THUNDER

, a noise in the lower region of the air, excited by a sudden explosion of electrical clouds; which are therefore called Thunder-clouds.

The phenomenon of Thunder is variously accounted for. Seneca, Rohault, and some other authors, both ancient and modern, account for Thunder, by supposing two clouds impending over one another, the upper and rarer of which, becoming condensed by a fresh accession of air raised by warmth from the lower parts of the atmosphere, or driven upon it by the wind, immediately falls sorcibly down upon the lower and denser cloud; by which fall, the air interposed between the two being compressed, that next the extremities of the two clouds is squeezed out, and leaves room for the extremity of the upper cloud to close tight upon the under; thus a great quantity of the air is enclosed, which at length escaping through some winding irregular vent or passage, occasions the noise called Thunder.

But this lame device could only reach at most to the case of Thunder heard without lightning; and therefore recourse has been had to other modes of solution. Thus, it has been said that Thunder is not occasioned by the falling of clouds, but by the kindling of sulphurous exhalations, in the same manner as the noise of the aurum fulminans. “There are sulphurous exhalations, says Sir I. Newton, always ascending into the air when the earth is dry; there they ferment with the nitrous acids, and, sometimes taking fire, generate Thunder, lightning, &c.”

The effects of Thunder are so like those of sired gunpowder, that Dr. Wallis thinks we need not scruple to ascribe them to the same cause; and the principal ingredients in gunpowder, we know, are nitre and sulphur; charcoal only serving to keep the parts separate, for their better kindling. Hence, if we conceive in the air a convenient mixture of nitrous and sulphurous particles; and those, by any cause, to be set on sire, such explosion may well follow, and with such noise and light as attend the firing of gunpowder; and being once kindled, it will run from place to place, different ways, as the exhalations happen to lead it; much as is found in a train of gunpowder.

But a third, and most probable opinion is, that Thunder is the report or noise produced by an electrical explosion in the clouds. Ever since the year 1752, in which the identity of the matter of lightning and of the electrical fluid has been ascertained, philosophers have generally agreed in considering Thunder as a concussion produced in the air by an explosion of electricity. For the illustration and proof of this theory, see Electricity, and Lightning.

It may here be observed, that Mr. Henry Eeles, in a letter written in 1751, and read before the Royal Society in 1752, considers the electrical fire as the cause of Thunder, and accounts for it on this hypothesis; and he tells us, that he did not know of any other person's having made the same conjecture. Philos. Trans. vol. 47, p. 524 &c.

That rattling in the noise of Thunder, which makes it seem as if it passed through arches, or were variously broken, is probably owing to the sound being excited among clouds hanging over one another, and the agitated air passing irregularly between them.

The explosion, if high in the air, and remote from us, will do no mischief; but when near, it may destroy trees, animals, &c.

This proximity, or small distance, may be estimated nearly by the interval of time between seeing the flash of lightning, and hearing the report of the Thunder, estimating the distance, after the rate of 1142 feet per second of time, or 3 2/3 seconds to the mile. Dr. Wallis observes, that commonly the difference between the two is about 7 seconds, which, at the rate above mentioned, gives the distance almost 2 miles. But sometimes it comes in a second or two, which argues the explosion very near us, and even among us. And in such cases, the doctor assures us, he has sometimes foretold the mischiefs that happened.

The noise of Thunder, and the flame of lightning, are easily made by art. If a mixture of oil or spirit of vitriol be made with water, and some filings of steel added to it, there will immediately arise a thick smoke, or vapour, out of the mouth of the vessel; and if a lighted candle be applied to this, it will take fire, and the flame will immediately descend into the vessel, which will be burst to pieces with a noise like that of a cannon.

This is so far analogous to Thunder and lightning, that a great explosion and fire are occasioned by it; but in this they differ, that this matter when once fired is destroyed, and can give no more explosions; whereas, in the heavens, one clap of Thunder usually follows another, and there is a continued succession of them for a long time. Mr. Homberg explained this by the lightness of the air above us, in comparison of that near, which therefore would not suffer all the matter so kindled to be dissipated at once, but keeps it for several returns.

