The Three Classes of Lightning.

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classes. The first includes those where the discharge appears like a long luminous line, bent into angles and zigzags, and varying in complexion from white to blue, purple, or red. This kind is known as forked lightning, because it occasionally divides into two branches before reaching the earth, as if anxious to double its damages. It has also been seen to sever into three. Charpentier supplied Arago with a case of tricuspidate lightning, where the southern fork set fire to a house in the suburbs of Freiberg, the middle struck a building near the cathedral at the distance of 3921 feet from the first point of descent, and the northern division of the flash wreaked its fury on a cottage in a neighbouring village situate 8531 feet from the cathedral. The same individual speaks of another instance, in which five trees, standing at some distance from each other, were smitten, though not more than a single peal of thunder was heard. Still more numerous furcations have been reported, for it is said, that during a tempest at Landerneau and Saint Pol de Leon, twenty-four churches were struck, though three distinct claps only were heard. This was hot work! Eight churches a-piece for the three explosions! Without, however, being assured that the observers had exhausted all probabilities of error, it would be premature to assert the existence of many-branched lightnings to anything like the extent implied in this anti-ecclesiastical storm.

The second class differs from the first in the range of surface over which the flash is diffused. From this circumstance, the discharge is designated sheet lightning. Sometimes it simply gilds the margin of the cloud from which it leaps, but at others it floods it with a lurid radiance, or else suffuses its surface with blushes of a rosy or violet hue. As this species of meteor, however, makes its appearance in every ordinary storm, it is too popular a phenomenon to require any description.

Turn we to the third class of lightnings. These are not only remarkable for their eccentricities, but they have been made the subject of considerable contention. They differ so widely from the vernacular sorts of flashes, that many meteorologists have denied their right to be treated as legitimate lightnings. They neither assume the form of long lines on the one hand, nor of sheets of flame on the other; but exhibit themselves as balls or globular lumps of fire. They are not momentary apparitions, but meteors which take their own time, and travel at such a slow rate that they might not improperly be styled the government class of lightnings. It is this circumstance which gives them such a dubious character. An electrical bolt is supposed to be one of the leading emblems of celerity. From Professor Wheatstone's ingenious experiments, it has been shown that an ordinary flash,

although darting as it may seem from horizon to horizon, does not occupy the thousandth part of a second in its transit. Nay, it has been calculated that the spark obtained from an electrical machine comes and goes in the millionth part of a second. Yet, spite of this characteristic velocity, lightnings of the third order have been seen strolling along at a leisurely pace, or traversing the air at an easy trot, such as the eye might readily follow, or the foot positively outstrip.

A striking illustration occurred to a M. Butti, at Milan. One summer's day, whilst a terrible thunder-storm was raging, this individual was seated in his apartment, when his attention was withdrawn from the commotion in the heavens to a little human hubbub in the street below. Guarda! guarda! cried a number of voices. On looking out of the window, he perceived a globe of fire moving along the middle of the street, at some distance from the ground, but with an upward slant in its course. Eight or ten persons were in chase of the meteor, and by advancing at a quick step they were enabled to keep up with its motion. It glided quietly past M. Butti's window. Anxious to know a little more of the strange traveller, he ran down stairs, and joined the hue and cry. On it went for about three minutes more, still sauntering along at the same cool pace; but at length it came in contact with the tower of a church, and vanished with a moderate detonation. Here, then, was an instance in which a man might easily have overtaken, shall we say a thunderbolt, and, if necessary, have beaten it hollow!

