394 THE CLIMATE OF IRELAND, AND 



The facts collected relating to these winds lead to the following 

 conclusions : 



1. The occurrence of cyclonic movements in the atmosphere is 

 not infrequent in Ireland, and may be traced even in the case of 

 moderate winds. 



2. The rotatory movement is invariably in the same direction 

 namely, that opposite to the diurnal movement of the sun in azimuth. 



3. This rotation is always accompanied by a considerable 

 disturbance of barometric equilibrium, which is greater in pro- 

 portion to the velocity of the movement, the pressure being a 

 minimum at the centre of the vortex, and increasing regularly 

 with the distance from that point. 



4. The vortex itself has a progressive movement, at the rate 

 of from 100 to 300 miles per diem, the average velocity of those 

 observed being 220 miles per diem. The direction of this move- 

 ment is generally from S. W. to N. E. 



5. If a line be drawn through the centre of Ireland, in the 

 direction from S. W. to N. E., the track of the centres of the 

 greater number of the cyclones, passing over or near Ireland, lies 

 to the north of that line. 



With reference to the first of these conclusions viz., the pre- 

 valence of cyclonic movements in the atmosphere over, or near, 

 this island I may here state, that of all the greater storms which 

 occurred during the period of observation, no fewer than two-thirds 

 were cyclonic ; while several cyclonic movements of smaller velo- 

 city were distinctly traced. The area of observation may be 

 thought, perhaps, to be too small to furnish conclusive evidence 

 relating to these great aerial movements. But I have the satis- 

 faction of knowing, that of the cases recorded in the Memoir on 

 the subject published in the Transactions of the Royal Irish Aca- 

 demy, the cyclonic character of most (if not all) has been since 

 confirmed by the late Admiral Fitzroy, from the records of the 

 logs of ships. 



The prevalence of such movements is just what we should 

 be led, a priori, to expect, from the vicinity of the GruH'-Stream. 

 The waters of the stream in mid-Atlantic have a temperature 

 of 80 Fahr., while the air on either side of it is, in winter, 

 at the freezing temperature. We have here all the conditions 

 required for the production of cyclonic storms. Aqueous vapour 

 is much lighter than air at the same temperature, and conse- 



