« ZurückWeiter »
and by the sudden variations of temperature with which these changes are accompanied.*
Atmospheric air, and all dry gases, are very perfect non-conductors or insulators of electricity; and hence when a cloud or mass of vapour, charged with free electricity, floats in the atmosphere, its electricity is not carried off, or conducted to the earth, by the air which is interposed. The cloud, therefore, retains its electricity in virtue of the insulating medium which surrounds it; but when its quantity becomes great, it induces an electric state opposite to its own in the particles of the air, making them negative when it is positive, and positive when it is negative; just as a loadstone or magnet produces by induction in a bar of soft iron, boreal magnetism in one-half, and austral magnetism in the other. The particles of the air in this state are said by Dr Faraday to be polarized; and the consequence of this state is, that the earth's surface finally assumes an electrical state opposite to that of the cloud. The cloud, therefore, the air, and the earth, are all in an unnatural or constrained state; and the tendency of the two electricities to unite, is a force which, when it becomes irresistible, terminates in what Dr Faraday calls a disruptive discharge. The free electricity of the cloud rushes to the earth, acting principally on the bodies through which it passes; or, what is not uncommon, the free electricity of the earth passes into the cloud, and both of these violent discharges is accompanied with the well-known phenomena of thunder and lightning.
This sudden interchange of powers is often prevented or modified by local causes. If the electrified cloud and the insulating medium are not in a state of extreme constraint, and if a pointed metallic rod projects into the medium, a discharge of electricity will take place from the particles of air touching the metallic point, and a beautiful brush of light will be produced, accompanied with a rushing noise. The whole electricity of the cloud may thus be quietly carried off, and a disruptive discharge completely prevented.
The very same phenomena take place when one charged cloud induces an opposite state in another cloud, through the intermedium of the air; and there is reason to believe, as maintained by Beccaria, that more complex discharges take place between 'such' distant clouds when the earth lies between them in the line of discharge.
* If Dr Faraday be correct in ascribing the electricity in the hydroelectric machine to the friction of the escaping steam, these views will admit of some modification.
The following account of a thunderstorm in the Gulf Stream, is a very instructive illustration of the preceding views. It was given by a passenger on board the splendid packet-ship New York, which was damaged by lightning on the 19th of April 1827, on her voyage to Liverpool:
About half-past five in the morning, we were roused by a sound like the report of heavy cannon close to our ears. From the deck the word was quickly passed that the ship had been struck by lightning, and was on fire. Every one ran on deck; there, all the elements were in violent commotion; it had been broad day, but so dark, so dense, and so close upon us were the clouds, that they produced almost the obscurity of night. There was just sufficient light to give a bold relief to every object in the appalling scene. The rain poured down in torrents, mingled with hailstones as large as filberts: these lay upon the deck nearly an inch thick. Overhead blazed the lightning on all sides, accompanied by simultaneous reports: the sea ran mountains high, and the ship was tossed rapidly from one sea to another. One appearance was peculiarly remarkable: the temperature of the sea was 74 deg. Fahrenheit, while that of the air was only 48 deg. This caused, by evaporation and condensation, immense clouds of vapour, which, ascending in columns all around us, exhibited the appearance of innumerable pillars supporting a massive canopy of clouds. In all directions might be seen waterspouts, which, rising fearfully to the clouds, seemed actually to present to the eye a combination of all the elements for the destruction of every thing on the face of the deep.'— (P. 54.)
This storm has been instanced by Mr Harris as a case of stationary disturbance of electrical equilibrium; but there are various other types of a thunderstorm arising from the motion of clouds. The following is an instance of a charged cloud driven by an upper current upon a comparatively tranquil air, possibly ' in a polarized state.' His Majesty's frigate Clorinde was damaged by lightning on the coast of Ceylon in the spring of 1813. Captain Briggs gives this account of it :—
The weather was moderate. About three in the afternoon a dark cloud approached the ship from the windward quarter. This induced me to clue up the topsails. About an hour afterwards the ship was struck by lightning. The cloud was charged with electricity, and had burst upon the ship. The mainmast was shivered in pieces; three men were killed, and many hurt.'-(P. 60.)
When highly electrified clouds are passing over the earth's surface, we may,' says Mr Harris, trace in their progress de• liberate discharges of a passing kind, few in number, in some 'instances not extending beyond one or two.' Such thunderstorms, if they deserve the name, bear scarcely any relation. to those wide-spread disturbances of the atmosphere which pass 2 H t
VOL. LXXXI. NO. CLXII.
over a great extent of country, destroying life and property in their career. In these cases, the atmosphere appears to receive an intense charge of electricity from the electrified masses of clouds, as they are hurried along by the wind.
'Such storms,' says Mr Harris, have been observed to pass from the southern shores of England to the north of Scotland and Ireland. A thunderstorm of this kind occurred in July 1827. It began on the S.W. coast of Devonshire on Sunday evening, reached Cheltenham the same night, and Glasgow the next morning, the atmosphere throughout this extent appearing to undergo a rapid and progressive change.'-(P. 62.)
We have already alluded to the case of a thunderstorm produced by the distant and oppositely electrified masses of clouds and air, in which the surface of the earth becomes involved as ' a line of discharge between them;' but as this case is only a hypothetical one, we must refer the reader to Mr Harris's brief notice of it.
