[m] By reflecting the Light of the heavenly Bodies to us, I mean that Whiteness or Lightness which is in the Air in the Day-time, caused by the Rays of Light striking upon the Particles of the Atmosphere, as well as upon the Clouds above, and the other Objects beneath upon the Earth. To the same Cause also we owe the Twilight, viz. to the Sun-beams touching the uppermost Particles of our Atmosphere, which they do when the Sun is about eighteen Degrees beneath the Horizon. And as the Beams reach more and more of the airy Particles, so Darkness goes off, and Day light comes on and encreaseth. For an Exemplification of this, the Experiment may serve of transmitting a few Rays of the Sun through a small Hole into a dark Room: By which means the Rays which meet with Dust, and other Particles flying in the Air, are render’d visible; or (which amounts to the same) those swimming small Bodies are rendered visible, by their reflecting the Light of the Sun-beams to the Eye, which, without such Reflection, would it self be invisible.
The Azure Colour of the Sky Sir Isaac Newton attributes to Vapours beginning to condense, and that are not able to reflect the other Colours. V. Optic. l. 2. Par. 3. Prop. 7.
[n] By the Refractive Power of the Air, the Sun, and the other heavenly Bodies seem higher than really they are, especially near the Horizon. What the Refractions amount unto, what Variations they have, and what Alterations in time they cause, may be briefly seen in a little Book called, The Artificial Clock-Maker, Chap. 11.
Although this inflective Quality of the Air be a great Incumbrance and Confusion of Astronomical Observations;——yet it is not without some considerable Benefit to Navigation; and indeed in some Cases, the Benefit thereby obtained is much greater than would be the Benefit of having the Ray proceed in an exact straight Line. [Then he mentions the Benefit hereof to the Polar Parts of the World.] But this by the by (saith he.) The great Advantage I consider therein, is the first Discovery of Land upon the Sea; for by means hereof, the tops of Hills and Lands are raised up into the Air, so as to be discoverable several Leagues farther off on the Sea than they would be, were there no such Refraction, which is of great Benefit to Navigation for steering their Course in the Night, when they approach near Land; and likewise for directing them in the Day-time, much more certainly than the most exact Celestial Observations could do by the Help of an uninflected Ray, especially in such Places as they have no Soundings. [Then he proposes a Method to find by these means the Distance of Objects at Sea.] V. Dr. Hook’s Post. Works. Lect. of Navig. p. 466.
[o] Cum Belgæ in novâ Zemblâ hybernarent, Sol illis apparuit 16 diebus citiùs, quàm revera in Horizonte existeret, hoc est, cùm adhuc infra Horizontem depressus esset quatuor circiter gradibus, & quidem aere sereno. Varen. Geog. c. 19. Prop. 22.
[These Hollanders] found, that the Night in that place shortened no less than a whole Month; which must needs be a very great Comfort to all such Places as live very far towards the North and South Poles, where length of Night, and want of seeing the Sun, cannot chuse but be very tedious and irksome. Hook Ibid.
[By means of the Refractions] we found the Sun to rise twenty Minutes before it should; and in the Evening to remain above the Horizon twenty Minutes (or thereabouts) longer than it should. Captain James’s Journ. in Boyl of Cold. Tit. 18. p. 190.
[p] Aer—in Nubes cogitur: humoremque colligens terram auget imbribus: tum effluens huc & illuc, ventos efficit. Idem annuas frigorum & calorum facit varietates: idemque & volatus Alitum sustinet, & spiritu ductus alit & sustentas animantes. Cic. de Nat. Deor. l. 2. c. 39.