500 EFFECTS OF LIGHTNING. [248. and scattered around, amidst the shattered fragments of the mast (Harris). One of the most instructive instances recorded is that of the Dido, which when off Java Head, in May, 1847, was struck soon after daylight, during a storm attended with heavy rain and little wind, by a tremendous bifurcated flash of lightning, which fell upon the main royal mast. One of the branches struck the extreme point of the royal yard-arm, and in its course to the conductor on the mast, demolished the yard, and tore in pieces or scorched up the greater part of the sail; the other part fell on the vane-spindle (the point of which showed marks of fusion) and truck, which last was split open on the instant that the discharge seized the conductor. From this point, however, the explosive action ceased, and the discharge freely traversed the whole line of the conductor from the mast-head downward, without doing further damage. One of the chief points of interest connected with this case is the entire destruction of the yard-arm, which was not supplied with a conductor, and the complete protection of the mast, which was furnished with one. It is also important as proving the incorrectness of the law of protection laid down by some French writers-viz., that a conducting-rod will protect a circular area having a radius double the height of the conductor above the highest point of the building. In all cases, the lightning will take the path of least resistance, and, from the recorded results of experience, it appears that that path of least resistance will in about seven times out of ten, be such that the lightning will strike the highest point, if it be furnished with a good conducting line to the earth or sea; but it is quite possible that instances may occur, in which the line of least resistance may be in a different direction, or, as in the case of the Dido, that there may be two such lines where the resistances are equal. If a break occur in any part of the conductor, explosion will take place at this spot when a discharge of lightning is directed upon the rod, producing, in many cases, fearful destruction. One of the most awful catastrophes of this kind occurred on the 18th of August, 1769, when the tower of St. Nazaire of Brescia was struck by lightning. Beneath this tower were vaults containing upwards of 90 tons of gunpowder, belonging to the Republic of Venice. The whole of this enormous quantity of powder exploded, destroying one-sixth part of the city of Brescia, and burying 3000 persons beneath its ruins. track followed by the electricity may be illustrated by sending a discharge through a series of interrupted conductors, such as gold-leaf pasted upon paper. The portions of gold-leaf in the On a small scale the 248.] ATMOSPHERIC ELECTRICITY. 501 line of the discharge will be burned up, whilst the contiguous portions not included in the track of the electricity remain unaltered. The peal of thunder which accompanies the lightning flash is due, like the snap which accompanies the discharge of a Leyden jar, to the sudden displacement of air, which, in the case of lightning, sometimes extends through a distance of a mile or more. The reverberation of the peal arises chiefly from the echoes produced by objects upon the earth, and by the clouds themselves. The flash from the thunder-cloud is exactly analogous to the discharge of the Leyden jar: the cloud and the surface of the earth form the two coatings to the intervening layer of air, which, as in the case of the condenser, supplies the place of the glass, whilst a church steeple, or any projecting object, acts the part of a discharging rod.* But it is not only during a storm that the atmosphere exhibits signs of electricity. In a cloudless sky, if a flame or a pointed rod be connected with an electroscope, the instrument diverges positively. Before rain, the instrument often assumes a negative state: in general, the rain that first falls after a depression of the barometer is charged negatively. It frequently happens that the rain is negatively charged, although the atmosphere, both before and after its fall, exhibits signs of positive charge. Fogs, snow, and hail, if unattended with rain, are nearly always positively charged in a high degree. It appears to be probable that the clouds are almost always positive. In most cases, when negative electricity is observed in the instruments it is simply due to an effect of induction. Palmieri has conducted observations on atmospheric electricity for twenty-five years in the observatory on the side of Mount Vesuvius, and has obtained some results which point to a definite law. If within a distance of about 50 miles there is no shower of rain, hail, or snow, the electricity of the air is always positive, except during the projection of ashes from the volcano. During a shower the electricity is positive, surrounded by a zone of negative, and beyond this the air is again positive. Whenever the air is negative, therefore, it indicates that there is a heavier shower not far off. If the storm is approaching the These electrical accumulations are often renewed with extraordinary rapidity. On the 6th of July, 1845, about 10 P.M., after a clear hot day, in the masses of vapour forming a bank of cumuli, I counted in two minutes 83 flashes unattended by thunder; and several times during the same evening, I observed between 30 and 40 discharges from one cloud to another, per minute. 502 ATMOSPHERIC ELECTRICITY. [248. observatory, the air becomes negatively charged, then positively when the storm is near, negatively again after it has passed over, and again positively when the shower has moved to a considerable distance (Nature, 1872, vi. 146). These results explain the frequent variations during storms which are often observed in other localities, although it must be noticed that meteorological phenomena in Italy take place with much greater regularity than in this country. In winter, the atmospheric charge is usually higher than in summer. According to Quetelet, whose conclusions are based upon a series of five years' uninterrupted observations, the atmospheric electricity attains an average maximum in January, and steadily decreases till June, when it is at its minimum: from this period it again increases progressively till January, in which month the tension of the electricity is thirteen times as high as it is in June. The electricity of the air may be stated generally to be higher in a cloudless than in a cloudy sky. Only once during the months of October, November, December, and January, has he obtained proof of negative electricity in the air. The tension of the charge varies likewise during each twentyfour hours; it has two maxima and two minima. The first maximum is before eight o'clock A.M. in summer, and before ten A.M. in winter; the second after nine P.M. in summer, and before six P.M. in winter. The first minimum is uniformly about four A.M., and the second about three P.M. in summer, and one P.M. in winter. The observations made for some years at the Kew observatory by Ronalds, furnish results closely according with those of Quetelet.