Strictures on Dove's Essay "On the Law of Storms." 143 



ward current for the supply of which they are required. It could 

 not but be admitted by the learned professor, that when, under 

 such circumstances, a fluid rushes from all quarters towards a 

 focal area, the consequent motion must quicken as it approaches 

 the ascending current. It must also be clear that when it moves 

 from all parts of the circumference, the velocity must increase 

 inversely as the square of the distance from the centre. Never- 

 theless, after the base of the ascending column is reached, evi- 

 dently the horizontal afflux must be superseded by a vertical 

 movement. Hence about the centre of the space around which 

 the upward currents prevail there may be a calm. 



119. It seems to be conceded that a tropical hurricane is a 

 gigantic tornado. Of course it may be assumed that the features 

 of these meteors are proportionable ; and that the focal area of a 

 hurricane, will be as many times greater than that of a tornado, 

 as the whole area of the former is greater than that of the latter. 

 In fact, if the focal areas be respectively bases of ascending col- 

 umns moving with equal velocities, the quantity of air requisite 

 to supply the upward currents thus constituted, 

 squares of the diameters of the columns severally. 



120. The diameters of the focal areas of tornadoes as observed 



■ 



in this country, seem in no instance to have exceeded five hun- 

 dred feet. The focal area of the Providence tornado was estima- 



1 



will be as the 



th 



To supply an upward 



columnar current of ten thousand feet diameter, would require four 

 hundred times as much wind as to supply an analogous current 

 of a diameter of five hundred feet. It follows that a hurricane, 

 equivalent to four hundred of the largest tornadoes, would not 

 require a focal area greater than two miles in breadth. To cross 

 this at the rate of progression attributed to great storms, by Prof. 

 Dove, (thirty miles per hour,) only four minutes would be neces- 

 sary. 



12 L Let us suppose the focal area of a hurricane for the base 

 of a vertical current to be in diameter ten thousand feet, and that 

 the space beyond the area be divided into zones by the circum- 

 ferences of equidistant circles, concentric with each other and 

 with the focal area. The circumferences being equidistant, the 

 quantity of air over these zones will evidentlv be as the sauares 



* See letter from Z. Allen, Esq. in this Journal, Vol. xixvm, p. 76. 