Still more singular is the story of a globular apparition which presented itself to a tailor in the Rue St. Jaques, in the neighbourhood of the Val de Grace, about the year 1843. M. Babinet was commissioned by the Academy of Sciences to investigate the facts, and reported substantially as follows:- After a loud thunderclap, the tailor being finishing his meal, saw the chimneyboard fall down, as if overset by a slight gust of wind, and a globe of fire, the size of a child's head, come out quietly and move slowly about the room, at a small height above the floor. The tailor said it looked like a good-sized kitten, rolled up into a ball, and moving without showing its paws. It was bright and shining, but he felt no sensation of heat. The globe came near his feet, like a young cat that wants to rub itself against its master's legs; but by moving them aside gently he avoided the contact. It appears to have played for several seconds about his feet, he bending his body over it and examining it attentively. After trying some excursions in different directions, it rose vertically to the height of his head, which he threw back to avoid its touching his face. The globe, elongating a little,

Globular Lightnings.

6

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then steered towards a hole in the chimney above the mantelpiece, which hole received a stove pipe in winter, but was now pasted over with paper. The thunder,' he said, 'could not see the hole; but, nevertheless, the ball went straight to the aperture, removing the paper without hurting it, and made its way into the chimney. Shortly afterwards, and when he supposed it had had time to reach the top, it made a dreadful explosion, which destroyed the upper part of the chimney, and threw the fragments on the roofs of smaller buildings, which they broke through. The tailor's lodging was on the third story; the lower ones were not visited at all by the thunderbolt.'

Here there is something quite piquant about the conduct of this suspicious visitor, if its proceedings are correctly narrated. The circumstances read like a romance. The entrance of the flash quietly into the poor man's dwelling, as if to make a mere morning call-the attempt to play with his feet, the tailor prudently declining its advances-the detection of the veiled aperture by the cunning meteor-the delicate unpasting of the paperand then, after indulging in this sportive behaviour, the terrible explosion with which the fire-phantom signalized its departureall this appears so anomalous that we might readily suppose the lightning to have been wandering about in a state of bewilderment, or rather of positive insanity.

It will be needless, however, to multiply illustrations. They are too numerous and too well authenticated to admit of lawful scepticism. Any attempt to explain them in the present state of electrical knowledge must be merely provisional. The likeliest solution is that of Sir W. Snow Harris, who conceives that the phenomenon arises from a species of 'glow' or 'brush' discharge —such as takes place, under certain circumstances, from the extremity of a conductor upon the nearest particles of air-these molecules being compelled in turn to transmit their electricity to the adjoining atoms, so that the fluid is propagated to a distance with comparative slowness, because with comparative difficulty. It is not, properly speaking, an ordinary flash folded up

* This eminent electrician, however, expresses an opinion from which we cannot but dissent, viz., 'that the greater number of discharges described as globular lightning are most probably nothing more than a vivid and dense electrical spark in the act of breaking through the air, which, coming suddenly on the eye and again vanishing in an extremely small portion of time, has been designated a ball of light.' If this 'spark' had rapid motion, it could not leave the impression of a mere ball on the retina, but would be drawn out into a long line like an ordinary flash, which is probably nothing more than a mere fiery point traversing the sky with such rapidity that it appears to be in all parts of its path at once. Whilst, on the other hand, if the spark had no apparent motion, it could not answer to the description which is given of the globular lightnings in question. But, if it had a moderate progress through the air, then the third class of Arago is virtually admitted.

into a ball, but a mitigated discharge (perhaps analogous to the well-known St. Elmo's fires) which precedes the restoration of the electric equilibrium by other and more legalized means. But, on the other hand, it must be confessed that this explanation bristles with difficulties; and though, with Arago, it may be necessary to admit these globular apparitions into the fraternity of lightnings, yet with Arago, too, it is necessary to regard them as one of the most inexplicable phenomena in the range of physics.'

