510

POWER OF FLIGHT POSSESSED BY BIRDS.

during the down-stroke; otherwise the effect of the former would neutralise that of the latter. This is partly accomplished by the great velocity of the down-stroke compared with the up-stroke, which causes the resistance of the air to be much greater against the former than against the latter.1 But it is by the alteration in the surface of the wing, as it acts upon the air, that the chief difference is made in Birds; the arrangement of their great feathers being such, that they strike the air with their flat sides, but present only their edges in rising. What is called "feathering the oar" in rowing, is a similar operation, performed with the same intention, and deriving its name from this resemblance.

672. The degree in which the wings act in raising the body or in propelling it through the air, varies considerably in different species, according to the way in which they are set. Thus in Birds of Prey, which require a rapid horizontal motion, the surface of the wings is very oblique, so that they strike backwards as well as downwards, and thus impel the body forwards whilst sustaining it in the air. Such birds find a difficulty in rising perpendicularly; and can in fact only do so by flying against the wind, which then acts upon the inclined surface of the wings just as it does upon that of a kite. On the other hand, the Lark, Quail, and such other birds as rise to great heights in a direction nearly vertical, have the wings so disposed as to strike almost directly downwards. It has been estimated that a Swallow, when simply sustaining itself in the air, is obliged to use as much force to prevent its fall, as would raise its own weight to a height of about twenty-six feet in a second. Hence, we may form some idea of the enormous expenditure of force which must take place, when the body is not only supported, but raised and propelled through the air. The Eider-duck is said to fly 90 miles in an hour, and the Hawk 150. The Swallow and Swift pass nearly the whole of the long summer days upon the wing, in search of food for themselves and their

1 This resistance varies as the square of the velocity of the stroke. Hence, if the down-stroke be made three times as fast as the up-stroke, the resistance it experiences will be nine times as great. But as this only operates during one-third of the time, it is in effect equal to three times that which operates against the up-stroke, and which would tend to lower the Bird in the air.

IMPOSSIBILITY OF HUMAN FLIGHT.

511

helpless offspring; and the rapidity of their flight is such, that they can scarcely traverse less than seven or eight hundred miles in that time, although they go but a short distance from home. The flight of Insects is even more remarkable for its velocity in proportion to their size; thus a Swallow, which is one of the swiftest-flying of Birds, has been seen to chase a Dragon-fly for some time without success; the Insect always keeping about six feet in advance of the Bird, and turning to one side and the other so instantaneously, that the Swallow, with all its powers of flight, and its tact in chasing Insects, was unable to capture it.

673. If the preceding estimate of the power expended by a Bird in sustaining itself in the air be correct, it may be easily proved that it would be impossible for a Man to sustain himself in the air by means of his muscular strength alone, in any manner that he is capable of applying it. It is calculated that a man of ordinary strength can raise 134 lbs. to a height of 3 feet per second; and can continue this exertion for eight hours in the day. He will then exert a force capable of raising (13× 60 × 60 × 8) 381,600 lbs. to a height of 34 feet; or one-eighth that amount, namely 47,700 lbs., to the height of twenty-six feet, which, as we have seen, is that to which the Bird would raise itself in one second, by the force it is obliged to exert in order to sustain itself in the air. Now if we suppose it possible that a Man could by any means concentrate the whole muscular power required for such a day's labour, into as short a period as the accomplishment of this object requires, we might find the time during which it would support him in the air, by simply dividing this amount by his weight, which we may take to be 150 lbs. The quotient is 318, which is the number of seconds, during which the expenditure of a force that would raise 47,700 lbs. to a height of twenty-six feet, will keep his body supported in the air; and this is but little more than five minutes. There is no possible means, however, by which a Man could thus concentrate the force of eight hours' labour, into the short interval in which he would have to expend it while supporting himself in the air. And we have elsewhere seen (MECHANICS, § 285), that by no combination of mechanical powers can force be created; as these only enable force to be more advantageously applied. Hence, the problem of human flight will never be

USE OF PREHENSILE ORGANS IN LOCOMOTION.

