706. The subject of animal locomotion has been discussed in two works recently published, namely Animal Mechanism by E. J. Marey, and Animal Locomotion by Dr Pettigrew. One object which is sought by both works is to prepare the way for the construction of flying machines, by a careful study of the motion of birds. From Dr Pettigrew's work some interesting facts may be borrowed. The Albatross, which weighs about 17 pounds, can sail for about an hour at a time without once flapping his wings. The great velocity and consequently great momentum which an animal can acquire is illustrated by the following statements: A sword-fish has been known to thrust his tusk through the upper sheathing of a vessel, a layer of felt, four inches of deal, and fourteen inches of oak plank. A wild duck terminated its career by coming violently into contact with one of the glasses of the Eddystone Lighthouse. The glass, which was fully an inch in thickness, was completely smashed. Advantage is taken of this circumstance in killing sea-birds, a bait being pinned on a board and set afloat with a view to breaking the neck of the bird when it stoops to seize the bait.

707. One of the earliest writers on the subject of the motion of fishes was Borelli; he gave an explanation and a diagram which have been since generally adopted. The half of a fish's body which contains the head is supposed to remain in a straight form, while the half which contains the tail moves to and fro like a pendulum; and this tail part by striking the water produces a reaction which urges the fish forward. According to Dr Pettigrew the fish really bends both the halves of its body, so as to form a figure like two sickles turned in opposite directions. This is the simplest case; in the long-bodied fishes, like eels, instead of two such curved portions there may be several; and the fish alternately straightens and bends them all.

LIX. WATER.

708. As water discharges so many functions in the life of man we may well devote a few pages to tracing the various forms which it assumes.

709. It is the same substance which constitutes seas, rivers, springs, clouds, rain, snow, hail and ice. The sun heating the surface of the oceans and rivers, raises from them vast quantities of water in the form of vapour; this ascends into the atmosphere and remains invisible as long as the temperature is high enough. But the temperature declines as the distance from the earth is increased, owing to the atmosphere being too rare to retain the sun's heat; and thus vapour at some height above the ground becomes mist or cloud. The formation of mists may be observed very frequently along the course of a river, or in a damp valley, in the evening; the vapour raised by the sun continues as vapour during the day, but when the temperature declines it is condensed into mist. There is at all times much vapour present in the air, but especially during the hot summer days, and then if any cold surface is presented to the air some of the vapour is immediately condensed on it; thus for instance, if we fill a tumbler with very cold water the outside of the tumbler on a hot summer day becomes covered with moisture, which arises from the condensation of vapour. In like manner the insides of window panes become covered with moisture when the air outside is suddenly chilled. In ball rooms in Russia and Norway it has sometimes happened that on the sudden opening of a window a slight shower of snow has fallen in the room.

710. Clouds are supported by the atmosphere, and they rise in it until they reach a stratum of air of about the same density as their own; there they remain in equilibrium until disturbed by some change in the temperature. The greatest height which clouds are known to have reached is about 10000 feet. Clouds may be suddenly condensed so that the particles unite and form drops of water, and these descend to the earth as rain. Sometimes the process is accompanied by thunder and lightning, and the

rain is then unusually violent. Snow and hail are forms which the descending water takes when the temperature becomes diminished below the freezing point of water; in the case of snow the diminution takes place before the condensation of the vapour into drops, and in the case of hail after the condensation.

711. The quantity of rain which falls at any specified place depends much on local circumstances. In the British Islands the western side catches the clouds which have passed over the Atlantic ocean, and have become laden with vapour; thus this side has a much greater rainfall than the eastern side has. Rains in England are also often introduced by a south-east wind. Vapour brought to us by such a wind must have been raised in countries to the south and east of us; and we may accordingly attribute it to the valleys watered by the Meuse, the Moselle, and the Rhino, and to some extent to the more remote regions of the Elbe, the Oder, and the Weser. It has been calculated that the quantity of rain which falls in England is thirtysix inches a year on the average; that is to say if we suppose the rain to be uniformly distributed it would amount to a volume having the area of England for a base, and a yard for height. Of this quantity it is supposed that thirteen inches flow off to the sea by rivers, and that the remaining twenty-three inches are raised again from the ground by evaporation. The thirteen inches which flow into the sea are restored by evaporation from the sea, and are carried back to the land through the atmosphere.

