saac 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.

THIS is a sure Sign and very well expressed, that is, clearly, and, in
few Words, which is the Excellency of such Aphorisms. In order to be
convinced of its good Sense and Certainty, we must consider a little
what _Mists_ are, whence they rise, and what becomes of them.

MISTS are, strictly speaking, uncompacted Exhalations, which while they
fleet near the Earth are styled _Mists_, but when they ascend into the
Air, are called _Clouds_. If therefore, rising out of low Ground, they
are driven along the Plain, and are soon lost to the Sight, it must
arise from some of these Causes. That there is an Air abroad sufficient
to divide and resolve them, or the Heat of the Sun has been strong
enough to exhale them, that is, to rarify them, so as to render them
lighter than the Air through which they were to pass. Whichever way
this happens the Maxi