disappear some and some gradually, as they come, for the bulk of
them seem to withdraw at once: only some stragglers stay behind a
long while, and do never, there is the greatest reason to believe,
leave this island. Swallows seem to lay themselves up, and to come
forth in a warm day, as bats do continually of a warm evening,
after they have disappeared for weeks. For a very respectable
gentleman assured me that, as he was walking with some friends
under Merton-wall on a remarkably hot noon, either in the last
week in December or the first week in January, he espied three or
four swallows huddled together on the moulding of one of the
windows of that college. I have frequently remarked that swallows
are seen later at Oxford than elsewhere: is it owing to the vast
massy buildings of that place, to the many waters round it, or to
what else?

When I used to rise in a morning last autumn, and see the swallows
and martins clustering on the chimnies and thatch of the
neighbouring cottages, I could not help being touched with a secret
delight, mixed with some degree of mortification: with delight to
observe with how much ardour and punctuality those poor little
birds obeyed the strong impulse towards migration, or hiding,
imprinted on their minds by their great Creator; and with some
degree of mortification, when I reflected that, after all our pains
and inquiries, we are yet not quite certain to what regions they do
migrate; and are still farther embarrassed to find that some do not
actually migrate at all.

These reflections made so strong an impression on my imagination,
that they became productive of a composition that may perhaps
amuse you for a quarter of an hour when next I have the honour of