THUNDER, 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-storms. Though the phenomena of lightning, therefore, have been at 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 show why rains are sometimes attended with thunder, 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 cracks will be heard at any particular place during the whole season; at others 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 a great tract of hot weather is very often observed without any thunder being heard; and besides, though not common, it 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 pressure 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 can have little motion
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 electrics 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 or 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 phosphoreal smell is smelt†; which shows, that though the electricity does not fly out through the air in the usual way, yet the fluid within the glass is agitated, and there is little reason to doubt that any conducting body inclosed within the substance of the glass would be electrified also. The vapours 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. But for a farther illustration of these matters, see the articles WEATHER, and WIND.