We might as well look to Mark Twain's own words for the meaning of "Mark
Twain."  In a footnote to Chapter 8 of "Life On The Mississippi," Twain
defines "Mark Twain" as:

Two fathoms. 'Quarter twain' is two-and-a-quarter fathoms,
thirteen-and-a-half
feet. 'Mark three' is three fathoms.

   "Mark Twain" didn't always mean safe water.  In Chapter 13 of "Life On
The Mississippi" is a very humorous story about a time when the young cub
pilot Sam Clemens was thrown into utter panic upon hearing the words "Mark
Twain."  (I'll attach the text at the end.)

   "Life On The Mississippi" is available on my website at:
   http://cadswes.colorado.edu/twain/

   About the original owner of the name "Mark Twain"--this has been
discussed
before in this forum, but we can look at chapter 50 of "Life On The
Mississippi" for Twain's own description of Captain Isiah Sellers, the
original "Mark Twain" and how and why Clemens appropriated the name.  I'll
attach part of that chapter at the end also.

   I'm paid by the word to write e-mail--hopefully you aren't charged the
same
way to receive it.

   Alan

(parts of chapter 13 and 50 follow)

===========================================
From Chapter 13, Life On The Mississippi
A pilot must have a memory; but there are two higher qualities which he must
also have. He must have good and quick judgment and decision, and a cool,
calm
courage that no peril can shake. Give a man the merest trifle of pluck to
start with, and by the time he has become a pilot he cannot be unmanned by
any
danger a steamboat can get into; but one cannot quite say the same for
judgment. Judgment is a matter of brains, and a man must start with a good
stock of that article or he will never succeed as a pilot.

The growth of courage in the pilot-house is steady all the time, but it does
not reach a high and satisfactory condition until some time after the young
pilot has been 'standing his own watch,' alone and under the staggering
weight
of all the responsibilities connected with the position. When an apprentice
has become pretty thoroughly acquainted with the river, he goes clattering
along so fearlessly with his steamboat, night or day, that he presently
begins
to imagine that it is his courage that animates him; but the first time the
pilot steps out and leaves him to his own devices he finds out it was the
other man's. He discovers that the article has been left out of his own
cargo
altogether. The whole river is bristling with exigencies in a moment; he is
not prepared for them; he does not know how to meet them; all his knowledge
forsakes him; and within fifteen minutes he is as white as a sheet and
scared
almost to death. Therefore pilots wisely train these cubs by various
strategic
tricks to look danger in the face a little more calmly. A favorite way of
theirs is to play a friendly swindle upon the candidate.

Mr. Bixby served me in this fashion once, and for years afterward I used to
blush even in my sleep when I thought of it. I had become a good steersman;
so
good, indeed, that I had all the work to do on our watch, night and day;
Mr. Bixby seldom made a suggestion to me; all he ever did was to take the
wheel on particularly bad nights or in particularly bad crossings, land the
boat when she needed to be landed, play gentleman of leisure nine-tenths of
the watch, and collect the wages. The lower river was about bank-full, and
if
anybody had questioned my ability to run any crossing between Cairo and New
Orleans without help or instruction, I should have felt irreparably hurt.
The
idea of being afraid of any crossing in the lot, in the day-time, was a
thing
too preposterous for contemplation. Well, one matchless summer's day I was
bowling down the bend above island 66, brimful of self-conceit and carrying
my
nose as high as a giraffe's, when Mr. Bixby said--

'I am going below a while. I suppose you know the next crossing?'

This was almost an affront. It was about the plainest and simplest crossing
in
the whole river. One couldn't come to any harm, whether he ran it right or
not; and as for depth, there never had been any bottom there. I knew all
this,
perfectly well.

'Know how to run it? Why, I can run it with my eyes shut.'

'How much water is there in it?'

'Well, that is an odd question. I couldn't get bottom there with a church
steeple.'

'You think so, do you?'

The very tone of the question shook my confidence. That was what Mr. Bixby
was
expecting. He left, without saying anything more. I began to imagine all
sorts
of things. Mr. Bixby, unknown to me, of course, sent somebody down to the
forecastle with some mysterious instructions to the leadsmen, another
messenger was sent to whisper among the officers, and then Mr. Bixby went
into
hiding behind a smoke-stack where he could observe results. Presently the
captain stepped out on the hurricane deck; next the chief mate appeared;
then
a clerk. Every moment or two a straggler was added to my audience; and
before
I got to the head of the island I had fifteen or twenty people assembled
down
there under my nose. I began to wonder what the trouble was. As I started
across, the captain glanced aloft at me and said, with a sham uneasiness in
his voice--

'Where is Mr. Bixby?'

'Gone below, sir.'

