Sidebilder
PDF
ePub

iron and steel magnetised, metals and rocks softened and fused, and combustible substances set on fire. Sir John Herschel mentions a remarkable instance of “a large oak-tree near Alton, Hants, which was rent into ribands, and every limb of which had been struck off as if by an axe, and had fallen around the tree as by mere privation of support, without lateral projection.” He adds: “In producing these effects, the electricity would seem to act immediately by the expansion of vapour generated by its violent heat." the thunderbolt falls upon sand it occasionally produces fulgurites or fulminary tubes, which are siliceous tubes of various sizes vitrified internally.

593. Return Shock.—This shock sometimes proves fatal to living beings, even at great distances from the place where the electric discharge takes place. It is caused by the inductive action of the electrified cloud on bodies within the sphere of its influence, by which they become charged with the electricity opposite to that of the cloud. Hence, when the cloud has discharged its electricity into the ground, the induction ceases and a rapid change takes place in bodies from the electrified to the neutral state, thus causing the concussion of the return shock.

594. Lightning-Rods.The lightning-rod was introduced by Franklin in 1755 as a means of protecting buildings from the destructive effects of electricity. The advantage gained by it consists not in protecting the building in case of a discharge by allowing a free passage for the electric fluid to escape to the earth, for it is but a poor protection in such a case; but in quietly and gradually keeping up the communication, it tends to maintain the electric equilibrium, and thus prevent the occurrence of a discharge. The best are made of copper not less than three-quarters of an inch thick, and pointed at the upper extremity. They should be of one piece throughout, fastened vertically to the roof of the building, and thence carried down into the ground. The lower extremity should part into two or three branches bent away from the house, and carried sufficiently far into the soil to meet water or permanently moist earth. The conductor should be connected with all metallic surfaces on the roof or other parts of the building, in order to prevent the occurrence of lateral discharges, or discharges from the conductor to these surfaces, which are often

very

destructive. 595. St Elmo's Fire.—This meteor is the Castor and Pol. lux of the ancients, and is frequently mentioned in classic writings from the Argonautic expedition downwards. Cæsar notices its appearance after a storm of hail in these words : “ Eâdem nocte legionis quintæ cacumina suâ sponte arsêrunt.” The finest and most beautiful displays of this most striking phenomenon occur at sea during storms, when it appears as a light resting on the masts. Mr W. Trail of Orkney gives a particular description of it as seen by him there during a storm in 1837. The mast was illuminated, and from the iron spike at the top a flame a foot in length pointed to a dense cloud rapidly advancing from N.N.W. As the cloud, accompanied with thunder and hail, approached, the flame increased, following the course of the cloud, till it reached 3 feet in length, when the cloud was passing overhead; after which it quickly diminished, but continued to point to the cloud as it was borne to the S.S.E. It lasted about four minutes. If in a dark room we bring a needle close to the conductor of an electric machine when charged, a light will be seen to play on the point of the needle caused by the combination of the electricity of the conductor and that of the needle which is charged by induction with the opposite electricity to that of the conductor. This simple experiment explains St Elmo's fire, which takes place when an electrified cloud approaches near the earth, so that the electricity of the cloud and that of the earth combine, not in flashes of lightning, but more slowly and continuously from different points, which therefore appear to glow with a bright flame.

596. HailstoRMS.-Hailstorms are modifications of the thunderstorm. Since they occur where the force of thunderstorms is most concentrated, they are of a more local character, and their occurrence and destructiveness would appear to

be to a large extent determined by the configuration and vegetable covering of the earth's surface. They seldom occur during the night or during winter; but most frequently in summer, and during the hottest part of the day. They are most frequent within the tropics, less so in temperate climates, unless in the vicinity of mountains, and still less in the arctic regions. They are more common in the neighbourhood of mountains than in plains. Thus the south of France, lying between the Alps and the Pyrenees, suffers much from hailstorms, the vines being often broken and destroyed by their violence. The annual loss from this cause has been estimated at above two millions sterling. So important is a proper knowledge of these and similar storms considered in France, that the services of upwards of 1200 observers have been secured in that country to note the chief features of storms (orages). The observations are transmitted to the Imperial Observatory at Paris, and the results published in a magnificent atlas of storm-charts. The following storm is one of the most remarkable.

