and know them so accurately that you can instantly
name the one you are abreast of when you are set down at random in
that street in the middle of an inky black night, you will then have a
tolerable notion of the amount and the exactness of a pilot's knowledge
who carries the Mississippi River in his head. And then if you will
go on until you know every street crossing, the character, size, and
position of the crossing-stones, and the varying depth of mud in each of
those numberless places, you will have some idea of what the pilot must
know in order to keep a Mississippi steamer out of trouble. Next, if you
will take half of the signs in that long street, and CHANGE THEIR PLACES
once a month, and still manage to know their new positions accurately on
dark nights, and keep up with these repeated changes without making any
mistakes, you will understand what is required of a pilot's peerless
memory by the fickle Mississippi.

I think a pilot's memory is about the most wonderful thing in the world.
To know the Old and New Testaments by heart, and be able to recite them
glibly, forward or backward, or begin at random anywhere in the book
and recite both ways and never trip or make a mistake, is no extravagant
mass of knowledge, and no marvelous facility, compared to a pilot's
massed knowledge of the Mississippi and his marvelous facility in the
handling of it. I make this comparison deliberately, and believe I am
not expanding the truth when I do it. Many will think my figure too
strong, but pilots will not.

And how easily and comfortably the pilot's memory does its work; how
placidly effortless is its way; how UNCONSCIOUSLY it lays up its vast
stores, hour by hour, day by day, and never loses or mislays a single
valuable package of them all! Take an instance. Let a leadsman cry,
'Half twain! half twain! half twain! half twain! half twain!' until
it become as monotonous as the ticking of a clock; let conversation be
going on all the time, and the pilot be doing his share of the talking,
and no longer consciously listening to the leadsman; and in the midst
of this endless string of half twains let a single 'quarter twain!' be
interjected, without emphasis, and then the half twain cry go on again,
just as before: two or three weeks later that pilot can describe with
precision the boat's position in the river when that quarter twain
was uttered, and give you such a lot of head-marks, stern-marks, and
side-marks to guide you