The tail is the principal instrument by which the progressive motion is effected. Thus--suppose that the tail is inclined to the right; if, in this situation, the muscles of the left side, tending to bring the tail in a right line with the body, are suddenly thrown into action, the resistance of the water, by reacting against the broad surface of the tail in the direction P R, perpendicularly to that surface, will cause the muscular action to give the whole body an impulse in that direction; and the centre of gravity, C, will move onwards in the direction C B, parallel to P R. This impulse is not destroyed by the farther flexion of the tail towards the left side, because the principal force exerted by the muscles has already been expended in the motion from R to M, in bringing it to a straight line with the body; and the force which carries it on to L is much weaker, and therefore occasions a more feeble reaction. When the tail has arrived at the position L, indicated by the dotted outline, a similar action of the muscles on the right side will create a resistance and an impulse in the direction of K L, and a motion of the whole body in the same direction, C A. These impulses being repeated in quick succession, the fish moves forward in the diagonal C D, intermediate between the direction of the two forces.
Upon the same principle a boat is impelled by paddling; and the action of the rudder of a ship in turning the vessel will be readily understood. In this latter case, however, there is an additional mechanical advantage; since the point round which the vessel turns, is beyond the middle and towards the prow, and hence the force applied at the extremity of the keel acts as by an arm of a lever.
Note 37, p. [236].--Flight of birds.
In appreciating the mechanical means by which a bird is enabled to direct its course, we must not omit to take into account the power it possesses of changing the position of the centre of gravity of its body, so that the reaction of the air may be modified with regard to each wing.
Note 38, p. [236].--Flight of insects.
The command possessed by insects in directing and changing their course, seems more perfect even than that of birds. Many of them travel on their wings to immense distances, and, considering their comparative size, they generally move through the air with greater velocity than that of birds. Bees have been known to fly great distances from their hive, in search of food; and the silk-worm moth has travelled more than a hundred miles in a very short space of time. Many of our readers have, no doubt, noticed with surprise the apparent facility with which gnats have accompanied them, although they may have been advancing on horseback at a full gallop; and the author during the last summer has been forcibly struck with the manner in which flies and other insects have kept up with a railway carriage, alternately flying in and out of the vehicles, as though they had been at perfect rest. Some species possess a remarkable power of poising themselves in the air, and hovering for a length of time over the same spot, without falling or rising, advancing or retreating; the Dragon-fly affords a striking example of this fact.
Note 39, p. [237].--Obliquity of the wings of birds.
In consequence of the manner in which the wings are affixed to the scapula, they give a stroke to the air in a direction both downwards and backwards; so that while the former supports the bird, the latter impels it forward. It is curious to notice that the degree of this obliquity varies in different birds, and is evidently adapted to their habits: thus, for instance, birds of prey have a great obliquity of wing, which better enables them to pursue their victims in a horizontal course; while those birds which soar to a considerable elevation, in a nearly vertical direction, as the Lark, have scarcely any obliquity of wing, but strike directly downwards.
Note 40, p. [238].--A mechanical proposition.