597. On the 7th May 1865 the north-east of France was visited by a hailstorm of unwonted violence, which has since been called the Storm of Câtelet, the place where it was most

It was preceded by six weeks of unusually dry and warm weather; after which, and for a few days before the storm, the temperature became quite scorching, and several times storms appeared to be forming, but they all passed off without hail or rain. At seven in the morning of the 7th the air freshened and the barometer rose. A wind from N.E. chased light clouds before it with astonishing speed; but higher up a S.W. wind prevailed, bearing slowly before it woolly-looking clouds, which grew denser towards noon. Still the air was calm at the surface of the earth, and continued so till three P.M., when dense heavy clouds, piled on each other, rose out of the south-west, and thunder began to be heard. Below, the mass of cloud was of a pale livid colour, out of which lightning darted continually; above, were many layers or banks of sombre-tinted clouds, forming a broad base to the lower electrically-charged cloud, which resembled an inverted pyramid. The storm ascended the valley of the Somme, but little damage was done till it had crossed the heights and descended into the valley of the Escaut, where it fell with terrible violence on Vend'huile, le Câtelet, and Beaurevoir. At Câtelet the hailstones were as large as pigeon's eggs, and some even appeared as large as hen's

severe.

eggs; but these latter, on examination, were found to be composed of several hailstones rolled together. Next morning, at one place the hail had piled together a mass of ice which was upwards of a quarter of a mile in length, 22 yards in breadth, and at certain points fully 16 feet in height; and at another place there was a similar mass of hailstones one mile and a quarter in length, and a furlong in breadth, which was not fully melted till after the 13th of the month. The damage done by this storm was immensethe tiles on the roofs of houses, the glass, and even the sashes of windows, were smashed to pieces; a mill was levelled with the ground ; large trees were torn up by the roots ; and crops of rye, barley, and wheat were beaten to the ground and destroyed. In the memory of the inhabitants of that part of France no storm had occurred fraught with such disastrous results.

598. This class of storms are found in France to be invariably bound up, or associated, with barometric depressions, and their general direction is influenced as is that of the wind in the same circumstances. The general direction in which they advanced in 1865 was from S.W. to N.E.; and otherwise on this point they resembled the storms of Europe already described. But when they were confined to the lower parts of the atmosphere they were diverted from their course on coming up against high table-lands and mountain ranges much in the same way apparently as rivers are when high banks oppose their course. The rate of their progressive motion varied from 12 to 45 miles an hour, thus showing in this respect also a close correspondence with the larger storms of Europe.

599. On the 28th July 1818, a hailstorm of unusual violence passed over Orkney, its course being marked over a district twenty miles in length by one and a half in breadth. It did not last longer at any place than nine minutes, during which 9 inches of ice fell. On the 13th July 1788, a hailstorm passed directly from the south-west of France to Utrecht. It moved in two parallel columns with very great rapidity, traversing the distance in less than nine hours. The length of the one column was 435 miles, and of the other 497 miles, and the breadth respectively 5 and 10 miles. Between the two tracts there was a space of twelve miles, where no hail, but heavy rain, fell. each place the storm lasted only a few minutes; and along its course property valued at above a million sterling was destroyed.

600. In rainy weather, such as frequently occurs in March and September, when, between the intervals of sunshine, a cloud appears in the west, overspreads the sky, and as it passes pours down a considerable quantity of rain, if the barometer be watched from the time of the cloud's appearance in the west to its disappearance in the east, it will be observed to fall a little and then rise, the fall being of a strictly local character. This may be considered typical of the class of storms under discussion. Though no barometric observations are recorded during the thunderstorms of France in 1865, it is highly probable that a local barometric depression accompanied each storm in its course. In all cases before the storm the air is close and sultry and highly charged with moisture. These storms may thus be regarded as secondary storms or sub-storms within the area of the more general storm or atmospheric disturbance passing over Europe at the time, with the wind in all probability circling round them as they are carried forward in the larger storm. Barometric and wind observations made almost every minute during thunderstorms would go far to explain their true character. For registering these small barometric fluctuations so as to arrive at some explanation of gusts of wind, heavy showers, and

« ForrigeFortsett »