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The diurnal arc is described during the daytime, the nocturnal arc during the night.

Indeed, the engineer inclined the rod towards the south or away from the sun, as it must not be forgotten that the colonists of Lincoln Island, being in the Southern Hemisphere, saw the orb of day describe his diurnal arc above the northern horizon.

By means of inclined mirrors attached to the instruments, the sun ought apparently to go back to the horizon itself; and the moment when it seemed to touch it with the lower side of its disc would be precisely that at which it would occupy the highest point of the diurnal arc, and consequently the exact moment when it would pass the meridian-in other words, it would be noon at the place where the observation was taken.

No trace remained of the solid ice-mountains of the winter but these moving relies, which the solar rays could scarcely dissolve, and which seemed, as it were, to protest against the brilliant but not very powerful Polar sun, now describing a diurnal arc of considerable length.

On this day, the sun which had risen at twenty minutes to six, set at forty minutes past five, having traced its diurnal arc for eleven hours above the horizon.