Thunder, a loud noise occasioned by the explosion of a flash of lightning, that is re-echoed from the inequalities on the surface of the earth, and frequently forms a continued rolling or rumbling sound.

Various theories have been proposed by naturalists, to account for this grand and awful phenomenon ; but, like that of lightning, it is now generally understood to arise from electricity.

Thunder is frequently productive of beneficial effects on animal and vegetable nature; as its violent concussions contribute to disperse the noxious vapours collected in the atmosphere, during sultry and dry summers : it may also be regarded as a prognostic of the weather. BradlEY remarks, that frequent thunder, without lightning, is generally followed by tempestuous winds 3 but, in the contrary case, it predicts copious and sudden showers.

With a view to prevent fatal accidents, during violent thunderstorms, the erection of Conductors has been strongly recommended. - For the same purpose, the electrical Kite, described under that head, may be usefully employed ; but it requires to be managed with great precaution : we shall therefore state the most proper manner of directing it, on these occasions.

After having constructed the electrical kite on the principles laid down vol. iii. pp. 51-53, it will be necessary to erect a pole or beam, with a moveable arm on Its upper part (nearly resembling in shape the letter F), so as to move on a pivot. If the height of the beam above ground be six feet, the arm requires to be two feet, or in a similar proportion, adapted to the length of the former. Near the extremity of the arm, a large stone, weighing from 7 to 14 lbs. should be suspended by means of a strong rope, so that it may balance about six inches from the ground. When the wind has sufficient strength to carry the kite, the main string, by which it is conducted, should be fastened to this stone, and part of the former be allowed to remain on the soil. By such contrivance, the assistance of any person for directing the machine is rendered unnecessary ; as the stone, depending on the moveable arm before alluded to, will yield to the different movements of the kite. Thus, when a thunder-cloud happens to hover above the apparatus, and the electric matter finds a conductor in the metallic point attached to the top of the drake (see vol. iii. p. 53), all violent explosions will be obviated, and such dangerous fluid will quietly glide down along the wire connected with the string, till it spend itself in the ground.

As, however, this method of conducting the electric fluid is attended with considerable danger, if managed by unskilful hands, we think it our duty to suggest an additional precautionary measure, by which the whole may be regulated with perfect security to the person thus employed. After the main string of the kite (which ought to be from two to three hundred yards long) has been fastened to the Stone in the manner above directed, its extremity ought to be extended on the ground : the other end of the leading-string should be previously passed through a small roller or pulley, connected with a ring, to which must be tied; 1. a silk cord coated with wax; and, 2, a small metallic chain of such length, that it may, when the kite is vising, touch the ground at the distance of 20 or 30 paces. Now the machine is suffered to ascend, till it begin to draw the main string; when the silk cord is allowed gradually to pass through the hand, till the chain, suspended on the ring of the pulley, be at a proper distance. Next, as the body of the drake rises, the operator ought to advance progressively towards the end of the string fixed to the stone, till at length the principal cord becomes completely distended. In this state of the machinery, the farther regulation may be safely entrusted to the balance of the stone, with its moveable arm turning on the pivot of the beam before described : by means of the waxed silk cord, the manager will be protected from the influence of the electric fluid gliding down the main string, while the metallic chain will afford security against accidental lightning. And, in order to promote the derivation of this fluid from the leading string of the machine, it will be advisable to connect another chain with the extremity of such string, projecting from the stone contiguous to the beam, so that it may thus be easily conducted into the ground, in the manner similar to that already pointed out, vol. ii. p. 48.

It has been remarked, by accurate observers, that the noise of thunder is not experienced oftener than 16 or 18 times in the year, at any fixed place. This phenomenon occurs most frequently in the months of May and July; namely, in the proportion of three to one, when compared with the months of April and September ; but only in that of two to one, when contrasted with June and August. During the months of April and September, the number of thunder-storms is equal; and likewise in June and August. They arise generally with a south-wind ; less frequently with a southwest ; still more rarely with east and west ; and scarcely ever with north, north-west, or north-east winds. It is also remarkable, that thunder-storms are the most violent, when proceeding from regions where they seldom occur; that they happen in the afternoon, or in the evening, more usually than in the morning; and are in mountainous districts more common than in a plain country.

When swallows pursue each other in flights, with a loud noise, early in the morning, it may be considered as an almost certain in-dication of an approaching thunder-storm : many persons, also, feel a peculiar heaviness and languor, previously to tempestnous weather; and, though such sensations subside only when the clouds break forth in torrents of fire or rain, yet we are inclined to ascribe these symptoms to some diseased action in the animal frame, or to a preternatural susceptibility of nervous impressions. - In short, if due attention were paid to the changes taking place among animals and plants, previously to the formation of thunder and lightning, we could, perhaps with the greatest precision, foretel their approach.

Lastly, to prevent unfortunate accidents during thunder-storms, we would seriously advise every person to keep at some distance from trees, and rather to expose himself to a shower. While in a room, it will be proper to avoid the fire-place, fire-irons, gilded mouldings, and all articles containing metallic ingredients. - See also vol. iii. p. 99.