V.
If small Clouds increase——Much Rain.
THIS and the following Observation cannot well be understood, without giving some Account of Clouds in general. The Atmosphere is supposed to extend itself about five Miles round this Globe of Earth, and within that Space move all kind of Vapours exhaled by the Sun's Force, or protruded by the subterraneous Heat. The ascending of these Vapours into the Air, depends upon many things, and therefore as different as its Causes; for instance, their ascent depends in the first place on the degree of Heat with which they are drawn up or forced out; next upon the Lightness of the Vapours themselves; thirdly, on the Density or Rarity of the Air through which they pass; and lastly, on the Force and Direction of the Winds, which they encounter in their Passage.
ACCORDING to the Nature of these Vapours, and the Circumstance attending their Passage, they appear to us differently below. For if they be extremely subtile they mount very high, and there, according to the Sentiment of Sir Isaac Newton, form by Refraction the Azure, or blue Colour, that over-spreads the Sky in serene Weather. Clouds, while they remain visible, do not rise above the Height of a Mile; and we always observe, that the highest are of a very light Colour, and hardly seen. If, therefore, small Clouds increase, it shews, that the Disposition of the Air is such, as that these Clouds cannot rise therein, either from their own Weight, the want of a protrusive Force, or from the falling of the Wind, which in cloudy Weather is always a Sign of Rain.
VI.
If large Clouds decrease——Fair Weather.
THE same kind of Reasoning accounts very clearly for this Prognostick, since it shews, that the Vapours are either exhaled by the Sun's Heat, or are driven off by Winds, and so resolved into smaller Clouds, capable of ascending higher in the Atmosphere; all which are Circumstances that secure us from Rain, and afford us a certainty of fair Weather.
IT is, however, to be observed, that large black Clouds are frequently, in a Summer Evening, melted into Dews; and this much more frequently happens in the Autumn, because the Evenings are then cooler, and the Vapours more easily condensed for that Reason. In all Observations of this Sort, there is a great degree of Prudence and good Sense required to apply them, and hence it very frequently happens that such Observations are condemned as treacherous and abusive, merely because those who would employ them want the Sagacity which is requisite to understand them clearly.
VII.
MISTS. If they rise in low Ground and soon vanish. } Fair Weather.