38 Proceedings. 



effect and the stronger is the wind. When pressure varies 

 greatly, that is, when barometer-readings are very different 

 in two comparatively near places, the wind will be strong; 

 and when pressure varies little, there will be little wind. In 

 the weather-charts the arrows fly with the wind, thus indi- 

 cating its direction ; the barbs and feathers on them indicate 

 its force ; they are drawn more barbed and more feathered to 

 denote stronger wind. Wind and storm, you will see, are 

 only relative terms. 



The term "gradient" is applied to the difference in height 

 of the barometer at two places ; gradients are always at right 

 angles to the isobar, that is, they ascend from the centre of 

 the depression to its outer edges. From the way in which 

 the gradient is described, we gather the direction of the wind 

 indicated. When we speak of a gradient from Valentia to 

 Holyhead being for N.W. winds, we imply that the reading 

 at Valentia is the higher of the two readings, the station with 

 the higher reading always being placed first ; and so a man 

 standing midway between the two stations, with the lower 

 barometer on his left-hand side, would face S.S.E. and have 

 his baclv to the N.N.W. ; the wind would therefore be N.N.W., 

 which brings us back again to Buys Ballot's law. 



We can now see its practical application : by observing the 

 direction of the wind, even without any telegraphic informa- 

 tion, we can tell the position in regard to ourselves of the 

 cyclonic depression. We know that cyclones generally travel 

 from S.W. to N.E., and we therefore can roughly foretell 

 coming weather. When a S.E. wind is blowing, the depres- 

 sion, on our left as we back the wind, is in the S.W., and so 

 is approaching us ; it will probably bring warm weather, 

 wind, and rain. With a northerly wind we shall have cold 

 and fine weather, for the depression is somewhere to the east 

 and has passed away from us. 



Hitherto we have been considering low-pressure systems or 

 cyclones ; we now come to the opposite conditions, high- 

 pressure systems or anii-cyclones. In these the isobaric lines 

 are still very much in circles, but pressure at the centre, 

 instead of being lowest, is now highest. The wind still 