THUNDERBOLT. When lightning acts with extraordinary violence, and breaks or shatters any thing, it is called a Thunderbolt, which the vulgar, to fit it for such effects, suppose to be a hard body, and even a stone.—But that we need not have recourse to a hard solid body to account for the effects commonly attributed to the Thunderbolt, will be evident to any one, who considers those of the pulvis fulminans, and of gunpowder; but more especially the astonishing powers of electricity, when only collected and employed by human art, and much more when directed and exercised in the course of nature.

When we consider the known effects of electrical explosions, and those produced by lightning, we shall be at no loss to account for the extraordinary operations vulgarly ascribed to Thunderbolts. As stones and bricks struck by lightning are often found in a vitrified state, we may reasonably suppose, with Beccaria, that some stones in the earth, having been struck in this manner, gave occasion to the vulgar opinion of the Thunderbolt.

Thunder-clouds, in Physiology, are those clouds | which are in a state fit for producing lightning and thunder.

From Beccaria's exact and circumstantial account of the external appearances of Thunder-clouds, the following particulars are extracted.

The first appearance of a Thunder storm, which usually happens when there is little or no wind, is one dense cloud, or more, increasing very fast in size, and rising into the higher regions of the air. The lower surface is black and nearly level; but the upper finely arched, and well defined. Many of these clouds often seem piled upon one another, all arched in the same manner; but they are continually uniting, swelling, and extending their arches.

At the time of the rising of this cloud, the atmosphere is commonly full of a great many separate clouds, that are motionless, and of odd whimsical shapes. All these, upon the appearance of the Thunder-cloud, draw towards it, and become more uniform in their shapes as they approach; till, coming very near the Thundercloud, their limbs mutually stretch toward one another, and they immediately coalesce into one uniform mass. These he calls adscititious clouds, from their coming in, to enlarge the size of the Thunder-cloud. But sometimes the Thunder-cloud will swell, and increase very fast, without the conjunction of any adscititious clouds; the vapours in the atmosphere forming themselves into clouds wherever it passes. Some of the adscititious clouds appear like white fringes, at the skirts of the Thunder-cloud, or under the body of it, but they keep continually growing darker and darker, as they approach to unite with it.

When the Thunder-cloud is grown to a great size, its lower surface is often ragged, particular parts being detached towards the earth, but still connected with the rest. Sometimes the lower surface swells into various large protuberances bending uniformly downward; and sometimes one whole side of the cloud will have an inclination to the earth, and the extremity of it nearly touch the ground. When the eye is under the Thundercloud, after it is grown larger, and well formed, it is seen to sink lower, and to darken prodigiously; at the same time that a number of small adscititious clouds (the origin of which can never be perceived) are seen in a rapid motion, driving about in very uncertain directions under it. While these clouds are agitated with the most rapid motions, the rain commonly falls in the greatest plenty, and if the agitation be exceedingly great, it commonly hails.

While the Thunder-cloud is swelling, and extending its branches over a large tract of country, the lightning is seen to dart from one part of it to another, and often to illuminate its whole mass. When the cloud has acquired a sufficient extent, the lightning strikes between the cloud and the earth, in two opposite places, the path of the lightning lying through the whole body of the cloud and its branches. The longer this lightning continues, the less dense does the cloud become, and the less dark its appearance; till at length it breaks in different places, and shews a clear sky.

These Thunder clouds were sometimes in a positive as well as a negative state of electricity. The electricity continued longer of the same kind, in proportion as the Thunder-cloud was simple, and uniform in its di- rection: but when the lightning changed its place, there commonly happened a change in the electricity of the apparatus, over which the clouds passed. It would change suddenly after a very violent flash of lightning, but the change would be gradual when the lightning was moderate, and the progress of the Thunder-cloud slow. Beccar. Lettere dell 'Elettricismo pa. 107; or Priestley's Hist. Elec. vol. 1, p. 397. See also Lightning.

Thunder-House, in Electricity, is an instrument invented by Dr. James Lind, for illustrating the manner in which buildings receive damage from lightning, and to evince the utility of metallic conductors in preserving them from it.

