Chapter 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.