After this slight disquisition on the various classes of lightning, let us inquire into the conduct of a bolt when hurled to the earth by the strong arm of the cloud-compeller. Foremost amongst the qualities of the electrical discharge we should notice the explosive power which it exhibits. When the fluid happens to meet with some obstruction in its course, it frequently evinces its dissatisfaction by shattering the non-conducting object. It is not guilty of mere linear violence, like a cannon-ball, but exerts a radiating force, like a bomb-shell, bursting substances asunder as if they had been charged with gunpowder. In 1762 the south-west pinnacle of the church at Breâg, in Cornwall, was demolished by a stroke of lightning. Amongst the fragments, one stone weighing 3 cwt. was hurled southwards over the roof to the distance of 60 yards; another was sent to the north for the space of 400 yards; whilst a third was projected in a south-west direction. In the forest of Nemours a tree was smitten in the year 1723: two pieces were rent from its trunk: the smaller-one which four men could not have raised-was tossed by the thunderbolt to the distance of about 50 feet; the larger, which a team of eight men could not move, was flung 16 feet on a contrary tack. In 1838 the topgallant mast of H.M.S. Rodney was hit by a flash, and literally cut up into chips, the sea being strewn with the fragments as if the carpenters had been sweeping their shavings overboard. The action of lightning on wood, indeed, is specially worthy of notice. In striking a tree or a mast, it will sometimes slice it into long shreds or filaments, so that it will appear like a huge broom or a mere bundle of laths. Some of the rafters of the Abbey of St. Médard de Soissons, which was damaged by a flash in 1676, were found to be cleft from top to bottom to the depth of 3 feet, 'into the form of very thin laths; others, of the same dimensions, were 'broken up into long and fine matches; and some were divided ' into such delicate fibres that they almost resembled a worn-out 'broom.' When H.M.S. Hyacinth fell under the displeasure of Jupiter in 1833, and was punished with a thunderbolt, her topmasts, for about 40 feet in length, were literally shaken into a mere fagot of sticks; and when the Thetis underwent a similar

Explosive Powers of Lightning.

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visitation in Rio harbour, Captain Fitzroy described the fore-topmast as a mere collection of long splinters almost like reeds.

Whence, then, comes this enormous explosive force, shivering trees, bursting the iron hoops from masts, whisking huge stones through the air, and projecting fragments to every point of the compass? Arago suggests that it may be due to steam. If lightning, as we shall presently see, can instantaneously raise the temperature of thin iron-wires, so as to render them incandescent, or even to fuse them entirely, what will be its effect on the sap which it finds in trees, or the moisture which lurks in the interstices of stones? The sudden conversion of water into vapour at a temperature of 500° F.-less than that at which iron melts— would develop steam with an elasticity equal to 45 atmospheres. There is much to be said in favour of this suggestion. The action of lightning upon trees is happily explained by assuming that, when struck, a fiery current dashes through the veins of the vegetable, and tears it into fibres by the action of the high pressure steam thus produced. It is possible, also, that the humidity contained in more solid objects may supply vapour of sufficient force to account for many of the ruptures which lightning effects. But there would be difficulty in applying this solution to all cases of electrical explosion; because there are many where no sensible amount of moisture can be presumed to exist, and others where the injury is on too extensive a scale to admit of resort to this agency. What is done to the green tree can hardly explain what is done to the dry. We can scarcely suppose that steam is responsible for the damage inflicted on the church of Guesnon, near Brest, where a stroke of lightning blew off the roof of the building, and laid its walls level with the ground. Nor can we charge it with lifting up the deck of the revenue cutter, Chichester, not less than 6 inches, in Kilkerran Bay. Besides, there is no proof that steam really exhibits itself in connexion with these catastrophes. It will, therefore, require more extensive observations to determine the accuracy of Arago's surmise; but we think that some of the expansive effects of lightning may be ascribed to the mere propagation of the shock which it inflicts upon air or water, when interrupted in its course, or constrained to follow too contracted a route-pressure being conveyed in all directions through fluid media.

The progressive force of lightning will seem more natural and intelligible than its radiating powers. But here, also, its doings are extremely remarkable. Some bolts will dash through resisting objects by tearing great openings, as was the case in a Cornish church, mentioned by Smeaton, where apertures were made in the solid wall of the belfry, one of which was fourteen