CHAP. XIX. Of Thunder, Lightning, and Coruscation, or Flashes.
I. Lightning and Corus∣cation fol∣low after Thunder. TO the end that the Nature of Lightning and Coruscation may be the better understood, we must premise something concerning THUN∣DER; for usually they are Concomitants of it, and depend on it, as their Cause. Now Thunder is produc'd, when many Clouds rush down upon others that lye directly under them. For it comes to pass sometimes, that the highest Clouds being condensed by Heat, and made more weighty, fall down upon others that lye under them, and cause that Noise which we call a Thunder-Clap. For it cannot be question'd, but that in the Sum∣mer time especially there are abundance of Nitrous, Fat and Sulphureous Exhalations, in the Air. It is certain also, that Snowy Clouds are condensed by Heat, and thereby become more Heavy. It is also as apparent, that all Clouds are not of the same height, but some higher, and others lower; since besides Reason, Sense it self comes to confirm it, which often shews us one Cloud moving over ano∣ther. These things being thus presuppos'd, we proceed to the Explication of Thunder, Lightning, and Coruscation.
II. What is the Cause of Thunder. When any Clouds condensed by Heat, and by this means being become more Heavy, rush down upon an Inferiour Cloud, the intermediate Air be∣ing thereby strongly agitated, is the cause of a dreadful Sound, which is called Thunder. For let us suppose two Clouds A and B, composed of loose and much dilated Snow; let us also suppose, that there is a warmer Air about the upper Cloud A, * 1.1than about the lower Cloud B; it is evident, that for this Reason it must be more and more con∣densed, and consequently become more Heavy: So that its most outward parts falling down first, push others they meet with in their way, and hurrying them along with them, do at last rush down all together, with great Violence, upon the Lower Cloud, with a dreadful rumbling Noise.
III. Heat con∣denseth the Clouds. I have said, that Heat condenseth the Clouds: For tho' Heat do rarefie some Bodies, yet it ordinarily is wont to condense Clouds, as hath before been said of Snow, which consists of the same matter as a Cloud doth. For if Snow be laid in a warm place, it contracts it self into a less Figure, before that any water drop from it, or its weight be diminished. The Clouds therefore being thus condensed, do easily descend, and op∣pose the passage of others that moved upwards, by reason of their Loosness, and thrust down∣wards whatsoever opposeth them in their way. And forasmuch as this cannot be without a violent concussion of the Air, therefore it is accompanied with that terrible Noise, the resounding Air making it still louder.
IV. What Thunder is. Thunder therefore is a Noise excited by the Air, which hath been intercepted between two Clouds, whereof the Upper falling upon the Lower, and which being forc'd to retire thence, by the weight of the middle part of the Upper Cloud, which continues to come down, forceth for it self narrow and irregular Gaps, to get out by, which produceth a dreadful Sound, being much augmented by the Rarefaction caused by the enkindling of the Nitrous Sulphurs betwixt these 2 Clouds.
V. Why Thun∣der is much more frequent in Summer, than in Winter. It Thunders but rarely with us in the Winter time, because then the Heat that is sufficient to dissolve the Upper Clouds, cannot reach so far. But in the Summer time, when the Heat is violent, and more especially when the North-wind makes the Heat to be more close and stifling, Thunder is produc'd; because this wind is wont to carry the heated Air, that is near the Earth, upwards, to those higher Clouds; by which means not only those Superiour Clouds are condensed and rush'd downwards, but also the Lower continue so loose and dilated, and by the Dilatation of the Air that is under them, are so thrust upwards, that they, as it were, meet the others falling down upon them, and stop them there, hindring any part of them from coming down to the Earth.
VI. Why every Clap of Thunder i•• commenly ••ollow'd by 〈…〉〈…〉 of 〈◊〉〈◊〉▪ Thunder is commonly follow'd by a Shower of Rain, and when the Rain is very copious, it is a sign that little more Thunder will follow▪ because when the Force wherewith the Upper Cloud rusheth down from the Lower, is strong enough to make it come down wholly, the Thunder must needs cease: And if it be no•• strong enough, and the Lower Cloud dissolves into Rain, the same thing will still happen; be∣cause the Air that is shut up betwixt the 2 Clouds, can now with ease get forth, because of the small resistance of the Lower Cloud dissolving into Rain.
VII. The 〈◊〉〈◊〉 of Bells to make Thunder cease. The Sound of Bells, and of Canons discharged, makes Thunder to cease; because thereby the Air is strongly agitated, and communicating its motion to the Neighbouring Air, and that again to the Intermediate, till it reach and shake the Snow, whereof the inferiour Cloud consists, occasions its descent. Which Experience makes evident to those, who are us'd to Travel in those Vallies, where the falling down of heaps of Snow from the Hills is apprehended; for fearing lest the Sound of their Voice, by shaking the Air, should bring down the Snow, they are afraid so much as to Cough, to prevent that danger.