The returning stroke, discovered by Lord Stanhope, and which has been considered as finely exemplified in the case of the spire of Rouvroi, (see p. 458,) is still beset with difficulties. The example given of it by his Lordship occurred in Scotland, and has been described by Mr Brydone in the Philosophical Transactions for 1787, to which we must refer the reader.
There is another class of thunderstorms, of an exceedingly interesting nature, to which Mr Harris has not directed his attention in this work, from the circumstance, no doubt, of their leading character being that of the hurricane and the tornado. We have previously treated of the statistics and philosophy of this class of storms, and have described many of the principal electrical phenomena which accompany them. In the Barbadoes hurricane of the 18th and 19th August 1831, these phenomena were so awfully grand in their nature, and so new and inexplicable in their character,† that it would be desirable to study the electrical separately from the mechanical phenomena of such hurricanes, and endeavour to obtain some general explanation of them. The meteors and lightning which accompany the gales of the East and West Indies have been overlooked amid the appalling dangers of the tempest, and in very few of the Mauritius gales, except in that of the Boyne in 1835, have the electrical phenomena been at all observed. So trivial, indeed, is the part which thunder and lightning plays in
See this Journal, January 1839, Vol. LXVIII. p. 406.
these tremendous convulsions of nature, that at Montego Bay, in the hurricane of the 3d October 1780, when an earthquake added its awful contingent to the general horrors of the scene, theprodigious flashes of lightning,' which followed in regular succession, were regarded, not as a source of danger, but as a 'real blessing, amid the midnight darkness which brooded over 'the general desolation.'
Our limits will not allow us to pursue this interesting subject further; and we must, therefore, devote our few remaining pages to a brief notice of the best method of protecting buildings from lightning, and of Mr Harris's system of conductors for ships.
Were our houses, powder-magazines, and ships, built of iron, or did they consist of a framework of iron, filled up with stone, brick, or wood, they might bid defiance to the ravages of accidental or wilful fire, as well as to all the lightning of the tropics. Strike where it might, the deadly fluid would be conducted quietly to the ground. In the mean time, however, we must have recourse to a less perfect system of protection, till advancing knowledge and receding prejudice shall have introduced iron buildings and iron ships, as well as iron ploughs, iron roads, and fron bridges.
As the conducting powers of lead, tin, iron, zinc, and copper, are as the Nos. 1-2-2-4-4 and 12, copper is the best material for conducting-rods. The quantity of metal in the rod should not be less than what is contained in a cylinder half an inch in diameter. If iron is used, the cylinder should be nearly an inch and two-tenths in diameter. The metallic rod should be flattened rather than round, so as to have the greatest surface that is consistent with strength. The conductor thus formed should communicate with all the detached masses of metal in the building, such as leaden ridges, gutters, and metallic pipes. It should be placed as near the wall as possible, and pass directly into the ground. It should be attached to the most elevated point of the building, and if the structure is to consist of numerous ranges, such as the new Houses of Parliament,* long pointed rods should project from the most prominent parts into the atmosphere.
* We earnestly hope that this splendid national structure, which is to be adorned internally by the genius of our Artists, will be protected externally by the science of our Philosophers. We fear, however, that the expression of this hope is not sufficiently early to enable the architect to embody a system of metallic conductors in the very walls of the edifice.
In place of adopting the usual method of external conductors, we would recommend the introduction of a vertical iron bar into the thickness of the principal walls of the building. These bars should communicate with a horizontal wall plate of iron uniting the whole; and from this wall plate should rise all the external conductors which are to project into the atmosphere. These iron plates and bars might be so united as to form a sort of carpentry, which would add to the strength of the edifice.*
The protection of ships from lightning is more difficult to accomplish than that of buildings; and we have no hesitation in saying that the method invented by Mr Harris far surpasses all others, and completely fulfils all the objects of its application. These conductors consist of parallel plates of copper, about twotenths of an inch thick, from 1 to 5 inches wide, and 4 feet long. They are placed in a shallow groove, ploughed out of the after side of each mast, and are fixed there by short copper nails. The plates are inserted in the groove in a double series in contact, so that the joints of the plates of one series are opposite to the middle of the plates of the other series; and this series of plates is so turned over the heads of their respective spars, and also round the termination of the mast in the step on the keelson, that a continuous metallic line is maintained, notwithstanding the sliding or even removal of the spars. From the bottom of the masts, the metallic plates extend to the copper sheathing of the ship, and all the metallic bands terminate in the sea, by bolts clenched on the copper sheathing. When a ship is thus defended, it is at all times, in all places, and under all circumstances, secure against the attacks of lightning. The conduc tors are always where they ought to be, independent of the officers and the crew. When the top-masts or topgallant-masts are partially lowered, the continuity of the metallic line is kept up, although the inferior part of the conductor of the lowered mast is thrown out of its place. The system of protection, indeed, is fixed and permanent, notwithstanding the change of position or even the removal of the moveable or sliding masts.
In order to exhibit more fully and distinctly the national value of this system of protection, Mr Harris has just published an interesting pamphlet, † in which he has detailed the damage
* Bell wires and metallic pipes for water and gas, in modern houses, require to be carefully connected with the principal conductors. Without this precaution, they are rifles directed against the lives of the inhabitants.
†The Meteorology of Thunderstorms, with a History of the Effects of Lightning on 210 Ships of the British Navy. 1844.