An entire body of philosophy according to the principles of the famous Renate Des Cartes in three books, (I) the institution ... (II) the history of nature ... (III) a dissertation of the want of sense and knowledge in brute animals ... / written originally in Latin by the learned Anthony Le Grand ; now carefully translated from the last corrections, alterations, and large additions of the author, never yet published ... by Richard Blome.

CHAP. XVII. Of Rain, Dew, Hoar-Frost, and the Cool Evening Air.

I. What Rain is. RAIN is nothing else, but those Drops into which a Vapour elevated above the Earth, is resolved. We have a resemblance hereof in distill'd Liquors; for the rising Vapour, being condensed by the Coolness of the Air, returns to its former state, and runs together into Drops, which at first are insensible, but in their fal∣ling do wn are commonly increased, by others joyning with them; and those commonly are the largest, that fall down from the highest place.

II. Whence the difference of Rain doth pro∣ceed. We must not therefore suppose, as some do, that Rain is produc'd in the Air, as Water, that by being poured from on high, becomes disperst into many Drops; or that it is generated in manner of a Pool, the Water whereof falling from the Clouds is dissolv'd first into greater, and then into less Drops; but only by degrees and by parts: For the Drops of Rain, are formed of the particles of Vapours joyning together, and being joyned fall down to the Earth. Hence Rain hath got several Names, according to the different bigness of the Drops, whereof it consists: For the Rain that falls down in small or midling Drops, is called by the common Name, Rain; if it drops very small, 'tis called a Drizling Rain; if it falls down in great Drops, and those close and thick, 'tis called a Shower; if it comes down in great Drops, and fiercely, 'tis called a Storm.

III. Whence the Greatness of the Drops of Rain proceed The Drops of Rain are great, when the Heat acts upon the upper part of the Cloud; because then the Drops that descend do increase in their passage, by the addition of other Drops they meet with from the top of the Cloud to the bottom Whereas on the contrary, when the action of Heat is only exerted on the lower part of the Cloud, as it commonly happens in the Winter, the Drops are so small, that they are not called Rain, but only a Mizling Dew.

IV. How the Rain falls down from the Clouds. Tho' it be evident enough how Clouds, that consist only of Watry Drops, come down in Rain, viz. either by their own weight, when the Drops by the coalition of many particles, have attained to a sufficient thickness; or, when that the Air that is under them, by its withdrawing, or that which is above, by pressing upon them, inclines them to a descent; or, when many of these Causes chance to concur: But yet it is not so evident to all, how Rain is generated at first out of a condensed Cloud.

V. Rain is sometimes caused by Wind. Now this will be easily made out, if we con∣sider how 2 contrary Winds do gather great abun∣dance of Vapours into the middle Space between them, and thereby cause the production of a Mist or Cloud there. For let us suppose, for Example, a Wind to blow upon the Earth AB, from the West D, and another to blow the contrary way, from the East C, so as that both these Winds come to stop one another about the Space FGP; * 1.1it must necessarily follow, that the Vapours inter∣cepted between the wind D, and the wind C, being driven together, must be there condensed, and make a kind of confus'd Mass. In like manner the Wind that drives against this Cloud, yet not with that Violence as to carry it along with it, must drive the parts of it closer together; by which means many of the watry Particles, which before were insensible, and separate from each other, are by the force of the wind driven together, and so become great Drops, which by their own weight fall down. For as in your common Stills, Drops are not formed as soon as the Vapour is got to the Head of them, for that other Vapours ascending to the top of the Still, must increase their bulk. So neither can Rain be generated, as soon as the Vapours are got up to the Region of the Clouds; but it is necessary, that those Particles of which the Vapours do consist, be by the Wind driven together, and more closely joyned; and that being thus become more heavy, so as to be able to vanquish the resistance of the Air, they fall down to the Earth.

VI. The more common Cause of Rain is Heat. Notwithstanding that by this way Clouds are often turned to Rain, yet is the Heat of the Air a more frequent and efficacious Cause of it. For this heated Air clinging to the Clouds, makes the subtil Snow, whereof they consist, to dissolve, and separates it into many little Flocks, which have then force enough to drive away the Air that opposeth their descent; and so by the action of the Heat they meet with in their passage, they are quite melted, and turn'd into Drops: Which sometimes are large, when the Cloud is thick and close, and comes down by the pressure only of the Air that is above it; because the upper Drops meet with others in their way downward, that increase them and make them larger, as before hath been mentiond.

VII. W••y the Drops of Rain are Round. The Drops of Rain, as they fall down, are made round: For a Liquid Body then becomes Round, when all its Parts are by an equal force driven towards some Common Center; for all the Parts of that Circumference being moved by a like force, are at an equal distance from the Center. Now Rain is a Liquid Body, all whose Parts, when the Air is calm and free from Winds, are comprest by an equal Virtue to some Common Center. For the Calm Air doth on all sides equally compress these Drops, there being no Reason to be given why it should press them one way, more than another.

