"THE CUERENTS OF THE ATLANTIC. 393 



entire year, nearly as 3 to 1. This preponderance of high winds 

 on the western coast holds at all seasons, the maximum occurring 

 at the north-western extremity in autumn, and at the south- 

 western in winter. 



But of these winds, there are two distinct kinds. In one of 

 these, the wind blows steadily in the same direction for a con- 

 siderable time, and the axis of the gale is a straight line, or a line 

 but slightly curved. In the other, the wind rotates round a centre, 

 while the centre itself has a progressive motion just as we see, on 

 a miniature scale, in the vortices of dust which sweep along the 

 road in gusty and unsettled weather. These rotating winds are 

 called cyclones. They are from 200 to 300 miles, and upwards, in 

 diameter, and their centres move with a speed of from 100 to 300 

 miles per diem. The direction of the rotation is invariably the 

 same. In the northern hemisphere the wind revolves in the 

 direction opposite to that of the sun in its daily course, i.e., in the 

 direction N. W. S. E. In the southern hemisphere, the direction of 

 rotation is with the sun, or N. E. S. W. When a cyclone passes 

 over any point on the earth's surface, the wind must veer. If the 

 centre of the cyclone passes directly over the place of observation, 

 the veering is through 180 degrees, or the wind changes to the 

 opposite. The amount of veering is less, the smaller the chord of 

 the circle which passes over the place; until, for places at the 

 circumference, there is no veering whatever. 



But there is another remarkable distinction between these two 

 classes of winds. In the case of the cyclone, the barometric 

 pressure diminishes to a point, which is the centre of the vortex. 

 In the non-rotatory gales, the pressures diminish to a line, which 

 is the axis of the storm. 



The most remarkable of the cyclonic movements traced in the 

 course of the simultaneous observations was that which occurred 

 on the 18th and 19th of November, 1851. The centre of the 

 vortex passed over two of the principal stations, Markree and 

 Armagh, at both of which all the changes were accurately ob- 

 served ; and the velocity of the wind reached a maximum of Go 

 feet per second. The lull oi the wind during the passage of the 

 central portion of the vortex, and the reversal of its direction, were 

 observed at Markree, Armagh, and Donaghadee. The diameter 

 of the vortex was about 400 miles ; that of the quiescent central 

 portion 40 miles. 



