These Vapours are demonstratively no other than small Bubbles, or Vesiculæ detached from the Waters by the Power of the Solar, or Subterraneous Heat, or both. Of which see [Book 2. Chap. 5. Note (b).] And being lighter than the Atmosphere, are buoyed up thereby, until they become of an equal Weight therewith, in some of its Regions aloft in the Air, or nearer the Earth; in which those Vapours are formed into Clouds, Rain, Snow, Hail, Lightning, Dew, Mists, and other Meteors.
In this Formation of Meteors the grand Agent is Cold, which commonly, if not always, occupies the superior Regions of the Air; as is manifest from those Mountains which exalt their lofty Tops into the upper and middle Regions, and are always covered with Snow and Ice.
This Cold, if it approaches near the Earth, presently precipitates the Vapours, either in Dews; or if the Vapours more copiously ascend, and soon meet the Cold, they are then condensed into Misting, or else into Showers of small Rain, falling in numerous, thick, small Drops: But if those Vapours are not only copious, but also as heavy as our lower Air it self, (by means their Bladders are thick and fuller of Water,) in this Case they become visible, swim but a little Height above the Earth, and make what we call a Mist or Fog. But if they are a Degree lighter, so as to mount higher, but not any great Height, as also meet not with Cold enough to condense them, nor Wind to dissipate them, they then form an heavy, thick, dark Sky, lasting oftentimes for several Weeks without either Sun or Rain. And in this Case, I have scarce ever known it to Rain, till it hath been first Fair, and then Foul. And Mr. Clarke, (an ingenious Clergyman of Norfolk, who in his Life-time, long before me, took notice of it, and kept a Register of the Weather for thirty Years, which his learned Grandson, Dr. Samuel Clarke put into my Hands, he, I say) saith, he scarce ever observed the Rule to fail in all that Time; only he adds, If the Wind be in some of the easterly Points. But I have observed the same to happen, be the Wind where it will. And from what hath been said, the Case is easily accounted for, viz. whilst the Vapours remain in the same State, the Weather doth so too. And such Weather is generally attended with moderate Warmth, and with little or no Wind to disturb the Vapours, and an heavy Atmosphere to support them, the Barometer being commonly high then. But when the Cold approacheth, and by condensing drives the Vapours into Clouds or Drops, then is way made for the Sun-beams, till the same Vapours, being by further Condensation formed into Rain, fall down in Drops.
The Cold’s approaching the Vapours, and consequently the Alteration of such dark Weather I have beforehand perceived, by some few small Drops of Rain, Hail, or Snow, now and then falling, before any Alteration hath been in the Weather; which I take to be from the Cold meeting some of the straggling Vapours, or the uppermost of them, and condensing them into Drops, before it arrives unto, and exerts it self upon the main Body of Vapours below.
I have more largely than ordinary insisted upon this part of the Weather, partly, as being somewhat out of the way; but chiefly, because it gives Light to many other Phænomena of the Weather. Particularly we may hence discover the Original of Clouds, Rain, Hail and Snow; that they are Vapours carried aloft by the Gravity of the Air, which meeting together so as to make a Fog above, they thereby form a Cloud. If the Cold condenseth them into Drops, they then fall in Rain, if the Cold be not intense enough to freeze them: But if the Cold freezeth them in the Clouds, or in their Fall through the Air, they then become Hail or Snow.
As to Lightning, and other enkindled Vapours, I need say little in this Place, and shall therefore only observe, that they owe also their Rise to Vapours; but such Vapours as are detached from mineral Juices, or at least that are mingled with them, and are fired by Fermentation.
Another Phænomenon resolvable from what hath been said is, why a cold, is always a wet Summer, viz. because the Vapours rising plentifully then, are by the Cold soon collected into Rain. A remarkable Instance of this we had in the Summer of 1708, part of which, especially about the Solstice, was much colder than usually. On June 12, it was so cold, that my Thermometer was near the Point of hoar Frost, and in some Places I heard there was an hoar Frost; and during all the cool Weather of that Month, we had frequent and large Rains, so that the whole Month’s Rain amounted to above two Inches Depth, which is a large Quantity for Upminster, even in the wettest Months. And not only with us at Upminster, but in other Places, particularly at Zurich in Switzerland, they seem to have had as unseasonable Cold and Wet as we. Fuit hic mensis——præter modum humidus, & magno quidem vegetabilibus hominibusque damno. Multum computruit Fœnum, &c. complains the industrious and learned Dr. J. J. Scheuchzer: Of which, and other Particulars, I have given a larger Account in Phil. Trans. Nᵒ. 321.
In which Transaction I have observed farther, that about the Equinoxes we (at Upminster at least) have oftentimes more Rain than at other Seasons. The Reason of which is manifest from what hath been said, viz. in Spring, when the Earth and Waters are loosed from the brumal Constipations, the Vapours arise in great Plenty: And the like they do in Autumn, when the Summer Heats, that both dissipated them, and warmed the superior Regions, are abated; and then the Cold of the superior Regions meeting them, condenseth them into Showers, more plentifully than at other Seasons, when either the Vapours are fewer, or the Cold that is to condense them is less.
The manner how Vapours are precipitated by the Cold, or reduced into Drops, I conceive to be thus: Vapours being, as I said, no other than inflated Vesiculæ of Water; when they meet with a colder Air than what is contained in them, the contained Air is reduced into a less Space, and the watery Shell or Case rendered thicker by that means, so as to become heavier than the Air, by which they are buoyed up, and consequently must needs fall down. Also many of those thickned Vesiculæ run into one, and so form Drops, greater or smaller, according to the Quantity of Vapours collected together.
As to the Rain of different Places, I have in some of our Transactions assigned the Quantities; particularly in the last cited Transaction, I have assigned these, viz. the Depth of the Rain one Year with another, in English Measure, if it was to stagnate on the Earth, would amount unto, at Townely in Lancashire, 42½ Inches; at Upminster in Essex 19¼ Inches; at Zurich in Switzerland 32¼ Inches; at Pisa in Italy 43¼ Inches; at Paris in France 19 Inches; and at Lisle in Flanders 24 Inches.