64 



It must not be forgotten too, that normal migration, according 

 to the author, is not only performed at a height of not less than 

 four miles, but is also carried out at a great speed, and to use his 

 own words — " in one unbroken and for the most part nocturnal 

 flight." This supposed possession of a sense enabling birds to 

 detect the state of the atmosphere at heights exceeding four 

 miles, must, to be of really much use, greatly exceed the power 

 of a barometer in order to enable them to ascertain the meteoro- 

 logical conditions they will encounter eight or nine hundred 

 miles or more distant from their starting point, and that they are 

 able to do this there is no evidence to prove. 



It must often happen, however, that changes of the weather ■ 

 at the earth's surface do take place after such indications merely 

 as coincidences, in the same manner as a mass migration supposed 

 to be induced by a desire to escape difi&culties in front, may 

 in reality be pushed forward into dangers ahead by causes ia 

 the rear. 



It must be by no means forgotten that all Herr Gatke's 

 observations on the weather have been conducted in the very 

 limited area in which he resides, and though there is no reason 

 for doubting the accuracy of his statement that birds migrate 

 nearest the surface of the earth during the prevalence of light 

 south-east winds, the fact must be taken to apply only locally. 

 A reference to the charts published in the daily papers referring 

 to the winds of the previous twenty-four hours, will very commonly 

 reveal the fact that the air-currents nearest the surface of the 

 earth are traversing semi-circular or even still more eccentric 

 paths. Thus a south-east wind at Heligoland may in the Baltic 

 have been travelling from the north-east and the same current in 

 the North Sea might be locally recorded as a south-west wind. 

 Herr Gatke also draws attention to the fact that the greater or 

 lesser quantity of moisture in the atmosphere has also an 

 important influence on the amount of visible migration. The 

 quantity of moisture in the air is generally determined by the 

 direction from which the wind has been travelling, and also the 

 comparative areas of sea or land over which it has passed ; thus, 

 in the British Isles, winds blowing from quarters between the 

 points of north, north-west and due south, are, as a rule, heavily 



