MIGRATION AND WEATHER 99 



flic ting forces of the easterly winds, the trend towards 

 the north-east of the rapidly travelling cyclone and 

 its own southward flight. If it is flying faster than 

 the speed of the cyclone it will drift westward but 

 gradually approach the low pressure centre. After 

 passing this its course will at once change and its 

 speed will be accelerated towards the east. 



Even violent storms move at varying rates, and 

 it is conceivable that a bird leaving Scandinavia on 

 favourable anticyclonic winds might at once come into 

 the influence of a large, slowly-moving, circular storm, 

 with a low-pressure centre to the west of Ireland, 

 and might, if the air currents were strong, be carried 

 westward at first, then south and finally eastward, so 

 that it would actually pass round the British Islands. 



I have taken this exceptional case from the actual 

 course of a storm, which varied between forces 9 and 



II on the Beaufort Scale (say an average of 50 

 miles per hour) but only travelled slowly eastward 

 at about 17 miles per hour. In some cases the 

 storm centres are nearly stationary for many 

 hours. 



It is easy to appreciate Herr Herman's statement 

 that spring immigration in Hungary is accelerated 

 on the good side of a mild cyclone ; the direction of 

 the bird, of the circulating air currents and of the 

 whole system may be coincident. Given a low- 

 pressure centre west of the Bay of Biscay, spring 



