tions of nature. Some years ago, a plan was fuggefted, of producing propulfion by reaction in this way. By the force of a steam engine, a stream of water was to be shot out of the ftern of a boat; the impulfe of which ftream upon the water in the river, was to push the boat itself forward: it is, in truth, the principle by which fky-rockets afcend in the air. Of the ufe or the practicability of the plan I am not speaking; nor is it my concern to praise its ingenuity; but it is certainly a contrivance. Now, if naturalifts are to be believed, it is exactly the device, which nature has made use of, for the motion of fome fpecies of aquatic infects. The larva of the dragon fly, according to Adams, fwims by ejecting water from its tail; is driven forward by the reaction of water in the pool upon the current iffuing in a direction backward from its body,

VII. Again; Europe has lately been furprifed by the elevation of bodies in the air by means of a balloon, The difcovery confifted in finding out a manageable fubftance, which was, bulk for bulk, lighter than air; and the application of the difcovery was, to make a body compofed of this fubftance bear

up, along with its own weight, fome heavier body which was attached to it. This expedient, fo new to us, proves to be no other than what the author of nature has employed in the gofamir Spider. We frequently fee this fpider's thread floating in the air, and extended from hedge to hedge, across a road or brook of four or five yards width. The animal which forms the thread, has no wings wherewith to fly from one extremity to the other of this line; nor mufcles to enable it to fpring or dart to fo great a distance. Yet its creator hath laid for it a path in the atmosphere; and after this manner. Though the animal itself be heavier than air, the thread which it fpins from its bowels is fpecifically lighter. This is its balloon. The spider left to itself would drop to the ground; but, being tied to its thread, both are fupported. We have here a very peculiar provifion: and to a contemplative eye it is a gratifying spectacle, to fee this infect wafted on her thread, fuftained by a levity not her own, and traverfing regions, which, if we examined only the body of the animal, might feem to have been forbidden to its nature.

I MUST now crave the reader's permiffion to introduce into this place, for want of a better, an obfervation or two upon the tribe of animals, whether belonging to land or water, which are covered by shells.

I. The fhells of fnails are a wonderful, a mechanical, and, if one might so speak concerning the works of nature, an original contrivance. Other animals have their proper retreats, their hybernacula alfo or winter quarters, but the fnail carries thefe about with him. He travels with his tent; and this tent, though, as was neceffary, both light and thin, is completely impervious either to moisture or air. The young fnail comes out of its egg with the fhell upon its back; and the gradual enlargement which the hell receives, is derived from the flime excreted by the animal's fkin. Now the aptnefs of this excretion to the purpose, its property of hardening into a fhell, and the action, whatever it be, of the animal, whereby it avails itself of its gift, and of the conftitution of its glands, (to fay nothing of the work being commenced before the animal is born,) are things, which can, with no probability, be referred to any other cause than to exprefs defign; and that not on the part of

the

the animal alone, in which defign, though it might build the houfe, could not have fupplied the material. The will of the animal could not determine the quality of the excretion. Add to which, that the shell of a inail, with its pillar and convolution, is a very artificial fabric; whilft a fnail, as it fhould feem, is the moft numb and unprovided of all artificers. In the midst of variety, there is likewife a regularity, which would hardly be expected. In the fame fpecies of fnail the number of turns is, ufually, if not always, the fame. The fealing up of the mouth of the fhell by the fnail, is alfo well calculated for its warmth and fecurity; but the cerate is not of the fame fubftance with the fhell.

II. Much of what has been obferved of fnails belongs to hell fish and their shells, particularly to thofe of the univalve kind; with the addition of two remarks. One of

which is upon the great ftrength and hardness of most of thefe fhells. I do not know, whether, the weight being given, art can produce fo ftrong a cafe as are some of these fhells. Which defenfive ftrength fuits well with the life of an animal, that has often to fuftain the dangers of a flormy element and a rocky bottom, as well as the attacks of vora

cious fish. The other remark is, upon the property, in the animal excretion, not only of congealing, but of congealing or, as a builder would call it, fetting in water, and into a cretaceous-fubftance, firm and hard. This property is much more extraordinary, and, chymically fpeaking, more fpecific, than that of hardening in the air; which may be reckoned a kind of exficcation, like the drying of clay into bricks.

III. In the bivalve order of fhell fish, cockles, mufcles, oyfters, &c. what contrivance can be so simple or so clear, as the infertion, at the back, of a tough, tendinous, fubftance, that becomes, at once, the ligament which binds the two fhells together, and the hinge upon which they open and fhut?

IV. The fhell of a lobster's tail, in its articulations and overlappings, reprefents the jointed part of a coat of mail; or rather, which I believe to be the truth, a coat of mail is an imitation of a lobfter's fhell. The fame end is to be answered by both the fame properties, therefore, are required in both, namely, hardness and flexibility, a covering which may guard the part without obftructing its motion. For this double purpofe, the art of man, exprefsly exercised upon the fubject, has not been able to de-.