712. The vicinity of mountains exercises considerable influence on the supply of rain, and often gives rise to special phenomena. Killarney in Ireland is noted for its luxuriant vegetation. The south-west wind is checked by the Kerry mountains, tilted up, and carried over them; the vapour which the wind has brought from the Atlantic is expanded on reaching this height: this causes a reduction of the temperature, which produces condensation of the vapour, and incessant rain. Again, a traveller sometimes descends from the Alps amidst a heavy fall of rain or snow, while the plains of Lombardy from which the wind is blowing are blue and cloudless. The wind is hot enough to

keep the vapour in the atmosphere in its transparent state over the plains; but when the vapour rises among the mountains the fall of temperature, owing to their cold summits and its own expansion, produces a condensation and the fall of rain or snow.

713. A striking phenomenon of a similar kind is frequently seen at the Cape of Good Hope, "where the south or south-easterly wind which sweeps over the Southern Ocean, impinging on the long range of rocks which terminate in the Table Mountain, is thrown up by them,...... makes a clean sweep over the flat table-land which forms the summit of that mountain (about 3850 feet high), and thence plunges down with the violence of a cataract, clinging close to the mural precipices that form a kind of background to Cape Town, which it fills with dust and uproar. A perfectly cloudless sky meanwhile prevails over the town, the sea, and the level country, but the mountain is covered with a dense white cloud, reaching to no great height above its summit, and quite level, which, though evidently swept along by the wind, and hurried furiously over the edge of the precipice, dissolves and completely disappears on a definite level, suggesting the idea, (whence it derives its name) of a Table-cloth." Herschel's Meteorology.

714. One form in which water presents itself to our notice is that with which we are familiar under the name of Dew. The air always contains vapour of water floating in it; and the higher the temperature of the air the more vapour can it support in an invisible state. But if the temperature of the air is lowered then some of this vapour is condensed and takes the form of globules of water. This condensation frequently happens at night, especially after a warm day; and then at early morning the fields. and gardens are found plentifully covered with this.. moisture which is called Dew; if the temperature is below the freezing point of water the moisture becomes frozen, and is called Hoar Frost.

715. The principal facts noticed with respect to dew are the following: (1) Surfaces on which dew is deposited are always colder than the neighbouring air. (2) Dew is

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not deposited on a cloudy night; but if the clouds withdraw even for a few minutes, and leave an open sky, the deposition of dew begins and on the other hand if in a clear night a large cloud passes suddenly over-head the deposition of dew is checked. (3) Dew is not deposited in a sheltered situation. (4) Dew is most copiously deposited on surfaces which part with their heat readily, and regain it slowly; and dew is very slightly deposited on surfaces which part with their heat slowly, and regain it readily: thus much more dew is deposited on grass and plants than on the bare earth or the stones. (5) Dew is not deposited when there is much wind.

716. All the facts stated in the preceding Article agree well with the principle that dew is the vapour in the atmosphere condensed by contact with surfaces colder than the air in which it floated. The earth gains heat from the sun during the day; during the night this heat escapes from the earth again into the air: thus the surface of bodies near the ground becomes colder than the surrounding air, and so dew is deposited. But if the sky is covered with clouds the heat is sent back to the earth, and so prevents the fall of temperature and the consequent deposition of dew. Again if a spot be sheltered the heat is prevented from escaping, and so the temperature is maintained too high to allow the deposition of dew. It belongs to the science of Heat to distinguish the two classes of bodies referred to in (4) of the preceding Article; and it is found that bodies behave with respect to dew precisely as might have been anticipated from the facts established in the science of Heat. Thus, for example, dew is plentifully deposited on such substances as cloth, wool, velvet, and cotton; now these substances are much used for clothing, because they have the property of impeding the passage of heat from the body; they allow their outer surfaces to be very cold while they remain warm within: so that these outer surfaces part with their heat readily and regain it slowly. A wind checks the deposition of dew on a surface, because it perpetually brings warm air over the surface, and so keeps up the temperature.