1920.] of the Migratonj Flight uf Birds. 933 



routes cross seas too wide to be bridged by the eye of a 

 bird flying at a sufficient altitude, and 5000 leet is 

 considered sufficient altitude for all practical purposes ; 

 but that birds do ascend to 25,000 feet is unquestion- 

 able." 

 Now calculating on the formula /3. V.l. in feet the 



V 2 



following are the horizontal distances in miles at which 

 the horizon is visible from heights in feet. Of course, high 

 ground showino- above the horizon would be visible from a 

 lower height, but it takes very high ground to appear above 

 the horizon at 100 miles. 



From 500 feet 27 miles. 



1000 „ 39 „ 



2000 „ 55 „ 



3000 „ 67 „ 



» 4000 „ 77 „ 



5000 „ 86 „ 



„ 10,000 „ 122 „ 



„ 20,000 „ 176 „ 



„ 26,700 „ 200 „ 



I do not know on what Dixon bases his statement that 

 birds unquestionably ascend to 25,000 feet. Nor in the first 

 part of his statement under criticism has he considered the 

 migrations of the Golden Plover of America during either 

 their Atlantic or Pacific 'passages, nor that of our late- 

 lamented Esquimaux Curlew, of Urodynamis taitiensis the 

 lonor-tailed Cuckoo of New Zealand, or of certain of the 

 Petrels. The least of these migrants woukl have to ascend 

 to an altitude of over 70 miles in order to satisfy Dixon's 

 theory, and the Golden Plover would be flying far o"tside 

 the cushion of air which surrounds the earth. 



But perhaps Dixon's interpretation of regular migration 

 routes is narrowed to those arbitrary and largely imaginary 

 lines which some naturalists have traced across the world, in 

 the fond hope that birds will abide by them. 



Just one word ou loug-distance visibility. The following 



