And then he perceived that, very slowly, the circular top of the
cylinder was rotating on its body.  It was such a gradual movement
that he discovered it only through noticing that a black mark that had
been near him five minutes ago was now at the other side of the
circumference.  Even then he scarcely understood what this indicated,
until he heard a muffled grating sound and saw the black mark jerk
forward an inch or so.  Then the thing came upon him in a flash.  The
cylinder was artificial--hollow--with an end that screwed out!
Something within the cylinder was unscrewing the top!