THONSCHIEFER, in mineralogy, slate, is divided into three sub-species : 1. The common argillaceous schistus, which is composed of silex, alumina, oxide of iron, and proportions of carbonated lime and magnesia : it is used for covering houses, and the straight-foliated bluish-grey va rieties are employed as writing slates : the softer and more compact varieties are made into slate pencils. See ScRts TO'S, also SLATE. 2. Hone slate, called by Kirwan novaculite : its colour is a greenish-grey, or smoke grey, passing to olive and mountain-green. It occurs in mass, and has a glimmering lustre : its fracture in the great is slaty ; 111 the small, 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 pro ducing 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 pres sure of the atmosphere, and friction of its parts against one another ; a calm en sues, 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 al most 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 pe riod, 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 west ward. Yet in either case the motion is so slow, that the most superficial ob servers cannot help taking notice of a considerable resistance in the atmo sphere.
When lightning acts with extraordina ry violence, and breaks or shatters any thing, it is called a thunderbolt, which the vulgar, to fit it for such effects, sup pose to be a hard body, and even a stone. But that we need not have recourse to a hard solid body, to account fbr the effects commonly attributed to the thunderbolt, will he evident to any one, who considers those of gunpowder, and the several che micalfulminatingpowders, but more espe, cially the astonishing powers of electri city, 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 pro duced by lightning, we shall be at no loss to account for the extraordinary ope rations vulgarly ascribed to thunderbolts, As stones and bricks struck by lightning are often found in a vitrified state, we may reasonably suppose, with Becearia, that some stones in the earth, having been struck in this manner, gave occasion to the vulgar opinion of the thunderbolt.
Thunder-clouds are those clouds which are in a state' fit for producing lightning and thunder. From Beccaria's exact and circumstantial account of the external appearances of thunderclouds, the fol low hig particulars are extracted. The first appearance of a thunder storm, which usually happens when there is little or no wind, is one dense cloud, or more, in creasing very fast in size, and rising into the higher regions of the air. The low
er surface is black, and nearly level ; but the upper finely arched, and well de fined. 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, that are motionless, and of odd whimsical shapes. All these, upon the appearance of the thunder-cloud, draw towards it, and be come more uniform in their shapes as they approach ; till, coming very near the thunder-cloud, their limbs mutually stretch towarcls one another, and they immediately coalesce into one uniform mass. These he calls adscititious clouds, from their coming in to enlarge the size of the thunder-cloud. But sometimes the thunder-cloud will swell, and increase very fast, without the conjunction of any adscititious clouds ; the vapours in the atmosphere forming themselves into clouds wherever 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 rag ged, particular parts being detached to wards the earth, but still connected with the rest. Sometimes the lower surface swells into various large protuberances bending uniformly downward ; and some times one whole side of the cloud will have an inclination to the earth, and the extremity of it nearly touch the ground. When the eye is under the thunder-cloud, after it is grown larger, 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 com monly hails. 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 an other, and often to illuminate its whole mass. When the cloud has acquired a sufficient extent, the lightning strikes be tween the cloud and the earth, in two op posite places, the path of the lightning lying through the whole body of the cloud and its branches. The longer this lightning continues, the fess dense does the cloud become, and the less dark its appearance ; till at length it breaks in different places, and shows a clear sky. These thunder-clouds were sometimes in a positive as well as a negative state of electricity. The electricity continued longer of the same kind, in proportion as the thunder-cloud was simple and uni form in its direction : but when the light ning changed its place, there commonly happened a change in the electricity of the apparatus over which the clouds passed. It would change suddenly after a very violent flash of lightning; but the change would be gradual when the light ning was moderate, and the progress of the thunder-cloud slow. See Priestley's History of Electricity.