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been principally relied on to show the probable direction of the lightning. And because pavements have been torn up, hair and hooks carried into trees, a hat transported to the roof, the barks of trees detached below, leaves crisped on the under side, which was convex, and sods turned up on all sides, it has been concluded that the flash sometimes ascends. But the double burr which is seen on a card through which a Leyden jar has been discharged, and the marks of explosive power which generally characterize the mechanical execution of lightning, should be our caution not to give too much weight to such facts as have been briefly alluded to. It will not be thought necessary now, however, to say, with Maffei, who, a century ago, advocated ascending thunderbolts, that he could reconcile his views with Scripture, which speaks of the “ fire falling from heaven.” If it is simply required to know whether the cloud or the earth represents the positive end of the discharging line, we have only to place a steel needle at right angles to its course, and observe the disposition of the poles after the flash. Beccaria attempted to do this, by placing the steel parallel to the course of the lightning, and applying the experiments of Franklin, Dalibard, and his own.
Arago has divided the spark of atmospherical electricity into three kinds. 1. The zigzag. 2. Sheet-lightning. 3. Ball-lightning. The zigzag path is commonly manifested, if at all, between the earth and a cloud, and not between cloud and cloud. Sometimes a barbed form, as in the point of an arrow, has been attributed to it. When it divides, as is occasionally the case, into two branches, it is called forked. Less frequently, three prongs have been seen. The division of the charge is often inferred from the simultaneous destruction of different objects, even when it has escaped detection by any visible branches in the illuminated track of the darting electricity. If the branches of the zigzag course of the lightning are very small, it produces an effect known under the name of chain-lightning.
Logan * believed that the zigzag, shape of forked lightning was an illusion, to be referred to the irregular refractions produced by clouds and vapors. But Arago justly remarks upon this, that astronomers, when they observe celestial objects through the same clouds and vapors, do not witness such extraordinary influences upon light. In this zigzag movement the angles are very acute, so that, if lightning were regarded as a projectile, the law of continuity would seem to be strangely violated. But if we consider lightning as moving by an undulation of some description, as light and heat are propagated, then these irregularities and this multiplicity of direction. may, perhaps, derive some explanation from the action of crystals on light. But, may it not be that the path of the lightning takes its direction from the accidental lines in which the conducting particles of vapor are arranged, as in the well-known experiment of the spotted tubes ? * Howard has seen lightning in its course double back upon itself, in a curve not unlike that of the planets in their changes from direct to retrograde motion, and back again. The zigzag flashes, which the Italians call “ saette,” carry generally destruction with them.
The second kind of lightning in the classification with which I started is sheet-lightning. “In the calmest nights,” says Seneca, “ with the stars shining bright, you may see lightnings flash, but doubt not in the direction of the lightning there will be found clouds which the spherical form of the earth hides from our view. The flash ascends on high, and appears in the bright and serene sky, being withal elaborated in some obscure and dark cloud." ^ Bergman says, that in Sweden these flashes are called “lightnings of the barley.” This silent lightning is rarely seen when the sky is cloudy. It is much fainter than streak-lightning, as we see when the iwo
* Phil. Trans. XXXIX. 240.
+ Q. N. II. 26.
kinds are visible at the same time. Lozeran de Fesc, in his dissertation on thunder, to which the Academy of Bordeaux awarded its prize in 1726, supposed these summer, heat, or silent lightnings to be reflected.
