E '74 3 



ESSAY IV. 



On the periodical Appearing and Difappearing of certain 

 Birds, at different Times of the Year a . 



To William Watson, M.D. F.R.S. 

 Dear Sir, 



AS I know, from fome converfation we had on this head, that 

 you confider the migration of birds as a very interefting 

 point in natural hiftory, I fend you the following refle&ions on 

 this fubjecl: as they have occured to me upon looking into moft of 

 the ornithologifts who have written on this quefhon. 



It will be firft neceffary in the prefent, as in all other difputes, 

 to define the terms on which the controverfy arifes. I therefore 

 premife that I mean, by the word Migration, a periodical paffage 

 by a whole fpecies of birds acrofs a confiderable extent of fea. 



I do not intend therefore to deny that a bird, or birds, may 

 pofliblv fly now and then from Dover to Calais, from Gibraltar 

 to Tangier, or any other fuch narrow ftrait, as the oppolite coafts 

 are clearly within the bird's ken, and the paffage is no more ad- 

 venturous than acrofs a large frefh-water lake. 



a This Effay was firft yrinted in the LXIId vol. of the Philofophical 

 Tranfachons ; but is now reprinted, with confiderable additions. 



I as 



