Sayfadaki görseller
PDF
ePub

POSITION OF THE EARTH IN SPACE.

13

will inquire if this particular distance from the sun be better for our earth and its creatures than a greater or less would have been. We may be mathematically certain that the heat of the sun is according to the density of the sun-beams, and is reciprocally proportional to the square of the distance from the body of the sun.* Now, by this calculation, suppose the earth

, , should be removed and placed nearer to the sun, and revolve, for instance, in the orbit of Mercury, there the whole ocean would even boil with extremity of heat, and be all exhaled into vapours; all plants and animals would be scorched and consumed in that fiery furnace. But suppose the earth should be carried to the great distance of Saturn; there the whole globe would be one frigid zone; the deepest seas under the very equator would be frozen to the bottom; there would be no life, no germination, nor anything that comes now under our knowledge or senses. It was much better, therefore, that the earth should move where it does, than in a much greater or less interval from the body of the sun. And if you place it at any other distance, either less or more than Saturn or Mercury, you

will still alter it for the worse, proportionally to the change. It was situated, therefore, where it is by the wisdom of some voluntary agent, and not by the blind motions of fortune or fate. If any one should think within himself, how, then, can any animal at all live in Mercury and Saturn in such intense degrees of heat and cold? let him only consider, that the matter of each planet may have a different density, and texture, and form, which will dispose and qualify it to be acted on by greater or less degrees of heat, according to their several situations; and that the laws of vegetation, and life, and sustenance, and propagation, are the arbitrary pleasure of God, and may vary in all planets according to the divine appointment and the exigencies of things, in manners incomprehensible to our imaginations. It is enough for our purpose to discern the tokens of wisdom

Newton, Principia, p. 415. VOL. IV.

a

B

in the placing of our earth ; if its present constitution would be spoiled and destroyed, if we could not wear flesh and blood, if we could not have human nature at those different distances.

WILLIAM DERHAM, D.D.

Like his neighbour, John Ray of Black Notley, Dr Derham * was a clergyman who cultivated with much zeal different branches of Natural History. In his parsonage at Upminster he collected a large museum, including an extensive series of ornitholigical specimens, and both by his own publications, and the affectionate zeal with which he edited the labours of others, he earned a just renown amongst investigators abroad, and amongst his brethren of the Royal Society at home. In 1711 he was invited to preach the Boyle Sermons, and he afterwards published them under the title, “ Physical Theology ; or, a Demonstration of the Being and Attributes of God from the Works of Creation." This work, with its companion volume, the “ Astro-Theology," and Ray's “Wisdom of God in Creation," long enjoyed a great and well-merited popularity, and all the three are interesting as the first specimens of a delightful literature in which British authorship abounds, and of which the Bridgewater Treatises are the most familiar, as well as the most finished specimens.

Derham's work being originally in the form of sermons, his detailed illustrations are given in foot-notes. In the last of the following notes it is hardly necessary to premise that the fisherman's story about swallows hybernating under water is apocryphal.

On Bírds.

As this tribe hath a different motion from that of other ani

a

* Born at Stoughton, near Worcester, November 26, 1657 ; died at Upa minster, Essex, April 5, 1735.

[blocks in formation]

mals, and an amphibious way of life, partly in the air, and partly on the land and waters, so is their body accordingly shaped, and all their parts incomparably fitted for that way of life and motion; as will be found by a cursory view of some of the particulars. And the

1. First and most visible thing, is the shape and make of their body, not thick and clumsy, but incomparably adapted to their flight: sharp before, to pierce, and make way through the air, and then, by gentle degrees, rising to its full bulk. To which we may add,

2. The neat position of the feathers throughout the body; not ruffled, or discomposed, or placed some this, some a contrary way, according to the method of chance; but all artificially placed for facilitating the motion of the body, and its security at the same time, by way of clothing: and for that end, most of the feathers tend backward, and are laid over one another in exact regular method, armed with warm and soft down next the body, and more strongly made, and curiously closed next the air and weather, to fence off the injuries thereof. To which purpose, as also for the more easy and nimble gliding of the body through the air, the provision nature hath made, and the instinct of these animals to preen and dress their feathers, is admirable; both in respect of their art and curiosity in doing it, and the oil-bag* glands and whole apparatus for that service.

And now, having said thus much relating to the body's motion, let us, 3. Survey the grand instrument thereof, the wings,-- which, as they are principal parts, so are made with great skill, and placed in the most commodious point of the body,* to give it an exact equipoise in that subtile medium, the air.

