556 SCIENCE OF AGRICULTURE. Part II. 



2324. The explosion. If high In the air and remote from us, will do no mlschlrf; but when near, it may, 

 and 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, estimating the distance after the rate of 1142 feet for 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. 



2325. 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 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 

 neighborhood, 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, 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 vapors 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 tem- 

 porary 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 resist- 

 ance in the atmosphere. 



2326. 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 efffects, suppose to be a hard body, 

 and even a stone. But that we need not to 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 elasticity, 

 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. 



2327. 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. The 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, sweUing, 

 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 appear- 

 ance 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 

 very fast, without the conjunction of any adscititious clouds ; the vapors 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 down- 

 ward ; and sometimes one whole side of the cloud will have an inclination to the earth, and the extre- 

 mity of it 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 commonjy 

 hails. " 



2328. 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 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 shows a clear sky. Those 

 thunder-clouds are sometimes in a positive as well as a negative state of electricity. The 

 electricity continues longer of the same kind, in proportion as the thunder-cloud is sim- 

 ple and uniform in its direction ; but when the lightning changes its place, there com- 

 monly happens a change in the electricity of the apparatus over which the clouds passed. 

 It changes suddenly after a very violent flash of lightning ; but gradually when the 

 lightning is moderate, and the progress of the thunder-cloud slow. 



2329. Lightning is an electrical explosion or phenomenon. Flashes of lightning are usually seen crooked , 

 and waving in the air. They strike the highest and most pointed objects in preference to others, as hills, 

 trees, spires, masts of ships, &c. ; so all pointed conductors receive and throw off the electric fluid more 

 readily than those that are terminated by flat surfaces. Lightning is observed to take and follow the 

 readiest and best conductor ; and the same is the case with electricity in the discharge of the Leyden 

 phial ; from whence it is inferred, that in a thunder-storm it would be safer to have one's clothes wet than 

 dry. Lightning burns, dissolves metals, rends some bodies, sometimes strikes persons blind, destroys ani- 



