about seven days in its ascertained course 

 from near the windward islands, a distance of more than 3000 

 miles, the rate of its progress being equal to eighteen miles 

 an hour. If we suppose the actual velocity of the wind in its 

 rotatory movement to be five times greater than this rate of 

 progress, which is not beyond the known velocity of such 

 winds, it will be found equal in this period to a rectilinear 

 course of 1 5,000 miles. The same remark applies in substance 

 to all the storms which are passing under our review. What 

 stronger evidence of the rotative action can be required than 

 is afforded by this single consideration ?" 



Now in the passages here quoted, after the existence of a 

 whirlwind has been assumed to account for certain pheno- 

 mena, each individual explanation it is capable of giving is 

 made a demonstration of its truth, without regard to the ac- 

 cordance between its further requirings and the facts found 

 by observation. Let us then try it by some very simple re- 

 sults deducible from it. The velocity of the air in a progress- 

 ing whirlwind must be very different on its opposite sides. 

 If its greatest velocity be ninety miles an hour, and the hur- 

 ricane progresses at the rate of eighteen, the velocity of the 

 wind on the side where it is opposed to the progressive mo- 

 tion will be thirty-six miles less, or fifty-four miles an hour. 



Hence the path of such a hurricane would be marked by 

 two distinct sides, the difference of velocity being far too great 

 not to afford a very decided characteristic ; and as the force 

 of the wind gradually lessens and the rate of progress in- 

 creases as the storm advances, the storm on one side, during 

 the latter part of its path, would be reduced to at least a mo- 

 derate breeze. No such result, however, appears from ob- 

 servation ; for although the same storm varies at different lo- 

 calities in force, these variations have no relation to the direc- 

 tion of the progressive movement. 



Again, the fall of the barometer during the first part of the 

 hurricane, and its rise on the change of the wind to the op- 

 posite quarter, are accounted for by supposing the air to be 

 carried by the centrifugal force of the whirlwind from the 

 centre towards the circumference. 



A whirlwind can only be conceived to be maintained in mo- 

 tion in three ways, — 1st, by an ascending column of air in the 

 centre; 2nd, by the deflection of a rectilinear current by the 

 resistance of the air on the outside of the whirl ; 3rdly, by the 

 deflection produced by a force directed to the axis of the whirl- 

 wind. In the first of these the explanation of the variation in 



