the noise occasioned by the explosion of a flash of lightning echoed back from the inequalities on the surface of the earth, in like manner as the noise of a cannon is echoed, and in particular circumstances forms a rolling lengthened sound.

Although thunder, properly speaking, is only a mere sound, capable of producing very little effect, yet the word is generally supposed to include the phenomena of lightning also; and electrified clouds are by universal consent called *thunder-clouds*, and the explosions of many flashes of lightning proceeding from them are generally called *thunder-forms*. Though the phenomena of lightning, therefore, have been at a great length explained and accounted for under the articles *Electricity* and *Lightning*, and though the immediate cause of electrical explosions from clouds is explained under the article *Rain*; yet the ultimate cause remains still to be shown, and properly belongs to the present article.

It is universally allowed, that the variation of the electricity in different parts of the atmosphere is the cause of thunder. Under the article *Electricity*, it has been shown why lightning explodes after the thunder-clouds are charged. Under the article *Lightning*, it is shown why that meteor puts on the various forms in which we see it, why it sometimes strikes houses or animals, and sometimes not, &c.; and under the article *Rain*, why the atmosphere in some cases parts with the vapours which at other times it so obstinately retains. It remains therefore only to mention the theory by which some philosophers explain the reason why rains are sometimes attended with thunders, and sometimes not; which, to those who attentively peruse the articles above-mentioned, may be done in few words.

In this part of Great Britain, and for a considerable way along the eastern coast, although thunder may happen at any time of the year, yet the month of July is that in which it may almost certainly be expected. Its duration is of very uncertain continuance; sometimes only a few peals will be heard at any particular place during the whole season; at other times the storm will return at the interval of three or four days for a month, six weeks, or even longer; not that we have violent thunder in this country directly vertical in any one place so frequently in any year, but in many seasons it will be perceptible that thunder-clouds are formed in the neighbourhood even at these short intervals. Hence it appears, that during this particular period there must be some natural cause operating for the production of this phenomenon, which does not take place at other times. This cannot be the mere heat of the weather, for we have often a long tract of hot weather without any thunder; and besides, though not common, thunder is sometimes heard in the winter also. As therefore the heat of the weather is common to the whole summer, whether there be thunder or not, we must look for the causes of it in those phenomena, whatever they are, which are peculiar to the months of July, August, and the beginning of September. Now it is generally observed, in the tract of country of which we now speak, that from the month of April an east or south-east wind generally takes place, and continues with little interruption till towards the end of June. At that time, sometimes sooner and sometimes later, a westerly wind takes place; but as the causes producing the east wind are not removed, the latter opposes the west wind with its whole force. At the place of meeting, there is naturally a most vehement prelude of the atmosphere, and friction of its parts against one another; a calm ensues, and the vapours brought by both winds begin to collect and form dark clouds, which Thunder can have little motion either way, because they are pressed almost equally on all sides. For the most part, however, the west wind prevails, and what little motion the clouds have is towards the east; whence the common remark in this country, that "thunder-clouds move against the wind." But this is by no means universally true; for if the west wind happens to be excited by any temporary cause before its natural period when it should take place, the east wind will very frequently get the better of it; and the clouds, even although thunder is produced, will move westward. Yet in either case the motion is so slow, that the most superficial observers cannot help taking notice of a considerable resistance in the atmosphere.

That when two streams of air are thus driven against each other, the space where they meet must become highly electrified, is as plain as that an electric globe must be excited when friction is applied. It is true, as the substances here to be excited are both electric per se, it may be objected, that no electricity could be produced; for we cannot excite one electric by rubbing it with another. Yet it is observed, that glass may be electrified by blowing strongly upon it, or by the explosion of cannon; and even when glass is strongly pressed upon glass, both pieces become electrified as soon as they are separated. When glass is rubbed upon glass, no attraction nor repulsion can be perceived, nor is any sign of electricity observed on bodies brought near to it; yet a very bright electric light always appears on the glasses, and a phosphorescent smell is felt; which shows, that though the electricity does not fly out through the air in the usual way, yet the fluid within the glass is activated; and there is little reason to doubt that any conducting body inclosed within the substance of the glass would be electrified also. The vapors therefore, which are the conducting substances in the atmosphere, become immediately electrified in consequence of the pressure above-mentioned, and all the phenomena described under the various articles already referred to take place.

In like manner, by the struggle of two other winds as well as those of the east and west, may a thunderstorm be produced; but it is always necessary that the resistance of the air to the motion of the clouds should be very great, and nearly equal all round. For if the vapour should get off to a side, no thunder would take place; the electricity would then be carried off as fast as it was collected, and rain would only be the consequence, by reason of the electrified vapours parting with their latent heat, as is explained under the article RAIN. In fact, we very often observe, that in the time of rain the clouds evidently move across the wind, and the nearer their motion is to a direct opposition, the heavier will the rain be; while, on the other hand, if they move briskly before the wind, let the direction be what it will, the atmosphere soon clears up.

That rattling in the noise of thunder which makes it seem as if it passed thro' arches, or were variously broken, is probably owing to the sound being excited among clouds hanging over one another, and the agitated air puffing 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 114 feet per second of time, or three two third seconds to the mile. Dr Wallis observes that commonly the difference between the two is about seven seconds, which, at the rate above mentioned, gives the distance almost two 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 burnt 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.

Respecting the phenomena of thunder, we have many observations to communicate; some of which, we flatter ourselves, are new, and all of them valuable; but our bounds obliges us, though with great reluctance, to pass them over.