VIII. How the Dew is formed. Having premised thus much concerning Rain, it will not be difficult to conceive how the DEW is formed, if we observe that when the Air is clear and not ruffled with Winds; many loose and thin little Bodies are disperst abroad, especially in the Summer time, when Vapours and Exhala∣tions are copiously elevated from the Earth:

Which small Bodies flying in the Air, in the form of Vapours, when the cold of the Night comes on, do lose their agitation, and many of them clinging together, produce insensible Drops, which falling down upon the Dry ground are suck'd up; but falling upon the Leaves of Trees, or the Spires of Grass, are there gather'd into greater Drops. This chiefly happens in the Night, or before Day-break; because then the Earth is most cold, as being turned from the Sun. Whence it follows, that the Dew is nothing else, but certain Vapours, that have by Heat been elevated in the Day time; and which being condensed by the Coldness of the Night, fall down in small insensible Drops upon the Leaves of Plants; where many of them joyning together, they become sensible.

IX. What Hoar-frost is, and ••ow it is distinguisht from Dew. HOAR-FROST is generated, when the Vapours, that are near the Earth, are congealed by the coldness of the Night: Which only happens in the VVinter, when Cold predominates. For tho' when the Coldness of the Night is yet tempe∣rate, it be sufficient to compress the watry Particles up and down disperst through the Air, into sen∣sible Drops, yet not to congeal them: But the Cold of VVinter-Nights being more intense, con∣geals the said Drops in the form of Snow or Hail. So that it appears, that the difference between Dew and Hoar-Frost is this, That Mists do turn to Dew, if they consist of Drops of water; but into Hoar-Frost, when they consist of Vapours that are frozen before, or are congealed in their passage down to the Earth.

X Why Hoar-Frostt makes Coleworts tender. It is observed, that the Hoar-Frost makes some Herbs, and particularly Coleworts, tender; the Reason whereof is, because the Cold acting upon them, doth very much condense them, and causeth many humid Particles to congeal together; which afterwards are easily resolved again by any super∣vening Heat, and by this means their fibres be∣come in a manner clean dissolved, which is the cause of the Tenderness of those Plants. The like also happens in Flesh, which having been frozen, doth thereby grow very Tender.

XI. What the Cool Even∣ing Breez is, which is conse∣quent upon clear Sum∣mer days. As to that COOL EVENING-AIR, or Breez, which is consequent to fair Summer-days, we find that it commonly happens when the Heat of the Sun hath been very intense all the Day long, and there∣by greatly agitated the Air; by which means the surface of the Earth also is moved to send forth abundance of subtil Exhalations upwards, to the Region of Vapours; and because these Exhalations do more easily lose their agitation, than the Vapours do, as soon as they are deprived of the Sun's Heat, the former of these fall down. And herein doth chiefly consist the Nature of this Cool Air, or Breez; which according to the difference of places, is endued with different Qualities: For it is pro∣bable, that those Particles which exhale from places Infected, or poisonous Plants, are far more hurtful, than simple Vapours that arise out of the Earth.

XII. When the Cool of the Evening is hurtful. The Cool Air, or Breez of the Evening, is never more hurtful, than when the Day fore-going hath been very clear, and the Heat of the Sun mode∣rate: For the immoderate Heat of the Day doth much extenuate these Vapours, and elevates them on high, so that they cannot so readily be con∣densed by the Cold of the Evening. It is also very hurtful after that the Sun it set, because the more heavy Exhalations, which have less of Vapour mix'd with them, as soon as they are left destitute of the Sun-beams, do immediately come down∣wards.

XIII. If when the Air is duskish, the Sun shines in the East, it is a sign of Rain. The Air cover'd with Clouds, is a sign of Rain, if the Sun shine in the Morning; for this is an evident Token, that there are no other Clouds in the Neighbourhood of our Air, towards the East, which can hinder the heat of the Sun from con∣densing those that hang over our Heads, or from raising new Vapours, whence they might be in∣creased, out of the Earth. But this Cause only takes place in the Morning; for if it do not Rain before Noon, it cannot afford any signification of what is to happen in the Evening. Neither are those signs, which are taken from the Croaking of Frogs, or Crows, the Praesentions of Beasts, the Redness of the Sun, the Paleness of the Moon, and other such like, any whit more certain, but generally very fallacious: And he that would take the pains to observe them diligently, will frequently find the contrary, to what is commonly expected from such like Presages.

XIV. When no Dew falls in the Morning, it is a sign of Rain. Yet we find, that when no Dew falls in the Night, or the Mist is carried upwards, leaving the Earth destitute of all Moisture, this is a sign of Rain; because this seldom happens, but when the Earth, having not been sufficiently cooled in the Night-season; or been agitated by too much heat in the Day time, doth send forth abundance of Vapours, which driving the Mist upwards, make its Parts to run together; by which means they become so great and weighty, as that they are forced to come down in Rain.

Notes

• * 1.1

  Figure 53.