Part 2

The rapidity of lightning, as measured by means of the camera lucida, M. Halvig estimates at probably eight or ten miles in a second, or about forty times greater velocity than that of sound; and according to M. Gay-Lussac, a flash sometimes darts more than three miles at once in a straight direction. M. Arago distinguishes three classes of lightning: First, luminous discharges characterized by a long streak of light, very thin, and well defined at the edges, of a white, violet, or purple hue, moving in a straight line, or deviating into a zigzag track, frequently dividing into two or more streams in striking terrestrial objects, but invariably proceeding from a single point. Secondly, he notices expanded flashes spreading over a vast surface without having any apparent depth, of a red, blue, or violet color, not so active as the former class, and generally confined to the edges of the clouds from which they appear to proceed. Thirdly, he mentions concentrated masses of light, which he terms globular lightning, which seem to occupy time, to endure for several seconds, and to have a progressive motion. Mr. Hearder of Plymouth describes a discharge of lightning of this kind on the Dartmouth hills, very near to him. Several vivid flashes had occurred before the mass of clouds approached the hill on which he was standing; and before he had time to retreat from his dangerous position, a tremendous crash and explosion burst close to him. The spark had the appearance of a nucleus of intensely ignited matter, followed by a flood of light. It struck the path near him, and dashed with fearful brilliancy down its whole length to a rivulet at the foot of the hill, where it terminated. Analogous to the discharges described as globular lightning are the fire-balls so often noticed, about which there has been no little scepticism; but the evidence cannot reasonably be doubted, that displays of electrical light have repeatedly occurred, conveying the impression of balls of fire to the observer. An instance is given by Mr. Chalmers while on board the Montague, of seventy-four guns, bearing the flag of Admiral Chambers. In the account read to the Royal Society, he states, that “on November 4th, 1749, while taking an observation on the quarter-deck, one of the quarter-masters requested him to look to windward, upon which he observed a large ball of blue fire rolling along on the surface of the water, as large as a mill-stone, at about three miles distance. Before they could raise the main-tack, the ball had reached within forty yards of the main-chains, when it rose perpendicularly with a fearful explosion, and shattered the main-topmast to pieces.” In an account of the fatal effects of lightning in June 1826, on the Malvern Hills, when two ladies were struck dead, it is stated, that the electric discharge appeared as a mass of fire rolling along the hill toward the building in which the party had taken shelter.

Mr. Snow Harris remarks upon the difficulty of explaining these appearances on the principles applicable to the ordinary electric spark. The amazing rapidity of the latter, and the momentary duration of the light, render it impossible that they should be identical with it; but he conjectures that there may be a “glow discharge” preceding the main shock, some of the atmospheric particles yielding up their electricity by a gradual process before a discharge of the whole system takes place. In this view, the distinct balls of fire of sensible duration which have been perceived, are produced in a given point or points of a charged system previously to the more general and rapid union of the electrical forces—a supposition which will apply as well to the Mariner’s Lights, or St. Elmo’s Fire, observed during storms of thunder and lightning at sea. Pliny mentions lights noticed by the Roman mariners during tempests, flickering about their vessels, to which Seneca likewise makes allusion. By the superstitions of modern times they have been converted into indications of the guardian presence of St. Elmo, the patron saint of the sailor, hence called _cuerpo sante_ by the Spanish mariners. During the second voyage of Columbus among the West India islands, a sudden gust of heavy wind came on in the night, and his crew considered themselves in great peril, until they beheld several of these lambent flames playing about the tops of the masts, and gliding along the rigging, which they hailed as an assurance of their supernatural protector being near. Fernando Columbus records the circumstance in a manner strongly characteristic of the age in which he lived. “On the same Saturday, in the night, was seen St. Elmo, with seven lighted tapers, at the topmast. There was much rain and great thunder. I mean to say that those lights were seen which mariners affirm to be the body of St. Elmo, on beholding which they chanted many litanies and orisons, holding it for certain, that in the tempest in which he appears, no one is in danger.” A similar mention is made of this nautical superstition in the voyage of Magellan. During several great storms the presence of the saint was welcomed, appearing at the topmast with a lighted candle, and sometimes with two, upon which the people shed tears of joy, received great consolation, and saluted him according to the custom of the Catholic seamen; but he ungraciously vanished, disappearing with a great flash of lightning which nearly blinded the crew.