* Mr Willoughby saith there are two glands for the secretion of the unctuous matter in the oil-bag. And so they appear to be in geese. But upon examination, I find, that in most other birds (such at least as I have inquired into) there is only one gland : in which are divers little cells, ending in two or three larger cells, lying under the nipple of the oil-bag. This nipple is perforated, and being pressed, or drawn by the bird's bill, or head, emits the liquid oil, as it is in some birds, or thicker unctuous grease, as it is in others.

And here it is observable, with what incomparable curiosity every feather is made; the shaft exceeding strong, but hollow below for strength and lightness sake; and above, not much less strong, and filled with a parenchyma or pith, both strong and light too. The vanes are nicely gauged on each side as

broad on one side, and narrower on the other; both which incomparably minister to the progressive motion of the bird, as also to the union and closeness of the wing.+

And no less exquisite is the textrine art of the plumage

made;

* In all birds that iy much, or that have the most occasion for their wings, it is manifest that their wings are placed in the very best part, to balance their body in the air, and to give as swift a progression as their wings and body are capable of. For otherwise, we should perceive them to reel, and fly unsteadily; as we see them do if we alter their equipoise by cutting the end of one of the wings, or hanging a weight at any of the extreme parts of the body.

of The wise Author of Nature hath afforded an example of the great nicety in the formation of birds, by the nicety observed in a part no more considerable than the vanes of the flag-feathers of the wing. Among others, these two things are observable. 1. The edges of the exterior or narrow vanes bend downwards, but of the interior, wider vanes upwards; by which means they catch hold, and lie close to one another, when the wing is spread, so that not one feather may miss its full force and impulse upon the air. 2. A yet lesser nicety is observed, and that is in the very sloping the tips of the flag-feathers. The interior vanes being neatly sloped away to a point, towards the outward part of the wing; and the exterior vanes, towards the body, at least in many birds; and in the middle of the wing, the vanes being equal, and but little sloped. So that the wing, whether extended or shut, is neatly sloped and formed, as if constantly trimmed with a pair of scissors.

# Since no exact account that I know of, hath been given of the mechanism of the vanes or webs of feathers, my observations may not be unacceptable. The vane consists not of one continued membrane, because if once broken, it would hardly be reparable ; but of many laminæ, which are thin, stiff, and somewhat of the nature of a thin quill. Towards the shaft of the

>

[blocks in formation]

also; which is so curiously wrought and so artificially interwoven, that it cannot be viewed without admiration, especially when the eye is assisted with glasses.

And as curiously made, so no less curiously are the feathers placed in the wing, exactly according to their several lengths and strength: the principals set for stay and strength, and these again well lined, faced, and guarded with the covert and secondary feathers, to keep the air from passing through, whereby the stronger impulses are made thereupon.

And lastly, to say no more of this part, that deserves more to be said of it, what an admirable apparatus is there of bones, very strong, but withal light and incomparably wrought! of joints, which open, shut, and every way move, according to the occasions either of extending it in flight, or withdrawing the wing again to the body! and of various muscles; among which the peculiar strength of the pectoral muscles deserves especial remark, by reason they are much stronger in birds than in man, or any other animal not made for flying.

feather (especially in the shaft-feathers of the wing) those lamina are broad, &c., of a semicircular form, which serve for strength, and for the closer shutting of the laminæ to one another, when impulses are made upon the air. Towards the outer part of the vane, these laminæ grow slender and taper. On their under side they are thin and smooth, but their upper outer edge is parted into two hairy edges, each side having a different sort of hairs, laminated or broad at bottom, and slender and bearded above the other half. I have, as well as I could, represented the uppermost edge of one of these laminæ with some of the hairs on each side, magnified with a microscope. These bearded bristles or hairs on one side the laminæ, have straight beards; those on the other side, have hooked beards on one side the slender part of the bristle, and straight ones of the other. Both these sorts of bristles magnified (only scattering and not close) are represented as they grow upon the upper edge of the laminæ. And in the vane, the hooked beards of one lamina always lie next the straight beards of the next lamina, and by that means lock and hold each other, and by a pretty mechanism brace the laminæ close to one another. And if at any time the vane happens to be ruffled and discomposed, it can by this pretty easy mechanism be reduced and repaired.

« ÖncekiDevam »