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»rc hatched together, the eggs are excluded singly, and at considerable intervals. Ten, 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. . w' ,
VIII. A principal topic of comparison between animals, is in their instruments of mo- tion. These come before us under three divisions; feet, wings, and fins. I desire any man to fay, which of the three is best fitted for its use: or whether the fame consummate art be not conspicuous in them all. The constitution of the elements, in which the motion is to be performed, is very different. The animal action must necessarily follow that constitution. The Creator therefore, if we might so speak, had to prepare for different situations, for different difficulties : yet the purpose is accomplished not less successfully, in one case than the other. And, as between wings and the corresponding limbs of quadrupeds, it is accomplished without desertingthe general idea. The idea is modified, not deserted. Strip a wing of its seathers, and it bears no obscure resemblance to the fore-leg of a quadruped. The
articulations articulations at the shoulder and the cubitus are much alike; and, what is a closer circumstance, in both cases the upper part of the limb consists of a single bone, the lower part of two.
But, fitted up with its furniture of feathers and quills, it becomes a wonderful instrument; 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 surface in opposite directions, either upwards and downwards, or estimated in, any oblique line, the bird would lose as much by one motion, as she gained by another. The skylark could never ascend by such an action as this: for, though the stroke upon the air by the under fide of her wing would tarry her up, the stroke from the upper side, when me 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 neceflary to suppose, that the surface of the wing, measured upon the fame
plane, plane, is contracted, whilst the wing is drawn up ; and let out to its full expansion, when it descends upon the air for the purpose of moving the body by the reaction of that element. Now the form and structure of the wing, its external convexity, the disposition, 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 mape and dimensions. Such a twist, for instance, or semirotatory motion, is given to the great feathers of the wing, that they strike the air with their flat side, but rife from the stroke slantwise. The turning of the oar in rowing, whilst the rower advances his hand for a new stroke, is a similar operation to that of the feather, and takes its name from the resemblance. 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 resolution of forces, does two things at the fame time; supports the body in the air, and carries it forward,
The jleerage 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 circumstance not a little remarkable. Birds with long legs have short tails; and, in their flight, place their legs close to their bodies, at the fame time stretching them out backwards as far as they can. In this position the legs extend beyond the rump, and become the rudder; supplying that steerage which the tail could not.
From the wings of birds, the transition is easy to the Jins of fish. They are both, to their respective tribes, the instruments of their motion; but, in the work which they have to do, there is a considerable difference, founded in this circumstance. Fish, unlike birds', h save very nearly the lame specific gravity with the element in which they move. In the case of fish, therefore, there is little or no weight to bear up: what is wanted, is only an impulse sufficient to carry the body through a resisting medium, or to maintain the posture, or to support or restore the balance of the body, which is always the most unsteady where there is no weight to sink it. For these offices the fins are as large as necessary, though much smaller than wings, their action mechanicals their posi3 tion, tion, and the muscles by which they are moved, in the highest degree, convenient. The following short account of some experiments upon fish, made for the purpose of ascertaining the use of their fins, will be the best confirmation of what we assert. In most fish, beside the great fin the tail, we find two pair of fins upon the sides, two single 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 side: if the ventral fin on the fame side be cut away, then it loses its equilibrium entirely: .if the dorfal and ventral fins Be cut off, the fish reels to the right and left. When the fish dies, that is, when the sins cease to play, the belly turns upwards. The use of the fame parts for motion is seen in the following observation upon them when put in action. The pectoral, and more particularly the ventral fins, serve to raise and depress the fish: when the fish desires to have a retrograde motion, a stroke forward with the pectoral fin effectually