4.  “And let them be for signs, and for seasons, and for days and years.” Gen. i. 14.  The signs which the luminaries give are necessary to human life.  In fact what useful observations will long experience make us discover, if we ask without undue curiosity!  What signs of rain, of drought, or of the rising of the wind, partial or general, violent or moderate!  Our Lord indicates to us one of the signs given by the sun when He says, “It will be foul weather to-day; for the sky is red and lowering.” St. Matt. xvi. 3.  In fact, when the sun rises through a fog, its rays are darkened, but the disc appears burning like a coal and of a bloody red colour.  It is the thickness of the air which causes this appearance; as the rays of the sun do not disperse such amassed and condensed air, it cannot certainly be retained by the waves of vapour which exhale from the earth, and it will cause from superabundance of moisture a storm in the countries over which it accumulates.  In the same way, when the moon is surrounded with moisture, or when the sun is encircled with what is called a halo, it is the sign of heavy rain or of a violent storm; again, in the same way, if mock suns accompany the sun in its course they foretell certain celestial phenomena.  Finally, those straight lines, like the colours of the rainbow, which are seen on the clouds, announce rain, extraordinary tempests, or, in one word, a complete change in the weather.