Cyclones. 337 



stationary, and about an equal time after the passage of the 

 centre it gradually ascends. In the other, or storms of great 

 depression, a further rapid fall of three to seven tenths of an 

 inch an hour takes place in the mercury when the centre is 

 near, or within three hours; and the mercury sometimes 

 reaches its lowest point a little before the passage of the 

 centre. It also immediately rises again as rapidly as it fell. 

 The lowest depression of the mercury in a barometer yet 

 experienced in a cyclone took place on board the " Duke of 

 York," in the storm already noticed, at Hidgelee, on the 

 21st of May, 1833, when the mercury disappeared in the tube, 

 behind the graduated scale of the instrument, at twenty-six 

 and a half inches, having fallen two and a half inches in the 

 preceding three hours, and remained invisible for half an hour. 

 The oil in the sympiesometer at the same time retired com- 

 pletely, when the mercury in the barometer disappeared, and 

 rose again a little before its reappearance. 



Another point on which there is nothing approaching to a 

 doubt in all the numerous investigations that have been made, 

 is the general law of rotation of cyclones. This, in the 

 northern hemisphere, is against the hands of,, a watch ; and a 

 seaman steering down the wind in the northern hemisphere 

 has the centre on his left. In the southern hemisphere the 

 case is reversed ; the direction of rotation is the same as that 

 of the hands of a watch ; and a seaman steering down the wind 

 has the centre of the storm on his right hand. In both cases 

 the shift of the wind is in the same direction as the shift of the 

 centre of the storm. By attending to these simple rules, ships 

 not only avoid the danger of falling into the centre of a storm, 

 but take advantage of its most favourable semicircles to 

 continue their voyages with a prosperous wind, or even to 

 accompany a storm upon its track. 



The general law of rotation of cyclones, may also be stated 

 briefly thus : — In both hemispheres it is against the apparent 

 motion of the sun ; or, in both hemispheres it is the same as 

 the real direction of the earth's daily rotation. The uniformity 

 of this simple law, Dove has shown, is the result of a law 

 equally uniform and simple — namely, the daily revolution of 

 the earth upon its axis. For the purpose of illustration, 

 a well-chosen example will serve, better than many diagrams, 

 to make this clear. Suppose, for example, that an exten- 

 sive depression of the barometer, arising from any cause, 

 should take place exactly at the north or south poles of 

 the earth. Were then the earth at rest, currents of air 

 would stream inwards in straight lines from all sides to supply 

 the void. But as the earth rotates about its axis once in 

 twenty- four hours, and with it its atmospheric covering, the air ; 

 NO. VIII. — vol. v. z 



