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 Phaenomenon 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.