METEOROLOGICAL CONDITIONS 89 



following note in regard to these phenomena : 13th of August, 

 wind south-south-east; on the previous evening, south-east. At 

 the beginning of the night, before midnight, of the 12th, Redstarts, 

 Wheatear, Plovers, Sandpipers, and similar species, were on the 

 move. Later on in the night a violent thunderstorm broke out, 

 putting an end to the migration. The morning of the 13th was 

 clear, fine, warm, but no birds were to be seen ; query, had the 

 thunderstorm prevented their setting out on their migration 

 journey ? A day later, the migration of all birds due at the time 

 was commenced afresh, and lasted, with light southerly and south- 

 easterly winds and fine weather, until the 20th, whereupon, again, 

 on the night of the 21st, a thunderstorm, followed by stormy north- 

 west winds, put an end to all migratory movements. These condi- 

 tions of weather prevailed until the 4th of September, on which day 

 calm and fine weather set in, with a light north wind which veered 

 round to north-east, and, later on, to south, whereupon Redstarts, 

 Warblers, Flycatchers, Chats, Dotterels, and many ' Waders ' made 

 their appearance in the full prosecution of their migratory journey. 



The sheet- or summer-lightning, which is often observed on 

 evenings after very hot days, has a similar deterrent effect on 

 migration, and the same may be said in regard to the powerful 

 electrical discharges, not usually followed by thunder, which 

 frequently take place during the nights of late autumn, and 

 regularly precede and accompany violent storms. 



Another very peculiar phenomenon also intimately connected with 

 thunderstorms is the regular but temporary appearance, in millions, 

 of the large Dragon-fly (Libellula quadripunctata) before such 

 disturbances. Countless swarms of these insects make their ap- 

 pearance all of a sudden during the calm, sultry hours preceding 

 the catastrophe, while thunder-clouds gather on the horizon, and, 

 heaped upon each other, project into the blue ether beyond, like so 

 many giant mountains of snow. 



The directions whence these insects proceed cannot be ascer- 

 tained, nor do they arrive in swarms or companies, but solitary 

 individuals or scattered groups probably congregate on the spot in 

 one vast throng. The assembling individuals or groups must, how- 

 ever, follow each other in very rapid succession, for in a short time 

 the face of the cliff, still illumined by the sun, all the buildings, 

 hedges, and dry twigs on the island, are covered with them. Nor 

 is it necessary for the occurrence of this phenomenon that the 

 storm should actually discharge itself over Heligoland, or even in 

 its immediate neighbourhood, but only that the thunder-clouds 

 extend over about two-thirds of the whole expanse of sky as 

 measured from the horizon to the zenith. The insects vanish 