A (fig. 1, pl. 35), is a board about 3/4 of an inch thick, and shaped like the gable end of a house. This board is fixed perpendicularly upon the bottom board B, upon which the perpendicular glass pillar CD is also fixed in a hole about 8 inches distant from the basis of the board A. A square hole ILMK, about a quarter of an inch deep, and nearly one inch wide, is made in the board A, and is filled with a square piece of wood, nearly of the same dimensions. It is nearly of the same dimensions, because it must go so easily into the hole, that it may drop off, by the least shaking of the instrument. A wire LK is fastened diagonally to this square piece of wood. Another wire IH of the same thickness, having a brass ball H, screwed on its pointed extremity, is fastened upon the board A: so also is the wire MN, which is shaped in a ring at O. From the upper extremity of the glass pillar CD, a crooked wire proceeds, having a spring socket F, through which a double knobbed wire slips perpendicularly, the lower knob G of which falls just above the knob H. The glass pillar DC must not be made very fast into the bottom board; but it must be fixed so that it may be pretty easily moved round its own axis, by which means the brass ball G may be brought nearer to or farther from the ball H, without touching the part EFG. Now when the square piece of wood LMIK (which may represent the shutter of a window or the like) is fixed into the hole so that the wire LK stands in the dotted representation IM, then the metallic communication from H to O is complete, and the instrument represents a house furnished with a proper metallic conductor; but if the square piece of wood LMIK be fixed so that the wire LK stands in the direction LK, as represented in the figure, then the metallic conductor HO, from the top of the house to its bottom, is interrupted at IM, in which case the house is not properly secured.

Fix the piece of wood LMIK, so that its wire may be as represented in the figure, in which case the metallic conductor HO is discontinued. Let the ball G be fixed at about half an inch perpendicular distance from the ball H; then, by turning the glass pillar DC, remove the former ball from the latter; by a wire or chain connect the wire EF with the wire Q of the jar P; and let another wire or chain, fastened to the hook O, touch the outside coating of the jar. Connect the wire Q with the prime conductor, and charge the jar; then, by turning the glass pillar DC, let the ball G come gradually near the ball H, and when they are arrived sufficiently near one another, you will observe, that the jar explodes and the piece of wood LMIK is | pushed out of the hole to a considerable distance from the Thunder-house.

Now the ball G, in this experiment, represents an electrified cloud, which, when it is arrived sufficiently near the top of the house A, the electricity strikes it; and as this house is not secured with a proper conductor, the explosion breaks part of it, i. e. knocks off the piece of wood IM.

Repeat the experiment with only this variation, viz, that this piece of wood IM be situated so that the wire LK may stand in the situation IM; in which case the conductor HO is not discontinued; and you will observe that the explosion will have no effect upon the piece of wood LM; this remaining in the hole unmoved; which shews the usefulness of the metallic conductor.

Farther, unscrew the brass ball H from the wire HI, so that this may remain pointed, and with this difference only in the apparatus repeat both the above experiments, and you will find that the piece of wood IM is in neither case moved from its place, nor will any explosion be heard; which not only demonstrates the preference of conductors with pointed terminations to those with blunted ones, but also shews that a house, furnished with sharpterminations, although not furnished with a regular conductor, is almost sufficiently guarded against the effects of lightning.

Mr. Henley, having connected a jar containing 509 square inches of coated surface with his prime conductor, observed that if it was so charged as to raise the index of his electrometer to 60°, by bringing the ball on the wire of the Thunder-house, to the distance of half an inch from that connected with the prime conductor, the jar would be discharged, and the piece in the Thunder-house thrown out to a considerable distance. Using a pointed wire for a conductor to the Thunder-house, instead of the knob, the charge being the same as before, the jar was discharged silently, though suddenly; and the piece was not thrown out of the Thunderhouse. In another experiment, having made a double circuit to the Thunder-house, the first by the knob, the second by a sharp-pointed wire, at an inch and a quarter distance from each other, but of exactly the same height (as in fig. 2) the charge being the same; although the knob was brought first under that connected with the prime conductor, which was raised half an inch above it, and followed by the point, yet no explosion could fall upon the knob; the point drew off the whole charge silently, and the piece in the Thunderhouse remained unmoved.

Phil. Trans. vol. 64, p. 136. See Points in Electricity.