It is a striking instance of the triumph of mind, that by the introduction of lightning conductors into different civilized states, the power of this most energetic agent of nature is controled, and comparative security provided for life and property, otherwise in imminent jeopardy, when a severe thunder-storm occurs. Experience has taught the prime importance of furnishing exposed or elevated structures with a conducting apparatus, and has sufficiently shown that the immunity from danger enjoyed by many an unprotected building has been merely accidental; for when the teeming thunder-cloud has been wafted within reach of the edifice hitherto unscathed, the delusion has vanished that man may carelessly and with impunity thrust up his handiwork into the region of storms, as if daring the fury of the tempest, and inviting down its vengeance. The fine tower of St. Mark’s, at Venice, rising to the height of 360 feet, terminates in a pyramid which was severely injured in 1388. In 1417 the pyramid was again struck, and set on fire, having been constructed of wood. The same event happened in 1489, when it was entirely consumed. After being rebuilt of stone, the fell lightning renewed its destructive stroke in 1548, 1565, 1653 and 1745; and on the last occasion the whole tower was rent in thirty-seven places, and almost destroyed. It was again ravaged in 1761 and 1762, but in 1766 a lightning rod was put up, which has since protected it from damage. At Glogau, in Silesia, an interesting example of the value of conductors occurred in the year 1782. On the 8th of May, about eight o’clock in the evening, a thunder-storm from the west approached the powder magazine established in the Galgnuburg. An intensely vivid flash of lightning took place, accompanied instantly with such a tremendous peal of thunder, that the sentinel on duty was stupefied, and remained for awhile senseless, but no disaster occurred. Some laborers at a short distance from the magazine saw the lightning issue from the cloud and strike the point of the conductor, which conveyed it in safety by the combustible material. A different result took place with reference to a large quantity of unprotected ammunition, belonging to the republic of Venice, deposited in the vaults of the church of St. Nazaire, at Brescia. The church was struck with lightning in the month of August, 1767, and the electric fluid, descending to the vaults, exploded upward of 207,600 lbs. of powder, reducing nearly one-sixth of the fine city to ruins, and destroying about 3000 of the inhabitants. The Indians, whenever the sky wears a lowering aspect, so as to threaten a severe thunder-storm, are said to leave their pursuits and take refuge under the nearest beech-tree, considering it a complete protection, as it is affirmed that no instance has occurred of the beech having been struck by atmospheric electricity, when other trees of the American forests have been shivered into splinters in its neighborhood.

For ages the inhabitants of the globe have seen the lightning flash and heard the thunder rattle; and some writers upon the occult sciences of the ancients, as Salverte, have supposed that, tutored by experience, without any understanding of the theory of the subject, they possessed the secret of warding off from their buildings the thunderbolt by a conducting apparatus. It is certain that extraordinary intimations to this effect may be culled from their writings. Pliny states that Tullus Hostilius, practicing Numa’s art of bringing down fire from heaven, and performing it incorrectly, was struck with lightning—a fate which Professor Richman of St. Petersburg experienced, while performing incautiously the sublime experiment of Franklin, measuring the strength of the electricity brought down by a metallic rod in a thunder-storm, being instantly killed. Pliny likewise mentions the laurel as the only earthly production which lightning does not strike; hence, as a protection, these trees were planted around the temple of Apollo. Columella, however, mentions white vines surrounding the house of Tarchon, the Etruscan, for the same purpose. These expedients may provoke a smile without deserving one; for there can be no doubt that trees sufficiently high around a temple, or succulent plants covering a dwelling, will exercise to some extent a protective power, and act as a regular system of conductors. Salverte mentions several medals which appear to have reference to this subject, particularly one which represents the temple of Juno, the goddess of the air, the roof of which is armed with pointed rods. He quotes also Michaelis, upon the temple of Jerusalem, to show that the Jews were not unacquainted with the art of protecting their public buildings—a position grounded upon the following facts: “1. That there is nothing to indicate that the lightning ever struck the temple of Jerusalem during the lapse of a thousand years.” This, of course, does not make the fact certain; but when, as M. Arago justly remarks, we consider how carefully the ancient authors recorded the cases in which their public buildings were injured by lightning, we may accept the silence observed respecting the temple of Jerusalem, as proof that it was never struck. For three centuries the cathedral of Geneva, the most elevated in the city, has enjoyed a similar immunity, although inferior buildings have been repeatedly damaged. Saussure discovered the reason of this, in the tower being entirely covered with tinned iron plates, connected with different masses of metal on the roof, and again communicating with the ground by means of metallic pipes. “2. That according to the account of Josephus, a forest of spikes with golden or gilt points, and very sharp, covered the roof of this temple; a remarkable feature of resemblance with the temple of Juno represented on the Roman medals. 3. That this roof communicated with the caverns in the hill of the temple, by means of metallic tubes, placed in connection with the thick gilding that covered the whole exterior of the building; the points of the spikes there necessarily producing the effect of lightning-rods. How are we to suppose that it was only by chance they discharged so important a function; that the advantage received from it had not been calculated; that the spikes were erected in such great numbers only to prevent the birds from lodging upon and defiling the roof of the temple? Yet this is the sole utility which the historian Josephus attributes to them.” Upon a sober review of these facts, it is difficult to resist the conclusion that the ancient world had some proficiency in the art of guiding the electric fluid from the bosom of the clouds, conducting it in a prescribed course, and thus disarming it of its terrors.

