Book III. OF THE ATMOSPHERE. 363 



give a north-east direction." According to the observations made by Captain Cook, the north-east winds 

 prevail in the Northern Pacific Ocean during the same spring months they do with us, from which facts 

 it appears the cold air from America and the north of Europe flows at that season into the Pacific and 

 Atlantic Oceans. 



2388. Other descriptions of winds may arise from a variety of causes. The atmosphere has been ascer- 

 tained to be composed of air, vapour, and carbonic acid and water ; and as it is well known that these fre- 

 quently change their aerial form, and combine with different substances, and the reverse, consequently 

 partial winds and accumulations must continually occur, which occasion winds of diflerent degrees of 

 violence, continuance, and direction. 



2389. The principal electrical phenomena of the atmosphere are thunder and lightning. 



2390. Thunder is the noise occasioned by the explosion of a flash of lightning passing 

 through the air : or it is that noise which is excited by a sudden explosion of electrical 

 clouds, which are therefore called thunder-clouds. 



2391. The rattling, in the noise of thunder, which makes it seem as if it passed through arches, is pro- 

 bably owing to the sound being excited among clouds hanging over one another, between which the 

 agitated air passes irregularly. 



2392. The explosion, if high in the air and remote from us, will do no mischief, but when near, it may ; 

 and it has, in a thousand instances, destroyed 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, reckoning the distance after the rate of 1142 feet to a second of time, or 3^ 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 to us, and even among us ; and in such cases, the doctor 

 assures us, he has sometimes foretold the mischiefs that happened. 



2393. Season of thunder. Although in this country thunder may happen at any time of the year, yet the 

 months of July and August are those in which it may almost certainly be expected. Its devastations are 

 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 intervals 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 iiitervals. 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 is 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, 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 are naturally a most vehement pressure of the atmosphere, and fric- 

 tion 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 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 the 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 con- 

 siderable resistance in the atmosphere. 



2394. Thunderbolts. 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 gunpowder, 

 and the several chemical fulminating powders, 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. 



2395. Thunder-clouds are those clouds which are in a state fit for producing lightning and thunder. 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. 'I'he lower sur- 

 face 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, which 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 thunder-cloud, their limbs mutually stretch towards one another, 

 and they immediately coalesce into one uniform mass. Sometimes the thunder-cloud will swell, and 

 increase veryfast, without the conjunction of any adscititious clouds; the vapours in the atmosphere 

 forming themselves into clouds whenever 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 will nearly touch the ground. When the eye is under the thunder-cloud, after it is grown 

 large 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. 



2396. Lightning. 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 suificient 

 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 



