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There feems to be but one method indeed, by which the height 

 of a bird in the air may be eftimated ; which is, by comparing its 

 apparent fize with its known one, when very near us ; and it need 

 not be faid that this method of calculating muft depend entirely 

 upon the fight of the obferver, who, if he happens not to fee ob- 

 jects well at a difbnce, will very foon fuppofe the bird to be loft 

 in the clouds. 



There is alfo another objection to the hypothefis of birds parting 

 feas at fuch an extraordinary height, arifing from the known rare- 

 faction of the air, which may poffibly be inconvenient for refpira- 

 tion, as well as flight ; and if this was not really the cafe, one 

 mould fuppofe that birds would frequently rife to fuch uncom- 

 mon elevations, when they had no occafion to traverfe oceans. 



The Scotch Ptarmigan frequents the higheft ground of any Bri- 

 tifh bird, and it takes but very fhoit flights. 



But it is alfo urged by fome, that the reafon, why feamen do 

 not regularly fee the migration of birds, is becaufe they choofe the 

 night, and not the day, for the paflage e . 



Now though it may be allowed, that poffibly birds may crofs 

 from the coaft of Holland to the Eaftern coafr. of England (for 

 example) during a long night, yet it mult be dark nearly as long 

 as it is within the Arctic circle to afford time for a bird topafsfrom- 



fpecies of eagle from the bottom of a mountain, and followed it to the 

 top, when the bird hath rifen fo high as to appear lefs than he did from 

 the bottom. Mr. Hunter however adds, that he could mil hear the cry, 

 and diftinguifh the bird. 



e Mr. Catefby fuppofes that they may thus pafs in the night-time, to 

 avoid birds of prey. Phil. Tranf. Abr. Vol, II. p. 887. But are not owls 

 then ftirring ? 



On the other hand, if they migrate in the day-time, kites, hawks, and 

 other birds of prey, muft be very bad iportfmen not to attend (like 

 Arabs) thefe large and periodical caravans. 



the 