The subject of electrical agency is intimately connected with that of magnetism, to which this is the fittest place to glance—one of the most recondite points of physical science. The relation between the two is evident, from the notorious fact that lightning often renders steel magnetic, and disturbs the magnetism of the magnetised needle, so that in thunder-storms the compass needles of a ship have frequently been seriously injured. The magnetic agency, like electricity, has a general distribution over the earth, but the phenomena differ in different parts of the world, and are subject to periodical differences in the same place, the cause of which is very little understood. Every one is acquainted with the polarity of a freely suspended magnetic needle, or its tendency to lie parallel with the earth’s axis, pointing nearly north and south in every region of the globe. What is called the _dip_ or _inclination_ of the needle is its divergence from a perfectly horizontal position. Thus the north pole of the needle inclines downward in the latitude of London at an angle of 70°, but conveyed toward the equator, the dip diminishes, till no inclination at all appears. Transported farther toward the south, the dip again discovers itself, but in an opposite direction, the south pole of the needle inclining downward. “To understand the reason of this dip of the magnetic needle, and of its general direction, we have only to consider that the earth itself operates as a great magnet, the poles of which are situated beneath its surface. The directive property of the needle is owing to these poles; and when the needle is on the north side of the equator, the north pole of the earth having the greatest effect, the needle is attracted downward toward the north pole; hence exactly over the magnetic pole the needle would be vertical. Similar phenomena occur in the southern hemisphere; but here the south pole predominates, and of course depresses the corresponding pole of the needle; while at the magnetic equator, from the equal action of both poles, the needle will assume an exactly horizontal position.”

But neither the magnetic equator nor the magnetic poles coincide precisely with the geographical equator and poles, and this difference constitutes what is termed the _variation_ of the needle. From calculation, the north magnetic pole had been fixed in latitude 70°, and longitude 98° 30′ west, a spot which Commander Ross approached within the distance of ten miles, in the year 1830, but was unable to verify the site, for want of the requisite instruments. Upon going through a long series of calculations afterward himself, he concluded the above position to have been erroneously assigned, and that the real point lay in latitude 70° 5′ 17″ north, and longitude 96° 46′ 45″ west, a spot on the western coast of Boothia, which he prepared to reach. On the first of June, 1831, at eight o’clock in the morning, he arrived at the site to which his calculations pointed, and found the same day the amount of the dip to be 89° 59′, only one minute less than 90°, the vertical position, which would have precisely indicated the polar station; and the horizontal needles, suspended in the most delicate manner possible, did not betray the slightest movement. The spot was an unattractive level site along the coast, rising into ridges from fifty to sixty feet high, about a mile inland. The wish expressed by the discoverer was natural, that a place so important had possessed more of mark or note, but Nature had erected no monument to denote the spot which she had chosen as the centre of one of her “great and dark powers.” A cairn of some magnitude was constructed by the adventurers, upon which the British flag was planted, and underneath, a canister was buried, containing a record of the interesting enterprise.

