lt)0 Dr. Hare's Strictures on Dove's Essay 



required. It could not but be admitted by the learned Pro- 

 fessor, 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 circum- 

 ference, the velocity must increase inversely as the square of 

 the distance from the centre. Nevertheless, after the base of 

 the ascending column is reached, evidently 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 must be superseded by a vertical movement.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. 



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

 gigantic tornado. Of course it may be assumed that the fea- 

 tures 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 columns moving with equal velocities, the 

 quantity of air requisite to supply the upward currents thus 

 constituted, will be as the squares of the diameters of the co- 

 lumns severally. 



124. The diameters of the focal areas of tornadoes as ob- 

 served in this country, seem in no instance to have exceeded 

 500 feet. The focal area of the Providence tornado was esti- 

 mated to be 300 feet in diameter*. To supply an upward 

 columnar current of 10,000 feet diameter, would require 400 

 times as much wind as to supply an analogous current of a 

 diameter of 500 feet. It follows that a hurricane equivalent 

 to 400 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 necessary. 



125. Let us suppose the focal area of a hurricane, forming 

 the base of a vertical current, to be in diameter 10,000 feet, 

 and that the space beyond the area be divided into zones by 

 the circumferences of circles equidistant from and concentric 

 with each other and with the focal area. The circumferences 

 being equidistant, the quantities of air over these zones seve- 

 rally will evidently be as the squares of their mean diameters. 

 Of course if the zone nearest the area, having a mean diameter 

 of 10,500 feet, move inwards with the velocity of 100 miles per 

 hour, the velocity at four times that distance, or 42,000 feet 

 cannot be more than one-sixteenth as great, or little more than 

 six miles per hour. Thus at only four miles from the centre, 

 the centrifugal velocity would scarcely be adequate to a breeze. 



* See letter from Z. Allen, Esq. in Silliman's Journal, vol. xxxviii. p. 76. 



