BOOK II. OF THE ATMOSPHERE. 263 



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 tee-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 tern- 

 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 

 resistance in the atmosphere. 



12(it>. 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 

 commouly 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. 



1267. 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, swell- 

 ing 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 

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

 tremity 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 commonly 

 hails. 



1268. 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. 



1269. 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- 

 mal life, deprives magnets of their virtue, or reverses their poles ; and all these are well-known properties 



1270. With regard to places of safety in times of thunder and lightning. Dr. Franklin's advice is to sit in 

 the middle of a room, provided it be not under a metal lustre suspended by a chain, sitting on one chair, 

 and laying the feet on another. It is still better, he says, to bring two or three mattresses or beds into the 

 middle of the room, and folding them double, to place the chairs upon them ; for as they are not so good 

 conductors as the walls the lightning will not be so likely to pass through them. But the safest place of all 

 is in a hammock hung by silken cords, at an equal distance from all the sides of the room. Dr. Priestley 

 observes, that the place of most perfect safety must be the cellar, and especially the middle of it ; for when 

 a person is lower than the surface of the earth, the lightning must strike it before it can possibly reaph him. 

 In the fields, the place of safety is within a few yards of a tree, but not quite near it. Beccaria cautions 

 persons not always to trust too much to the neighborhood of a higher or better conductor than their own 

 body, since he has repeatedly found that the lightning by no means descends in one undivided track, but 

 that bodies of various kinds conduct their share of it at the same time, in proportion to their quantity and 

 conducting power. < 



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