The magnetic needle has frequently exhibited violent disturbance when the Aurora Borealis has appeared. This has led to the surmise that these brilliant lights are connected with the electric and magnetic properties of the earth, though in a manner which we cannot explain. It has been remarked that during the appearance of the aurora the electric fluid may often be readily collected from the air. If a current of electricity also be passed through an exhausted receiver, a very correct imitation of the auroral light will be produced, displaying the same variety of color and intensity, and the same undulating motions. It is highly probable, therefore, that the beautiful and fantastic meteoric display is connected with electricity; but great obscurity rests upon this department of meteorology.

Of all optical phenomena, the Aurora Borealis, or the northern day-break, is one of the most striking, especially in the regions where its full glory is revealed. The site of the appearance, in the north part of the heavens, and its close resemblance to the aspect of the sky before sunrise, have originated the name. The “Derwentwater Lights” was long the appellation common in the north of England, owing to their display on the night after the execution of the unfortunate earl of that name. The scene in the illustration is a picture of the auroral light, as observed from the neighborhood of Loch Leven—a scene in itself admirably calculated to exhibit the phenomenon; and to convey any adequate idea of its magical aspect, as seen in high latitudes, the painter’s hand and the poet’s art are needed. A native Russian, Lomonosov, thus refers to the spectacle:—

“Where are thy secret laws, O Nature, where? Thy torch-lights dazzle in the wintry zone; How dost thou light from ice thy torches there? There has thy son some sacred, secret throne? See in your frozen sea what glories have their birth; Thence night leads forth the day t’ illuminate the earth.

“Come then, philosopher, whose privileged eye Reads Nature’s hidden pages and decrees: Come now, and tell us whence, and where, and why, Earth’s icy regions glow with lights like these, That fill our souls with awe; profound inquirer, say, For thou dost count the stars, and trace the planet’s way.

“What fills with dazzling beams the illumined air? What wakes the flames that light the firmament? The lightning’s flash: there is no thunder there, And earth and heaven with fiery sheets are blent; The winter’s night now gleams with brighter, lovelier ray Than ever yet adorned the golden summer’s day.

“Is there some vast, some hidden magazine, Where the gross darkness flames of fire supplies? Some phosphorous fabric, which the mountains screen, Whose clouds of light above those mountains rise? Where the winds rattle loud around the foaming sea, And lift the waves to heaven in thundering revelry?”

The appearances exhibited by the aurora are so various as to render it impossible to comprehend every particular in a description that must be necessarily brief and general. A cloud, or haze, is commonly seen in the northern region of the heavens, but often bearing toward the east or west, assuming the form of an arc, seldom attaining a greater altitude than 40°, but varying in extent from 5° to 100°. The upper edge of the cloud is luminous, sometimes brilliant and irregular. The lower part is frequently dark and thick, with the clear sky appearing between it and the horizon. Streams of light shoot up in columnar forms from the upper part of the cloud, now extending but a few degrees, then as far as the zenith, and even beyond it. Instances occur in which the whole hemisphere is covered with these coruscations; but the brilliancy is the greatest, and the light the strongest, in the north, near the main body of the meteor. The streamers have in general a tremulous motion, and when close together present the appearance of waves, or sheets of light, following each other in rapid succession. But no rule obtains with reference to these streaks, which have acquired the name of “the merry dancers,” from their volatility, becoming more quick in their motions in stormy weather, as if sympathizing with the wildness of the blast. Such is the extraordinary aspect they present, that it is not surprising the rude Indians should gaze upon them as the spirits of their fathers roaming through the land of souls. They are variously white, pale red, or of a deep blood-color, and sometimes the appearance of the whole rainbow as to hue is presented. When several streamers emerging from different points unite at the zenith, a small and dense meteor is formed, which seems to burn with greater violence than the separate parts, and glows with a green, blue, or purple light. The display is over sometimes in a few minutes, or continues for hours, or through the whole night, and appears for several nights in succession. Captain Beechey remarked a sudden illumination to occur at one extremity of the auroral arch, the light passing along the belt with a tremulous hesitating movement toward the opposite end, exhibiting the colors of the rainbow; and as an illustration of this appearance, he refers to that presented by the rays of some molluscous animals in motion. Captain Parry notices the same effect as a common one with the aurora, and compares it, as far as its motion is concerned, to a person holding a long ribbon by one end, and giving it an undulatory movement through its whole length, though its general position remains the same. Captain Sabine likewise speaks of the arch being bent into convolutions, resembling those of a snake in motion. Both Parry, Franklin, and Beechey agree in the observation that no streamers were ever noticed shooting downward from the arch.