* An ingenious experiment by Becquerel shows that the inten sity of the charge increases with the elevation above the earth's surface, and according to Quetelet's observations, the increase in intensity is proportional to the height. This law of Quetelet has, however, been verified only for heights not exceeding 16 feet (5 metres). Becquerel's experiment was the following:- Having ascended Mount St. Bernard, he placed an electroscope upon a piece of varnished silk, on which he arranged by a loose cord about 80 metres of gilt thread. One end of this thread he attached to the shaft of an arrow, and connected the other ex * For an interesting discussion of the theory of the development of atmo spheric electricity, the reader is referred to Delarive's Traité de l'Electricité, iii. 188 et seq. 248.] ATMOSPHERIC ELECTRICITY. 503 tremity with the cap of an electroscope by a running knot. The arrow was then discharged in a vertical direction by means of a bow; as it ascended the leaves expanded gradually till they struck the sides of the glass. When the full length of the thread was attained, the upward motion of the arrow detached it altogether from the electroscope, leaving the instrument charged positively. On repeating the experiment, shooting the arrow horizontally, no charge at all was obtained. Similar results may be obtained on a clear day by ascending a lofty eminence or building, to avoid the induction of near objects, and taking a gold-leaf electroscope, terminating above in a ball. The electroscope being now in a neutral state, it will, if elevated only for a foot or two, diverge with positive electricity. On bringing it back to its original position, the leaves collapse, and on depressing it below this point, the leaves again separate with the opposite electricity. Two methods for collecting atmospheric electricity have been devised by Sir William Thomson. When a pointed conductor is raised in the air, a charge is induced upon it which is opposite to that contained in the atmosphere. If the atmospheric charge is considerable, there will be, as in the lightning conductor, a discharge from the point; but when the charge is very feeble, no such discharge may take place. If, however, the conductor loses some of its matter from the point, the induced charge will be carried off, and the other end of the conductor will exhibit the same kind of electricity, and of the same potential, as that contained by the air surrounding the point. This loss of matter may be effected by the burning of a slow match, made by soaking a roll of filter paper in a solution of plumbic nitrate, or by employing a fine jet of water, breaking into drops at the point in the air at which it is desired to measure the charge. By means of the water-dropping collector connected with a quadrant electrometer, a continuous photographic record of atmospheric electricity is made at the Kew observatory. Electricity develops itself in the atmosphere in other forms; for example, luminous brushes, stars, and glows, have been frequently observed in stormy weather on the extremities of the masts and yard-arms of ships, on the points of weapons, and occasionally even on the tips of the fingers. These phenomena are, in fact, cases of brush discharge upon a large scale, and are in many instances attended with a roaring noise like that of a burning portfire. Appearances of this description formerly went by the name of St. Elmo's fire; our own sailors term them comazants. 504 AURORA BOREALIS. [249. (249) Aurora Borealis.-Another very beautiful meteor which is sometimes seen in this country in clear frosty nights, but which is observed very frequently in higher latitudes, has probably an electrical origin. This is the aurora borealis. It has been supposed to be occasioned by the passage of electricity through the rarefied portions of the upper regions of the atmosphere from the poles towards the equator, but the explanation is unsatisfactory, and not adequate to account for the effects observed. The varieties of coloured light exhibited by the aurora may, however, be imperfectly imitated on a small scale by discharging a continued or an intermittent supply of electricity through a vessel partially exhausted of air. The forms which the aurora assumes are very varied, and of extraordinary beauty; there is, however, usually some general similarity in its aspect at the same locality. Commonly, streams of light are seen shooting upwards from the northern horizon. These streams are frequently observed to meet together in the zenith, and produce an appearance as if a vast tent were expanded in the heavens, glittering with gold, rubies, and sapphires. A remarkable connexion has been observed between the aurora and the magnetism of the earth; the magnetic needle being very generally disturbed during a display of the aurora. The arches of the aurora most commonly traverse the sky at right angles to the magnetic meridian, though deviations from this direction are not rare. Sir J. Franklin found that the disturbance of the needle was not always proportionate to the agitation of the aurora, but was always greater when the quick motion and vivid light were observed to take place in a hazy atmosphere. The aurora is most frequent and vivid in high latitudes towards either pole; but the meteor is not confined to these parts, as Dr. Hooker states that one of the most brilliant displays he ever witnessed was under the tropical sky of India; and other observers have recorded instances of its appearance in the equatorial districts of the globe. The altitude of the aurora varies considerably; there is no doubt, however, that it frequently occurs at small elevations. Both Franklin and Parry record examples of its appearing below the level of the clouds, which they describe as concealed behind the masses of its light, and as reappearing when the meteor vanished. There appear to be two distinct kinds of aurora, one dependent upon local causes, as in the cases last mentioned, while, in the other, the causes are probably cosmical, and the |