Prof. Dove on the Changes of Wind in a Cyclone. 471 



first a north wind, but the further north the place from whence 

 this north wind comes the more will it change into a N.E. wind, 

 as the velocity of rotation of the earth is less in the higher than 

 in the lower latitudes ; therefore the wind that left Edinburgh 

 as a N. wind reaches London, we will say, as a N.N.E. wind; 

 if it left the latitude of Shetland as a N. wind, it reaches London 

 as a N.E. wind; and if it left the polar circle as a N. wind, it 

 reaches London as an E.N.E. wind. It is plain that in these 

 cases we have only to do with one unaltered current, which, the 

 further it proceeds, the more its original direction appears de- 

 flected. Winds between N. and E. are therefore, properly speak- 

 ing, N. winds; in other words, a N.E. wind is a N. wind, which 

 comes from further N. than does the wind which arrives at the 

 place itself as a N. wind; and the N.E. is therefore the heaviest, 

 coldest and driest. 



If, then, we suppose that after a northerly current gradually 

 changing into an east wind, a southerly current sets in, we see 

 the wind change from E. through S.E. to S., the barometer fall, 

 and the air become warmer and moister. The longer this south 

 current lasts, the greater, therefore, is the distance from which 

 the wind comes, and the more the S. wind will change through 

 S.W. to W. ; for a S.W . wind is no other than a S. wind which 

 has its origin further to the S. than a wind which arrives as a 

 south wind. The winds between S. and W. are therefore south 

 winds. If the cause which produced them continues, the south 

 wind, which has become a west one, will keep back the direct 

 air from the south ; and there will be often repeated unsteady 

 changes between S. and W. with thick weather. If at last the 

 polar current presses forward again, we see the barometer rise 

 with the wind passing through N.W. to north, while the tempe- 

 rature sinks and the clouds break. 



This regular course prevails in the northern hemisphere, and 

 the opposite in the southern one. We may therefore say that, 

 on the average, the wind, from these causes, turns in both hemi- 

 spheres with the sun, and that therefore the seaman^s adage 

 truly says — 



" When the wind veers against the sun. 

 Trust it not, for back it will run." 



It follows, then, in regard to tempestuous movements of the 

 atmosphere, that storms are either " gales or hurrioanes.^^ Both 

 cause the wind- vane to turn round, but in the " gales '' it turns 

 only with the sun ; in the rotatory storms or hurricanes it turns 

 with the sun on one side of the storm, and against the sun on the 

 opposite side. If a ship in the northern hemisphere is on the 

 east side of a hurricane or cyclone advancing towards the north, 

 the navigator cannot tell simply from the way in which the wind 

 turns whether he is in a cyclone or in a gale ; but if he is on the 



