of a different density from that of the body, by which means the rays of light in their passage are arrested by innumerable refractions and reflections, become extinct, and are absorbed. It hence appears that all opaque bodies become transparent when their pores are filled with a substance of nearly equal density with their parts; and thus paper that has imbibed oil acquires transparency.

Mr. Joshua Bamford also answered this query.

QUERY 63.

Answered by Mr. J. B

-n, London.

This question has often been discussed, and upon the subject there are three opinions. 1st. That they migrate to a warmer climate. 2nd. That they retire to hollow trees and caverns, where they lie in a torpid state. 3rd. That they lie in the same state at the bottom of lakes, aud under the ice.

Those who assert that these birds migrate to a warmer country in winter, argue, That as it is a fact fully proved by the observations of natural historians, that many kinds of birds regularly migrate, is it not therefore more probable that swallows and martins, which disappear regularly every season, retire to some other country, than that they lie in a state of torpor in caverns and lakes? But this opinion is founded on facts; we often see them collected in great flocks, on churches, rocks, and trees, about the time when they annually disappear, and the direction of their flight has invariably been observed to be to the southward.

"Aloft they fly, and melt in distant air.
Far o'er the British sea, in westering course,
O'er the Biscayan mountain waves they glide:
Then o'er Iberian plains, through fields of air,
Perfum'd by orchard groves, where lowly bends
The orange bough beneath its juicy load,
And over Calpe's iron-fenc'd rock their course,
To Mauritania's sunny plains they urge.
There are, who doubt this migratory voyage,
But wherefore from the distance of the flight,
Should wonder verge on disbelief; the bulk
Se small, so large and strong the buoyant wing?"

Grahame's "Birds of Scotland."

They have also been found on their passage at a great distance from land. Mr. Adamson informs us, that about fifty leagues from the coast of Senegal four swallows settled upon the ship, that these birds were taken, and that he knew them to be European swallows, which he conjectures were returning to the coast of Africa; he also asserts, that during his long residence at Senegal, he constantly observed the swallows arrive there, about the same time that they leave Europe. Sir Charles Wager also mentions a great flock of swallows which settled on his rigging as he entered the channel. Since therefore swallows have been seen assembled in great flocks in autumn flying off in company towards southern climes, since they have been found both on their passage from Europe and returning again, can there be any doubt of their annual migration? The second notion has great antiquity on its side. Aristotle * and Pliny + say, that swallows do not remove very far from their summer habitation, but winter in the hollows of rocks. Between the town of Caen and the sea, along the river Orme, there are many caverns, where clusters of swallows are found hanging in the form of grapes. The same observation has long since been made in Italy. Albinovanus, in the elegant elegy which he wrote on the death of Mæcenas, describes the swallows retiring to the rocks as a sign of the approach of winter.

"Conglaciantur aquæ, scopulis se condit hirundo:
Verberat egelidos garrula, vere lacus."

"Frost binds the streams, in rocks the swallows lie;
In spring to cooling streams they twittering fly."

Of late several proofs have been brought of some spe-
cies having been discovered in a torpid state. 1st. In
the chalky cliffs of Sussex, as was seen in the fall of a
great fragment some years ago. 2nd. In a decayed
hollow tree, that was cut down near Dolgelli in Merio-
nethshire. 3rd. In a cliff near Whitby in Yorkshire,
where, in digging out a fox, whole bushels of martins
were found in a torpid condition. 4th. The Rev. Mr.
Conway, of Sychton, Flintshire, a few years ago in
*Arist. Hist. Animal. Lib. viii. cap. 12.
Plinii Hist. Nat. Lib. x. cap. 24

looking down an old lead mine in that county, observed numbers of swallows clinging to the timbers of the shaft, seemingly asleep, and on throwing some gravel on them, they moved, but never attempted to fly; this was between All Saints and Christmas. The third opinion has had a thousand testimonies brought in support of it, but they all seem to carry their own refutation along with them, it has, however, the authority of Linnæus in its favour, as appears from the following passage:— "Hirundo (Rustica) habitat in Europæ domibus intra tectum, cum urbica demergitur, vereque emergit." It is clear from the expression of demergitur (though perhaps not classical) that this naturalist conceived these birds hid themselves under water, during the winter. Dr. Wallesius, the cele brated Swedish chemist, likewise informs us, that he has more than once seen swallows assembling on a reed in a lake, till they were all immersed, and went to the bottom, previous to which they sung a dirge of a quarter of an hour's length. He had also seen a swallow caught during the winter out of a lake, with a net, drawn (as is common in northern countries) under the ice; this bird was brought into a warm room, revived, fluttered about, and soon after died. The fact, however, of swallows having been found in lakes and ponds is extremely doubtful, for though there are abundance of lakes in Britain, yet there is no well attested fact of swallows having been found therein in a state of torpor and immersion. Till such are found, we are surely excusable in suspending our belief. Upon the whole, we may conclude, that the swallows and martins in Spain, Italy, France, and perhaps some from England, remove to warmer climates; and that some English ones, and some in Germany, and other cold climates, retire into clefts and holes in the rocks, and remain there in a torpid state.

