332 



NATURE 



[August 2, 1883 



analysed, and the remarkable result has been arrived at that the I areas of rain or blue sky which are mapped out by the isobaric 

 greater number of prognostics are simply descriptive of the lines. 



weather and appearance of the sky in the different portions of The^e charts not only show the success of the prognostic-, but 

 the various shapes of isobars seen on synoptic charts ; and that also explain wherein they sometimes fail, by tracing the changes 

 they indicate foul or fair weather just as they precede the shifting of each particular condition of weather. Hitherto the only prog- 



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nostics which have been accounted for have been those due to ex- 

 cessive damp, but by means of isobaric charts many others can 

 be readily explained. It must not be supposed that the modern 

 methods diminish the value of prognostics, for even in forecast- 

 ing weather from synoptic charts they are of great value, and will 



always be exceedingly useful to solitary observers who have omy 

 a single barometer to depend upon besides these prognostics, as 

 for instance on board ship. 



Though this way of treating prognostics is a great advance on 

 the older methods, still there remains whatgnay be called a 



higher line of explanation. There is no doubt that the different 

 shapes of isobars are the product of different phases of atmo- 

 spheric circulation, just like the eddies and backwaters of a river, 

 and that the appearance of the weather is the product of the com- 

 plex vertical and lateral movements thus set up. For instance, 



there is no doubt that the principal cause of rain in a cyclone is 

 the condensation of the ascentional current of air round its centre, 

 while Ley and others have shown that many of the well-known 

 forms of clouds are due to the action of upper currtnts moving in 

 a different direction to those on the surface, and with a different 