This silent lightning has frequently been supposed to be the reflection of distant storms below the horizon of the observer. It has been objected to this view, that a reflected light, inferior to common lightning in the same proportion that twilight bears to daylight, would be too feeble to affect the eye. But Arago summons to the aid of the first supposition the fact, that, in 1739, while Cassini and Lacaille were making experiments on the velocity of sound, a discharge of cannon near the light-house of Cette was seen where both the town and light-house were concealed by Mount St. Bauzeli. Again, in 1803, Baron Zach was flashing gunpowder on the Brocken, as a signal for longitudes. The flashes were seen on Mount Kenlenberg, 180 miles off, although the mountain itself was below the horizon. Moreover, when guns are fired at the Hotel des Invalides, in Paris, the light is seen in the gardens of Luxembourg, where no part of the first building is in view. In many cases, it is known that a storm has been raging below the horizon, betraying itself to the observer by no clouds or noise, but only by the reflected light. On the 10th of July, 1783, the town of Geneva was visited by a terrible thunder-storm. From the Hospice du Grimsel, Saussure saw the light, without any clouds or noise, in that direction. It is not so easy to dispose of those instances in which heat-lightning has played for a whole night on all sides of the horizon. Can we suppose a storm all around, while over our heads is an oasis of serenity ? Moreover, Deluc mentions instances in which one flash from a visible cloud was attended by a stunning noise, and the next, though equally bright, was inaudible. May it not be, that in some cases the thunder is inaudible because the electric discharge occurs between cloud and cloud, in regions of highly rarefied air ? Arago proposed to test the reflection of the light by his polariscope.
Arago says, in regard to ball-lightning, that many questions might be asked of it, in presence of which science would stand mute. From the works of Boyle, he has gleaned an accident which occurred to the ship Albemarle, near Cape Cod, in 1681. A flash of lightning was seen, and something fell upon deck which the men could not extinguish or sweep overboard. Deslandes relates, that a church was struck near Brest, and three balls of fire were seen, each three and one half feet in diameter. In 1772, such a ball was seen to oscillate in the air, and then fall. On the 7th of December, 1838, the Royal ship Rodney was struck, with a sound equal to that of a thirty-two-pounder. Two men were killed, and their clothes burnt off. Their comrades said they saw balls of fire, and ran after them to throw them overboard. In 1848, such a ball came slowly up and exploded upon the mainmast of a United States ship in the Gulf-Stream. Joseph Wasse, in Northamptonshire, thought that, in 1725, he heard the noise of the motion of one ball through the air. These balls are visible from one to ten seconds. They are said sometimes to strike the earth and rebound. Are they subjective phenomena, originating in a dazzling brilliancy of the lightning, or are they agglomerations of ponderable substances? Fusinieri states, that he has often found iron in various degrees of oxidation, and sulphur, in the powdery deposits around the fissures through which the lightning has entered. As pertinent to the statement that thunder-stones, so called, are found in the trunks of trees, Arago asks the question, whether thunder has introduced toads into the trunks of trees.
To ascertain the duration of lightning in its various phases, Arago proposes to use a wheel of a definite number of spokes, which shall be turned by clock-work. The duration will be given either by the velocity necessary to make the whole circular area appear illuminated, or by the arc illuminated with a fixed velocity. Arago credits this contrivance to Wheatstone. I will remark, in regard to the color of lightning in general, that
when the discharging clouds are near the earth, the light is white; and when they are at a great height, the light is reddish or violet.
I may premise what I have to say on the subject of thunder, by observ. ing that sound, in general, is a vibration, sometimes originating in an aerial disturbance, and, at least, generally transmitted by the air, whatever its origin. Some physical writers have been anxious to determine the way in which the original disturbance is created. Is thunder produced in the cloud ? or is it produced by the passage of the electricity from cloud to cloud, or from a cloud to the earth? There are those who lay stress upon the exceeding velocity of electricity, and imagine that, as it rushes along in the air, it leaves behind itself a vacuum into which the air dashes with a great noise, as in the bladder-glass experiment with the air-pump: Others attribute the noise of thunder to the sudden compressions and dilatations which the air undergoes. Pouillet thinks the passage of a cannon-ball through the air with the same speed would make as great a sound as that of thunder. He also suggests, whether the conduction of electricity by such a substance as the earth's atmosphere may not consist in a rapid induction from particle to particle ; and whether the alternate decompositions and recompositions involved in these successive molecular inductions may not be the violence which produces the sound. If, in a single instance, the elevation of a thunder-cloud were computed by the interval between the flash and the report, and on the assumption that the sound originated in the cloud, and this calculated height compared with the true height as known in other ways, - as, for example, by the position of the cloud in respect to a steeple or other object whose height was known,- it would be possible to determine at least where, if not how, the sound was made.
