Reflecting on the circumstances which had accompanied his entrance to the marshes, the migration of the birds seemed almost the most singular.  They were evidently flying from some apprehended danger, and that most probably would be in the air.  The gale at that time, however, was blowing in a direction which would appear to ensure safety to them; into, and not out of, the poisonous marshes.  Did they, then, foresee that it would change?  Did they expect it to veer like a cyclone and presently blow east with the same vigour as it then blew west?  That would carry the vapour from the inky waters out over the sweet Lake, and might even cause the foul water itself to temporarily encroach on the sweet.  The more he thought of it, the more he felt convinced that this was the explanation; and, as a fact, the wind, after dropping, did arise again and blow from the east, though, as it happened, not with nearly the same strength.  It fell, too, before long, fortunately for him.  Clearly the birds had anticipated a cyclone, and that the wind turning would carry the gases out upon them to their destruction.  They had therefore hurried away, and the fishes had done the same.

The velocity of the gale which had carried him into the black waters had proved his safety, by driving before it the thicker and most poisonous portion of the vapour, compressing it towards the east, so that he had entered the dreaded precincts under favourable conditions.  When it dropped, while he was on the black island, he soon began to feel the effect of the gases rising imperceptibly from the soil, and had he not had the good fortune to escape so soon, no doubt he would have fallen a victim.  He could not congratulate himself sufficiently upon his good fortune.  The other circumstances appeared to be due to the decay of the ancient city, to the decomposition of accumulated matter, to phosphorescence and gaseous exhalations.  The black rocks that crumbled at a touch were doubtless the remains of ancient buildings saturated with the dark water and vapours.  Inland similar remains were white, and resembled salt.

But the great explosions which occurred as he was leaving, and which sent heavy rollers after him, were not easily understood, till he remembered that in Sylvester’s “Book of Natural Things” it was related that “the ancient city had been undermined with vast conduits, sewers, and tunnels, and that these communicated with the sea”.  It had been much disputed whether the sea did or did not still send its tides up to the site of the old quays.  Felix now thought that the explosions were due to compressed air, or more probably to gases met with by the ascending tide.

CHAPTER XXV

THE SHEPHERDS