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273. Ice-foot, Floes, Pack.-A belt of ice often thirty or forty feet thick forms along the sea-coast in Arctic regions, and is known as ice-foot. Blocks of stone and other debris fall from the cliffs; and as large portions become detached in summer, they float away with their load, which is deposited when the ice melts or is broken up. Many erratic blocks, as they are called, are believed to have had this origin. Outside the ice-cliffs of the South Polar land a similar fringe of ice is formed in winter, and broken up in summer. A floating field of unbroken ice is called a floe. When a floe is broken up by storms, and the fragments drifted close together, it forms the ice-pack or pack, of which we read in Polar expeditions. 'The Polar basin,' says Sir Wyville Thomson, or at all events such portions of it as have been hitherto reached, is neither open sea nor continuous ice, but a fatal compromise between the two, an enormous heavy pack formed by the piling up and crushing together of the floe of successive years, in frequent movement, breaking up and shifting according to the prevailing direction of the wind, and leaving open, now here and now there, lanes and vistas of deceptive open water, which may be at any moment closed and converted into a chaotic mass of hurtling floe-bergs by a hurricane from another direction.'

ATMOSPHERIC AND TERRESTRIAL
ELECTRICITY.

274. Ordinary Electricity of the Atmosphere.-It is commonly said, when one sees a sultry cloudy sky, that the air is electrified; yet in all kinds of weather, clear as well as cloudy, it is more or less electrical. The cause of this is not very well known, but it is usually ascribed

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to friction and evaporation. One theory is, that mere contact of the particles of aqueous vapour with those of air, as they fly about, and impinge according to the modern kinetic theory of gases, produces a separation of the two electricities; just as, when zinc and copper are brought into contact, the zinc becomes positively electrified, and the copper negatively. Thus the electrification is supposed to be the result of chemical affinity. When vapour is precipitated in the form of cloud, it takes billions of vapour-particles, we are told, to form a cloud-particle or molecule of water. However small, then, the electric charge on a single vapour-particle, if it exists in any degree, the accumulation of electricity on the mass of water-particles forming a cloud would be enormous.

A clear sky is always positively electrical; in cloudy weather, it is sometimes positively, sometimes negatively electrical and rain, snow, hail, &c., when caught on an electroscope, are found sometimes positive, sometimes negative; nay, not unfrequently changing suddenly during the same shower.

275. Thunder-storms.-In summer the usual result of a few days' strong heat and copious evaporation is to produce vast masses of cloud highly charged with electricity, sometimes positive, sometimes negative, no cause being yet assignable for the difference. When two oppositely charged clouds are borne by the wind within attracting distance of each other, they rush together, and their electricities combine with terrific vividness; and this is the extraordinary development of atmospheric electricity that we call a thunder-storm. An electrified cloud always induces the opposite electricity on the part of the earth under it, and when the cloud descends sufficiently low, the discharge takes place between the cloud and the earth, and the storm is then most dangerous and awful.

276. Lightning.-Franklin was the first to identify the lightning flash with the electric spark, by his famous kite experiment. The same disruptive, burning, and luminous effects are common to both, though in vastly different degrees. An electric spark three feet long is considered something great even for a powerful machine; but the lightning-flash often exceeds a mile in length, and sometimes extends to four and five miles.

There are several kinds of flashes. First, there is the forked-lightning, or zigzag line of bright light which may be seen darting between clouds, or from the clouds to the earth. Often it splits up, as it approaches the earth, into branches several thousand feet apart. Its zigzag form is owing to the unequal resistance of the air, the path taken being that of least resistance through the different strata of air.

Second, there is the large indefinite blaze of sheet-lightning, which we see more than twenty times for once that we see forked lightning. It is ascribed to an electric discharge within the clouds themselves, which illuminates their mass for a moment. When it occurs at a great height or distance, no thunder is heard, and a vague flash passes across our field of view; and it is highly probable that weak discharges occur without any audible sound, Such lightning is common in summer evenings.

A third and less frequent form is that of ball-lightning, which rather resembles a meteoric than an electric phenomenon. It is said to occur in this way. After a violent explosion of lightning, a ball is seen bounding like a bomb to the earth. When it reaches the ground, it either splits up at once and disappears, or it rebounds like an elastic ball before doing so. It may last as long as ten seconds, and is thus very different from the flash,

which has been proved to last less than one ten-thousandth of a second. It is very dangerous, and readily sets fire to any building in its way. This form of electric discharge is, if possible, more difficult to explain than the other form, nothing of the kind being produced by the electric machine.

277. Thunder.-The snap of the electric spark is heard on a grand scale in the thunder-clap, which accompanies the lightning. The origin of the sound is to be found in the instantaneous dilatation of the air along the track of the lightning flash, caused by the excessive rise of temperature that renders the air for the moment so brilliantly incandescent. There is thus a sudden compression of the air all round the track of the spark, and then a rapid rush of the air into the partial vacuum thus produced.

The sound must originate at the same instant along the whole track of the flash. Whence, then, the prolonged rolling and strange rising and falling of the peal? Two chief causes may be assigned. In nine cases out of ten, we must be nearer one end of the lightning track than the other. Suppose the one end a mile farther off than the other; owing to the time sound takes to travel, the sound from the farthest point will come to the ear five seconds after that from the nearest, and the arrivals from the intermediate points will form a continuous roar. Further, sound is reflected not only from clouds, but also from undulating surfaces of the earth, and even from the bounding surfaces of masses of air of different density. The primary sound is thus repeated again and again with every variety of loudness. The distance of the flash, as a whole, from the observer is roughly estimated by allowing five seconds to a mile for the sound to travel. Thunder is not heard at such a distance as

we might expect from its loudness; nine or ten miles seem to be the ordinary limit, and twelve miles the maximum limit. But the discharge of a single cannon has been often heard fifty miles off; and the cannonading at Waterloo is stated to have been heard at Creil, in the north of France, about 115 miles from the field.

278. Lightning-conductors.-Electricity, like any other force, always takes the easiest route. If we hold a pointed rod of metal to the prime conductor of a machine, its electricity will be all drawn silently off to the ground, as fast as it is produced. In the same way, if we present a pointed rod near to a charged cloud, its electricity will pass silently and harmlessly to the earth. This is the principle of the lightning-conductor. It is a pointed rod of copper or galvanised iron, reaching from eight to thirty feet above a building, and carried down to the earth, in which it is carefully buried. The part above the building is called the rod, and the rest the conductor, which is now generally in the form of a metallic ribbon. The whole must be very carefully constructed, otherwise its presence is the opposite of protective. It is led down two feet or so into the ground, and then turned away into a well or other water, if possible, or else into a drain filled with charcoal for twelve or sixteen feet. In large buildings, there are several rods all connected to one common conductor. In ships, there is one on each mast, leading to the metal of the keel. When properly made, they are, beyond all doubt, sufficient protection from the ravages of lightning. A striking illustration of this effect is furnished by what we are told of Pietermaritzburg, in Natal: 'Till lightning-rods became common in that town, it was constantly visited by thunder-storms at certain seasons. They still come as

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