VI.] THE ATMOSPHERE. 95 



Hence a gradient of 4 means that over a distance of 60 

 nautical miles the barometer rises y^ or -i- of an inch. If 

 the isobars run close together it shows that the gradient is 

 high, and therefore the winds will be strong; if they are 

 wide apart the gradient is low, and the winds are light. 

 Thus, in Fig. 22, the isobars indicate only light winds. 



Although much may be learnt about winds by studying 

 the isobaric lines, it must not be supposed that the air blows 

 directly from regions of high pressure to those of low 

 pressure. Prof. Buys Ballot of Utrecht has however laid 

 down a law which gives the exact relation of winds to 

 pressure, and which may be thus expressed : " Stand with 

 your back to the wind, and the barometer will be lower on 

 your left hand than on your right." Thus expressed, how- 

 ever, the law is true only for the northern hemisphere ; in the 

 southern it will be reversed, the barometer being lower on 

 the right hand than on the left. The same principle may 

 be enunciated in another form. If you stand v/ith the 

 high barometer to your right and the low barometer to 

 your left, the wind will blow on your back. The course 

 of the isobars in the chart therefore indicates the direction 

 of the wind, just as the distances between these lines 

 indicate its strength} Every Thursday the Times issues a 

 weather diagram giving a graphic representation of the 

 meteorology of the week, sufficiently explained, however, 

 by the description which is annexed to it. 



Whilst the Times publishes daily charts with isobaric 

 curves, the other morning papers give their meteorological 

 reports in different shapes. Fig. 23 is copied from the 

 Daily News of to-day (March 31, 1877). It represents 



1 The arrows on Fig. 22 fly with the wind ; the asterisl^s indicate 

 the position of meteorological stations ; and the figures give tde shade 

 temperature. For further information see Mr. Scott's " Weather 

 Charts." 1876. 



