Not P but Moby-Dick (61)

[Mike Jing]

>From Chapter 81:

This puts me in mind of fastening to an elephant in a tilbury on a
plain—makes the wheel-spokes fly, boys, when you fasten to him that way;
and there’s danger of being pitched out too, when you strike a hill.
Hurrah! this is the way a fellow feels when he’s going to Davy Jones—all a
rush down an endless inclined plane! Hurrah! this whale carries the
everlasting mail!”

What is this "everlasting mail" about?