Not P but Moby-Dick (26)

[Mike Jing]

>From Chapter 44:

Though, in these cases, the direction taken by any one whale be straight as
a surveyor’s parallel, and though the line of advance be strictly confined
to its own unavoidable, straight wake, yet the arbitrary vein in which at
these times he is said to swim, generally embraces some few miles in width
(more or less, as the vein is presumed to expand or contract); but never
exceeds the visual sweep from the whale-ship’s mast-heads, when
circumspectly gliding along this magic zone.

What's doing the gliding in "when circumspectly gliding along this magic
zone"? i.e. Whats' the subject of the verb "gliding"?