to be rough, and the Waves swell and rise, when at the same time the Air is calm and clear.

"THIS boiling Fermentation of the Sea causes the Vapours to rise, which by the Intenseness of the circumambient Cold are condensed into thick Clouds, and fall down in Storms of Wind and Rain, first upon the Sea, from whence they rose, and then the attractive Power of the Mountain-cold, by a secret Magnetism between Vapour and Cold, attracts the waterish Vapours, intermixt with nitrous Particles, to the high Tops of Mountains and Hills, where they hang hovering in thick Fogs and waterish Mists, until the atmospherical Heat rarefies the nitrous Part of the Fog (which is always uppermost, and appears white and translucent) into brisk Gales of Wind, and the Intenseness of atmospherical Cold having attracted the Vapours into the colder Regions of the Air, where being condensed into Clouds, the Wind breaks, dissipates, and drives them before it, till they fall down in Rain, and water the Surface of the Earth. And this seems to be the Reason why, in Egypt, and those level Countries where they have no Mountains, they have little Wind and less Rain."

XVIII.

CLOUDS. In Summer or Harvest, when the Wind has been South two or three Days, and it grows very hot, and you see Clouds rise with great white Tops like Towers, as if one were upon the Top of another, and joined together with black on the nether Side, there will be Thunder and Rain suddenly.

WE may very easily account for this Observation, because in Fact the Signs here mentioned are no other than Nature's apparatus for a Storm of Thunder and Lightning, which will be perfectly understood by attending a little to the Causes of these Meteors. Lightning is a great flame, very bright, extending every way to a great distance, suddenly darting upwards, there ending, so that it is only momentaneous. The Matter which produces the Fire, is the Oil of Plants, attenuated by the heat of the Day, and raised on high. Then whatever has exhaled from the Earth that is sulphureous or Oily, which is dispersed up and down in the Atmosphere, and is not continuous, is set on Fire by Turns, and the Flame dilates itself as far as the Tract of that Exhalation reaches. Some other Substance pendant and floating in the Air meets with this also, with which it excites an effervescence, takes Fire and flashes along with it. Thunder is another bright Flame, rising on a sudden, moving with great Velocity through the Air, according to any Determination upwards from the Earth horizontally, obliquely, downwards in a right Line, or in several right Lines as it were in serpentine Tracts joined at various Angles, and commonly ending with a loud Noise or Rattling.

IT is observed that it thunders most when the Wind blows from the South, and least when it blows from the East. The great Principle of Thunder is Sulphur, as is evident from the Smell it leaves behind it; but in order to occasion such an Explosion, there must be other Ingredients mixed therewith, especially Nitre, of which the Air is always full, besides other Things, of which it is impossible to give any Account. The Tracts of this Sort of Matter fly about in the Air, and are as it were Lines of Gunpowder, and as in the firing of that Powder, the Fire begins at one End, and pursuing its Aliment proceeds to the other Extremity, and so the whole Mass of Powder is fired; we may from thence account for the Phænomenon of Thunder. For in like Manner those inflamed Tracts which are suspended in the Air, flash from a Flame that runs from one Extreme to the other, wherever the Vein of Nourishment leads it. Hence those Rays of Thunder, which seem to be brandished through the Air, and sometimes to be split in two or more Tracts, and sometimes to return back, at other Times to be projected in Lines that are joined by various Angles, and this only because the Flame meets with Tracts lying in various Situations that cohere one with another. Therefore Thunder seems now to run horizontally, now from above downwards, now upwards from the Earth, for if the Matter of Thunder pressing out of the Earth is enflamed near the Ground, the Flame darting upwards, the Thunder will seem to be projected out of the Earth. If the same Tract be set on Fire at its upper end, the Flame will move downwards, and the Thunder will seem to descend out of the Sky.

HENCE we easily understand how it comes to thunder oftener in one Place than another, but most frequently in those where the Soil produces odoriferous Herbs, and abounds with Sulphur, and where the People are much exposed to the extreme Heat of the Sun. Thunder is less frequent in Places where there are few odoriferous Herbs, very little Sulphur, or where the Climate is watery and moist. For Instance, it thunders very much in Italy and Sicily, and very rarely in Egypt, and the adjacent Countries. If it be demanded how it comes to thunder in the midst of the Ocean? The Answer is easy, because from the Bottom of the Ocean vast Tracts of sulphureous Matter are cast up through the Waters; as it happens to spring Waters in several Places, the Streams of which will take Fire from a lighted Candle. For sulphureous Exhalations bursting out together with the Waters, the fulmineous Matter in the Air is set on Fire when it meets with Exhalations or Vapours with which it can excite a vehement Effervescence. It is very clear from this Account, that the Clouds mentioned at the Top of the twenty-eighth Page are thunder Clouds, or Clouds big with the Materials of Thunder.

XIX.

If two such Clouds arise, one on either Hand, it is Time to make haste to shelter.

AS this Observation is of the same Nature with the former, we shall continue our Remarks. The Reason why it seldom thunders in Winter is, because the exterior Parts of the Earth are so contracted by the cold Snow and Ice, that Sulphur cannot perspire in any great Quantities, but as soon as the Earth begins to be opened by the Sun in the Spring, something expires in the Month of April which takes Fire. But by the greater Heat of the Sun penetrating deeper into the Earth, the Cortex is more opened in May, and now there is a more copious Expiration of the fulminating Matter, and whatever was collected and shut up in Winter, is now released and snatched up in the Air, and thence proceeds the most frequent Thunders in the Month of May, and chiefly when a very hot Day or two has gone before. A less Quantity of the same Matter remains in the upper Cortex of the Earth for the Month of June, but in the mean Time a Stock arises out of the deeper Bowels of the Earth, which is attenuated and prepared, so that by the very fervent Heat of July it is elevated, as it were in heaps, and set on Fire. Hence Thunder is as frequent in July as in May. And the Heat decreasing in the succeeding Months, the Exhalation of the fulminating Matter out of the Earth is more sparing, and thence, also, the thunder is less frequent, till in October, and the other winter months, the earth is bound up with us, and hardly expires any more. Hence we see why it very seldom thunders when the northerly winds blow; for these winds constringe the earth with their cold, and so hinder the fulminating matter from bursting forth; and when they are burst forth and floating in the air, they hinder their effervency. But on the contrary, when the warm and moist south winds blow, which open every thing, the earth likewise is opened, and abundance of fulminating matter is expired and ascends on high, which is there easily inflamed.

AS the flame runs very swiftly, it seems to carry along with it particles, which it could not so easily set on fire, and when any of these particles are drawn together, and heated to a certain degree, they at last take fire, with a sudden and great explosion, and thereby produce what we call a thunder Clap. Now, though this be only a single sound, yet it is often heard in the form of a great