( 25 ; ) 
The daik Arcular Segment next the Horizon ap- 
pearing like a heavy black Cloud, or Mill, is form'd 
out of the denfeft and fpecifically heavieft Parts of the 
Zodiacal Matter , which in their Defcent muft fink 
deepeft into the Earth’s Atmofphere, and are leaft in- 
flammable in their Nature, while the rarer and light- 
er Parts, which are more inflammable and luminous, 
if not adually inflam’d, form the Arc or Arcs that 
lie above the dark Segment. The ingenious Author 
fpeaks of a Fort de /’ incendie, a Place where the 
Zodiacal Matter collefted together, and moving or 
palling thro’ it, is actually turn’d into Flame. Thus 
long Trains of defcending Zodiacal Matter arriving 
in their Defcent at this Place, being kindled, or at 
leaft reflecting the Light of that Incendium , produce 
the feveral Columns or Streams of Light that appear 
above, or behind the obfcure circular Bafe, or lu- 
minous Arches. 
The Breaks that are fometimes vifible in thefe 
Arches, are occafioned by the Defcent and Paffage of 
feveral difcontinued Trains and Flakes of the denfer 
and leaft inflammable Parts of the Zodiacal Matter , 
between the Eye of the Spectator and the luminous 
Arch. 
The various Colours arife from a Separation of 
the Rays of Light from each other, either by a fort 
of Filtration in palling thro’ Mediums of different 
Denfities, or by the Divergence of the differently 
refrangible and coloured Rays (or rather from the dif- 
ferent Celerities of thofe Rays, as the Author fays he 
has explained more at large in anotherPlace) after the 
manner that the Colours are formed in Clouds near the 
Horizon about the riling or letting Sun. 
To 
