ance or rushing, that they may be enabled to
steal through the air unheard upon a nimble and watchful quarry.

While I am talking of owls, it may not be improper to mention what I was
told by a gentleman of the county of Wilts. As they were grubbing a vast
hollow pollard-ash that had been the mansion of owls for centuries, he
discovered at the bottom a mass of matter that at first he could not
account for. After some examination he found that it was a congeries of
the bones of mice (and perhaps of birds and bats) that had been heaping
together for ages, being cast up in pellets out of the crops of many
generations of inhabitants. For owls cast up the bones, fur, and
feathers of what they devour, after the manner of hawks. He believes, he
told me, that there were bushels of this kind of substance.

When brown owls hoot their throats swell as big as a hen's egg. I have
known an owl of this species live a full year without any water. Perhaps
the case may be the same with all birds of prey. When owls fly they
stretch out their legs behind them as a balance to their large heavy
heads, for as most nocturnal birds have large eyes and ears they must
have large heads to contain them. Large eyes, I presume, are necessary
to collect every ray of light, and large concave ears to command the
smallest degree of sound or noise.

I am, etc.

* * * * *

[It will be proper to premise here that the sixteenth, eighteenth,
twentieth, and twenty-first letters have been published already in the
"Philosophical Transactions;" but as nicer observation has furnished
several corrections and additions, it is hoped that the republication of
them will not give offence; especially as these sheets would be very
imperfect without them, and as they will be new to many readers who had
no opportunity of seeing them when they made their first appearance.]

"The hirundines are a most inoffensive, harmless, entertaining, social,
and useful tribe of birds; they touch no fruit in our gardens; delight,
all except one species, in attaching themselves to our houses; amuse us
with their migrations, songs, and marvellous agility; and clear our
outlets from the annoyances of gnats and other troublesome insects. Some
districts in the south seas, near Guiaquil, are desolated, it seems, by
the infinite swarms of venomous mosquitoes, which fill the air, and
render those coasts insupportable. It would be worth inquiring whether
any species of hirundines is