Chapter 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 murmuring noise of a long continuance; sometimes for thirty or forty seconds, because of its various repercussions by the clouds and terrestrial obstacles. Hence it is, that in vales, which are surrounded by mountains of a different Height, there is a terrible and long continued Bellowing of thunder Claps. Whereas for one Explosion it has been observed that there is but one Clap. Yet however if the Flame set Fire to two, three, or more fulmineous Tracts, each of them at last will end in a Clap, and thus several Sounds may be heard together, or quickly succeeding one another.