512 solved, until some source of power shall be discovered, far surpassing that which his muscular strength affords, and so portable in its nature as not materially to add to his weight.

674. The only other organs of locomotion which we have to consider, are those of prehension. Of these, the principal have been elsewhere noticed, with reference to their use in laying hold of food and conveying it to the mouth (§ 172), and with regard to the differences between the hand of Man and the claspers of the Quadrumana (§ 643). The hand of Man is seldom employed to assist in his locomotion, except in swimming (where it serves the purpose of a fin), and in climbing; neither of which kinds of movement can be said to be natural to him. But the claspers of the Quadrumana

(fig. 254) are most efficient instruments of locomotion; enabling them not only to grasp the branches of the trees which they climb to despoil them of their fruit, but also to catch hold of them at the end of a long leap. This they do with the most wonderful agility; as all who have seen Monkeys in circumstances at all like those of their natural habitations, must have observed. The Gibbons, or long-armed Apes of the East Indies, are probably the most remarkable in this respect. The Author has seen the Ungkaputi leap round and round a room of about fifteen feet square, catching at each side by some small support attached to the wall; and taking its next leap (if such it could be called) by merely swinging itself from this, without touching anything solid with its feet. There are many of the Monkey tribe, however, especially in the New World, whose hands are less efficient as instruments of prehension; and these are furnished with a prehensile tail; that is, a tail which can be coiled round the branch of a tree, and by which the animal can suspend itself

PRODUCTION OF SOUNDS BY ANIMALS.

513

(fig. 255). A similar tail is possessed by some of the Opossum tribe; and by the Chameleon among Reptiles.

OF THE PRODUCTION OF SOUNDS: VOICE AND SPEECH.

675. It is not by their movements alone, that Animals are enabled to influence one another. Were it so, their communication would be restricted to the small amount which can be effected by signs and gestures. This, however, is necessarily the case amongst aquatic animals in general; since they are prevented by the nature of the medium they breathe from producing sounds through its means. Some of them appear to have the power of communicating with each other by the vibrations which they can excite in the water; of this we have already noticed an example among the Whale tribe (§ 491); and there is reason to believe that certain Mollusks possess a similar means of communication.

LL

514

PRODUCTION OF SOUNDS BY INSECTS.

676. Many Insects have the power of producing a continuous sound, which probably serves the purpose of intimating to each other the neighbourhood of their own kind; and even, in some instances, of expressing their feelings: some of these sounds are produced only during flight. Of this kind is the sharp hum of the Gnat, Mosquito, Gad-fly, &c., which, though often a source of extreme annoyance to man and beast, serves to give warning of the proximity of these blood-thirsty Insects, and is therefore of real service to the animals they attack. From recent experiments, however, it appears that in Bees and Flies, at least, the sound is not produced so much by the vibrations of the wings (to which it

is commonly attributed), as by those of a little membranous plate, situated in one of the spiracles or stigmata (§ 321) of the thorax; for if the apertures of these be stopped, no sound is heard, though the wings remain in movement. But in Cockchafers, and other noisy Beetles, Butterflies, &c., no such apparatus can be discovered. Other sounds are produced while the insect is feeding; that occasioned by the armies of Locusts, when incalculable millions of powerful jaws are in action at the same time, has been compared to the crackling of a flame of fire driven by the wind. Certain two-winged Flies, distinguished by a long proboscis (fig. 256), make a humming sound whilst sucking honey from flowers; and the same is the case with some of the Hawk-moths.

Fig. 256.-BOMBYLIUS.

677. Some Insects are remarkable for a peculiar mode of calling, commanding, or giving an alarm. The neuters or soldiers among the White Ants make a vibrating sound, rather shriller and quicker than the ticking of a watch, by striking against hard substances with their mandibles; this seems intended to keep the labourers, who answer it by a hiss, upon the alert and at their work. The well-known sound termed the "death-watch" is produced by a small beetle termed Anobium (fig. 257), that burrows in old timber; and