Aristotle says of the sound, “For thus in clouds, a separation of the pneumatic substance taking place, and falling against the density of the clouds, produces thunder.” Pliny suggests, whether thunder may not be caused by shooting-stars, hissing as hot iron does when put in water. But he wisely adds, " These things are hidden with the majesty of nature, and reserved within her cabinet. Lucretius compares thunder to the sound which accompanies the tearing of paper, silk, or parchment. He thought violent winds squeezed it out of the clouds. Descartes thought that an upper and a lower stratum rushed together, as he had sometimes seen to bappen in the Alps. And we might say, with Seneca, “ If clapping the hands makes such a noise, what must we hear when two clouds come together with a rush ?” Peytier and Hossard observed that the thunder from clouds in which they were immersed sounded like the blaze of powder when set on fire in an open space. Richard, in his Histoire de l'Air, compares it to the sound made by the rolling of a heap of nuts upon wooden planks. But as soon as he rose above the clouds, the thunder was loud again.
Aristophanes ridicules the meteorological speculations of the ancients in the following passage from the Clouds :
" Strepsiades. But tell me, who is it that thunders ? That makes me terribly afraid.
“ Socrates. The clouds, as they roll along, give birth to the thunder. “ Strep. How ? O most audacious man !
“ Soc. When they are saturated with much moisture, and are compelled to be borne along, and, full of showers, lower themselves from necessity; if, in this heavy state, they dash against each other, they explode and crack.
Strep. But is it not Jupiter that compels them to be borne along? " Soc. By no means; but the etherial vortex.
“ Strep. 'Vortex ? It certainly had escaped my notice that Jupiter had ceased to be, and that Vortex now reigned in his stead. But you have, as yet, told me nothing concerning the noise of the thunder.
" Soc. Have you not heard me say, that the clouds, when full of moisture, dash against each other, and resound by reason of their density?
Strep. How am I to believe this? • Soc. I will prove it to you from your own case. Have you not, after you have been stuffed with broth at the Panathenaic festival, then felt a disturbance in your belly, and a rumbling has suddenly resounded through it ?
Strep. Yes, by Apollo, I have ; and it has played the mischief with my inside.
“ Soc. And is it not probable that the air, being boundless, should make a much more mighty thundering ?
Every one distinguishes between a clap of thunder and the pealing sound which frequently is heard. This prolonged noise sometimes lasts from thirty-six to forty-five seconds. Captain Scoresby, near Lake Killarney, observed that the sound of a pistol-shot continued thirty seconds. In the neighborhood of Paris, where the echo is not remarkable, the report of a cannon was audible from twenty to twenty-five seconds. Many think the rolling sound of thunder sufficiently explained, when they refer it to a complicated system of echoes. It is not a fatal objection to this view that the thunder rolls also at sea, because the clouds can reflect as well as the solid mountains of the earth. The report of a cannon or pistol is repeated in a lowering sky, when it is not in clear weather. The French acade. micians, while making their experiments upon sound, observed that, when. ever clouds were between their two stations, the signals were reverberated so as to sound like thunder. Peclet, however, argues that the rolling of thunder cannot proceed from the reflection of sound from the clouds, because at sea the report of a cannon is never repeated in that way.
Dr. Hooke, in 1706, started the explanation given in Herschel's Treatise on Sound.* He rests his theory upon the moderate velocity with which sound travels through the air. This distinction between the velocity of the luminous and acoustic radiations of bodies is thus described by Pliny, though referred to the wrong cause : “ That the lightning is seen before the thunder-clap is heard, although they come indeed jointly together, it is certainly known. And no marvel, for the eye is quicker to see light than the ear to hear a sound. And yet nature doth so order the number and measure, that the stroke and the sound should accord together; .... neither is any man stricken who either saw the lightning before or heard the thunder-clap.” Lucretius knew better why the sound comes after the flash. But the question has been raised, whether the lightning strikes before it is visible. Arago brings forward many cases of persons who were struck, and yet heard and saw nothing.
