Pagina-afbeeldingen
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are hatched together, the eggs are excluded fingly, and at confiderable intervals. fifteen, or twenty young birds may be produced in one cletch or covey, yet the parent bird have never been encumbered by the load of more than one full grown egg at one time.

VIII. A principal topic of comparison between animals, is in their inftruments of motion. These come before us under three divifions; feet, wings, and fins. I defire any man to fay, which of the three is beft fitted for its ufe or whether the fame confummate art be not confpicuous in them all. The conftitution of the elements, in which the motion is to be performed, is very different. The animal action must neceffarily follow that conftitution. The Creator therefore, if we might fo speak, had to prepare for different fituations, for different difficulties: yet the purpose is accomplished not lefs fuccessfully, in one case than the other. And, as between wings and the corresponding limbs of quadrupeds, it is accomplished without deferting the general idea. The idea is modified, not deferted. Strip a wing of its feathers, and it bears no obfcure refemblance to the fore-leg of a quadruped. The articulations

articulations at the fhoulder and the cubitus are much alike; and, what is a closer circumftance, in both cafes the upper part of the limb confifts of a fingle bone, the lower part of two.

But, fitted up with its furniture of feathers and quills, it becomes a wonderful inftrument; more artificial than its first appearance indicates, though that be very striking: at least, the use, which the bird makes of its wings in flying, is more complicated, and more curious, than is generally known. One thing is certain; that, if the flapping of the wings in flight were no more than the reciprocal motion of the fame furface in oppofite directions, either upwards and downwards, or estimated in any oblique line, the bird would lofe as much by one motion, as fhe gained by another. The skylark could never afcend by such an action as this for, though the stroke upon the air by the under fide of her wing would the stroke from the upper fide, carry her up, when the raised her wing again, would bring her down. In order, therefore, to account for the advantage which the bird derives from her wings, it is neceffary to fuppofe, that the furface of the wing, measured upon the fame plane,

plane, is contracted, whilst the wing is drawn up; and let out to its full expanfion, when it defcends upon the air for the purpose of moving the body by the reaction of that element. Now the form and ftructure of the wing, its external convexity, the difpofition, and particularly the overlapping, of its larger feathers, the action of the muscles and joints of the pinions, are all adapted to this alternate adjustment of its fhape and dimenfions. Such a twift, for inftance, or femirotatory motion, is given to the great feathers of the wing, that they ftrike the air with their flat fide, but rife from the ftroke flantwife. The turning of the par in rowing, whilft the rower advances his hand for a new ftroke, is a fimilar operation to that of the feather, and takes its pame from the refemblance. I believe that

this faculty is not found in the great feathers of the tail. This is the place also for observing, that the pinions are so set on upon the body, as to bring down the wings, not vertically, but in a direction obliquely tending towards the tail: which motion, by virtue of the common refolution of forces, does two things at the fame time; fupports the body in the air, and carries it forward.

The fleerage of a bird in its flight is effected partly by the wings, but, in a principal degree, by the tail. And herein we meet with a circumftance not a little remarkable. Birds with long legs have short tails; and, in their flight, place their legs close to their bodies, at the same time ftretching them out backwards as far as they can. In this pofition the legs extend beyond the rump, and become the rudder; fupplying that fteerage which the tail could not.

From the wings of birds, the transition is eafy to the fins of fish. They are both, to their respective tribes, the inftruments of their motion; but, in the work which they have to do, there is a confiderable difference, founded in this circumstance. Fish, unlike birds, have very nearly the fame fpecific gravity with the element in which they move. In the cafe of fish, therefore, there is little or no weight to bear up what is wanted, is only an impulse fufficient to carry the body through a refifting medium, or to maintain the posture, or to support or restore the balance of the body, which is always the most unfteady where there is no weight to fink it. For thefe offices the fins are as large as neceffary, though much smaller than wings, their action mechanical, their pofi

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tion, and the muscles by which they are moved, in the highest degree, convenient. The following fhort account of fome experiments upon fish, made for the purpose of ascertaining the ufe of their fins, will be the best confirmation of what we affert. In most fish, beside the great fin the tail, we find two pair of fins upon the fides, two fingle fins upon the back, and one upon the belly, or rather between the belly and the tail. The balancing use of these organs is proved in this manner. Of the largeheaded fish, if you cut off the pectoral fins, i. e. the pair which lies close behind the gills, the head falls prone to the bottom: if the right pectoral fin only be cut off, the fish leans to that fide: if the ventral fin on the fame fide be cut away, then it loses its equilibrium entirely if the dorsal and ventral fins be cut off, the fish reels to the right and left. When the fifh dies, that is, when the fins cease to play, the belly turns upwards. The use of the same parts for motion is feen in the following obferthem when upon in action. The pectoral, and more particularly the ventral fins, serve to raise and deprefs the fish: when the fish defires to have a retrograde motion, a stroke forward with the pectoral fin effectually

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