Mr. John Baines after enumerating various instances of swallows having been observed in their annual migrations, and likewise of their being found in a state of torpidity in hollow trees, caverns, &c. adds, "The experiments of the Count de Buffon, and Mr. John Hunter, render the third opinion very improbable"-he sup

poses, that the swallows found in a torpid state in caverns, &c. are only the latter hatches, or those young birds which are incapable of distant migrations; and that the main bulk pass the winter in some part of the continent of Africa, which opinion, he says, is supported by Adamson, Buffon, Catesby, Marsigli, Ray, Reamer, Willoughby, and other eminent naturalists. Mr. Bamford's opinion exactly coincides with Mr. Baines'.

Miss Groves says, "There are four species of the swallow tribe which visit England, and that it is highly probable, three of these species annually migrate to a warmer climate; but that the fourth, viz. the sandmartin, from its habit of building its nest in the sandy banks of rivers, and in cliffs and quarries, may probably pass the winter in a torpid state."

cr

Answered by Mr. A. Nesbit of Farnley.

How swallows dispose of themselves during the winter is a question which has long puzzled the scientific, and amused the speculative world; nor does it appear that the point is yet clearly 'settled." Mr. Nesbit here enumerates the three different opinions which have been formed, (see Mr. J. B-n's communication) and adds, "But I am persuaded, that if we give the following assertions and conjectures their due weight, it will appear pretty evident that these birds dispose of themselves different ways."

Olaus Magnus, in his Breviar. Hist. Septent. Lib.xix. Chap. 11. says, "That swallows take up their winter quarters, in lumps and clusters under the ice, in the northern seas," and Etmuller affirms, "That he has found swallows in clusters, among the reeds and rushes. of lakes and fish-ponds.

Dr. Derham says, "That at a meeting of the Royal Society, Feb. 12, 1711-12, Dr. Colas declared, That he saw sixteen swallows drawn out of the lake of Samdrot, and about thirty out of the king's great, pond in Rosinielin, and that at Schlebitten, he saw two swallows come out of the water, with their wings, hanging on the ground, and very weak, not being able to fly.

See Derham's Physico Theology. Book vii. chap. 3.

note 4.

Mr. A. Aikin in his "Natural History for the Year," says, "That probably some of the latter broods of swallows remain with us during the winter, for the most part in a state of torpidity; but that the main body migrates across the channel to France, thence to Spain, and passes at Gibraltar, to the northern shores of Africa, returning by the same road, in the spring, to Great Britain."

It does not appear, however, that this is always their route, for it is well known that they have been frequently seen in very numerous flocks, by mariners, at a great distance from land; and have occasionally converted into a resting place, the rigging and yards of vessels.

Answered by Mr. John Smith.

Different opinions have been entertained by naturalists respecting the winter retreat of the birds in question. Some have thought that the chimney swallow (hirundo rustica) never quits this country; and others have inclined to the same notion with respect to the whole tribe of hirundines. It has been affirmed of these birds, (but more particularly of swallows), that, at the time of their disappearance, they congregate and immerge into rivers, ponds, &c. in the mud, at the bottrn of which, they lie torpid during winter, and thence

erge on the approach of summer. This opinion pecting the submersion, and after-resuscitation of swallows, has, however, been controverted. From the similarity of their respiratory organs to those of other birds, it has been concluded, that they could not remain under water for any length of time without drowning; and since they moult, (if they moult at all) after their disappearance, it has been thought to require too great a stretch of credulity to believe that they acquire new feathers in those subaqueous abodes. Perhaps the most probable opinion is, that both swallows and martins are migratory; and as voyagers have observed large flights of them making their way towards Africa,