XX. An essay on the periodical appearing and disappearing of certain birds, at different times of the year. In a letter from the Honourable Daines Barrington, Vice-Pref. R. S. to William Watson, M. D. F. R. S

Dear Sir, As I know, from some conversations we have had on this head, that you consider the migration of birds as a very interesting point in natural history, I send you the following reflections on this subject as they have occurred to me upon looking into most of the ornithologists who have written on this question. It will be first necessary in the present, as in all other disputes, to define the terms on which the controversy arises. I therefore premise that I mean by the word migration, a periodical passage by a whole species of birds across a considerable extent of sea.