If we suppose an electric disturbance to take place, not at a single focus, but along a great length of cloud or moist air, the audible effects of this disturbance will reach the ear from the different points of its origin in successive instants ; so that a sound which, at its departure, is contemporaneous in time, but diffused in space, produces an impression upon the organ of sensation, local in space, but prolonged in time. Dr. Robinson illustrates this view by a very long file of soldiers, and by the multiplied sound which would be heard by one placed in the same line beyond, if their guns were all fired together. Lardner has objected to this analogy, that in the latter case we should not have a succession of sounds, but a note of a certain pitch.
If Hooke's account of rolling thunder is adopted, it will be necessary to suppose the train over which the electric discharge runs to be three or four leagues long, in some remarkable storms. As all the peculiarities of sound, and the combination of sudden claps and rolling peals, depend on the con
* Encycl. Metr.
figuration of the clouds with respect to the point addressed by the noise, we may say, with Kaemtz, that every observer hears his own thunder as he sees his own rainbow.
It has already been stated, that silent lightning is not unfrequent. It is no less true that there is invisible thunder ; that is, thunder without lightning, or even clouds. Seneca says, that it thunders sometimes without lightning. In 1751, this was frequently observed at Martinique. We must exclude from the account earthquake countries. In St. Fé de Bogota, the thunder-mass is pronounced every year. The obvious explanation of invisible thunder is, that it proceeds from clouds below the horizon. In pursuing this view, we are arrested by the fact, that thunder is never heard at any very great distance, and that clouds in which the discharge of electricity is audible, but invisible, must therefore be excessively near to the earth's surface. De l'Isle once counted thirty-two seconds between the flash and the report. Arago finds no instance recorded greater than fortynine seconds. If this method of calculation is accurate, it would appear that thunder has never been heard to a greater distance than fifteen miles. The remarkable limitation of this maximum distance is proved by other means, perhaps less exceptionable. On the 25th of January, 1757, a steeple in Cornwall was struck. The great engineer, Smeaton, who was only thirty miles distant, saw the light, but heard no noise. Muschenbroek says it thunders at the Hague when no sound is heard at Leyden or Rotterdam, which are only ten and thirteen miles off. Also, thunder at Amsterdam is not heard at Leyden, which is removed from it twenty-two miles and a half. It certainly is strange that the sound of thunder, which, in many cases, has been compared to one or two hundred pieces of artillery booming at once, should be inaudible at distances exceeding fifteen or twenty miles, especially when we consider that canno
nonading has been heard two hundred miles. The Emperor Kanghi * was surprised that thunder could be heard only ten leagues, when he had heard artillery thirty leagues. The distinguished meteorologist, Howard, relates that, in 1812, when a continuous stratum of mist prevailed, he could hear the carriages on the stones of London streets, when he was five miles away. The great bell of St. Paul's cathedral is heard at Windsor, over a distance of twenty-four miles.
Now, in a level country, an object can be seen at the distance of fifteen miles, if it is vertically raised as much as one hundred feet above the earth's surface. Hence we are driven to the conclusion, either that invisible thunder comes from clouds which are less than one hundred feet in elevation, or else that the electric discharge can take place in an apparently serene sky, and that it may be accompanied with a heavy report without a corresponding flash. Can there be an electric discharge from a clear and serene sky? In reply to this question, Arago has marshalled many cases related by Pliny, Suetonius, and Crescentius, in which lightning was described as flashing from a clear heaven; but nothing is said about the thunder. Anaximander believed that it might thunder from a serene sky, for he attempted to find out the cause. There is not so much difficulty when thunder, unaccompanied by lightning, is heard in the presence of clouds, for then possibly the discharge may be in higher regions of clouds, the view of which is screened from the hearer by intervening strata too dense to be penetrated by the lightning's flash. But many would prefer the alternative of supposing that thunder-clouds are sometimes less than one hundred feet above the earth's surface, to admitting that it can thunder with or without lightning from a serene blue sky; especially if, soon afterwards, clouds appear. Volney relates, that, at Pontchartrain, he heard peals of thunder, but saw no clouds, even in the horizon. But in the course of an hour, majestic hail-clouds rose into sight.
* Mem. of Miss. to China, IV.