But that did the business for me. My imagination began to construct dangers
out of nothing, and they multiplied faster than I could keep the run of
them. All at once I imagined I saw shoal water ahead! The wave of coward
agony
that surged through me then came near dislocating every joint in me. All my
confidence in that crossing vanished. I seized the bell-rope; dropped it,
ashamed; seized it again; dropped it once more; clutched it tremblingly one
again, and pulled it so feebly that I could hardly hear the stroke
myself. Captain and mate sang out instantly, and both together--

'Starboard lead there! and quick about it!'

This was another shock. I began to climb the wheel like a squirrel; but I
would hardly get the boat started to port before I would see new dangers on
that side, and away I would spin to the other; only to find perils
accumulating to starboard, and be crazy to get to port again. Then came the
leadsman's sepulchral cry--

'D-e-e-p four!'

Deep four in a bottomless crossing! The terror of it took my breath away.

'M-a-r-k three!... M-a-r-k three... Quarter less three!... Half twain!'

This was frightful! I seized the bell-ropes and stopped the engines.

'Quarter twain! Quarter twain! Mark twain!'

I was helpless. I did not know what in the world to do. I was quaking from
head to foot, and I could have hung my hat on my eyes, they stuck out so
far.

'Quarter less twain! Nine and a half!'

We were drawing nine! My hands were in a nerveless flutter. I could not ring
a
bell intelligibly with them. I flew to the speaking-tube and shouted to the
engineer--

'Oh, Ben, if you love me, back her! Quick, Ben! Oh, back the immortal soul
out
of her! '

I heard the door close gently. I looked around, and there stood Mr. Bixby,
smiling a bland, sweet smile. Then the audience on the hurricane deck sent
up
a thundergust of humiliating laughter. I saw it all, now, and I felt meaner
than the meanest man in human history. I laid in the lead, set the boat in
her
marks, came ahead on the engines, and said--

'It was a fine trick to play on an orphan, wasn't it? I suppose I'll never
hear the last of how I was ass enough to heave the lead at the head of 66.'

'Well, no, you won't, maybe. In fact I hope you won't; for I want you to
learn
something by that experience. Didn't you know there was no bottom in that
crossing?'

'Yes, sir, I did.'

'Very well, then. You shouldn't have allowed me or anybody else to shake
your
confidence in that knowledge. Try to remember that. And another thing: when
you get into a dangerous place, don't turn coward. That isn't going to help
matters any.'

It was a good enough lesson, but pretty hardly learned. Yet about the
hardest
part of it was that for months I so often had to hear a phrase which I had
conceived a particular distaste for. It was, 'Oh, Ben, if you love me, back
her!'


=======================================================
>From Chapter 50, "Life On The Mississippi" (about Captain Sellers)
(Excerpts)

We had some talk about Captain Isaiah Sellers, now many years dead. He was a
fine man, a high-minded man, and greatly respected both ashore and on the
river. He was very tall, well built, and handsome; and in his old age-- as I
remember him--his hair was as black as an Indian's, and his eye and hand
were
as strong and steady and his nerve and judgment as firm and clear as
anybody's, young or old, among the fraternity of pilots. He was the
patriarch
of the craft; he had been a keelboat pilot before the day of steamboats; and
a
steamboat pilot before any other steamboat pilot, still surviving at the
time
I speak of, had ever turned a wheel. Consequently his brethren held him in
the
sort of awe in which illustrious survivors of a bygone age are always held
by
their associates. He knew how he was regarded, and perhaps this fact added
some trifle of stiffening to his natural dignity, which had been
sufficiently
stiff in its original state.

...

The old gentleman was not of literary turn or capacity, but he used to jot
down brief paragraphs of plain practical information about the river, and
sign
them 'Mark Twain,' and give them to the 'New Orleans Picayune.' They related
to the stage and condition of the river, and were accurate and valuable; and
thus far, they contained no poison. But in speaking of the stage of the
river
to-day, at a given point, the captain was pretty apt to drop in a little
remark about this being the first time he had seen the water so high or so
low
at that particular point for forty-nine years; and now and then he would
mention Island So-and-so, and follow it, in parentheses, with some such
observation as 'disappeared in 1807, if I remember rightly.' In these
antique
interjections lay poison and bitterness for the other old pilots, and they
used to chaff the 'Mark Twain' paragraphs with unsparing mockery.

It so chanced that one of these paragraphs became the text for
my first newspaper article. I burlesqued it broadly, very broadly, stringing
my fantastics out to the extent of eight hundred or a thousand words. I was
a
'cub' at the time. I showed my performance to some pilots, and they eagerly
rushed it into print in the 'New Orleans True Delta.' It was a great pity;
for
it did nobody any worthy service, and it sent a pang deep into a good man's
heart. There was no malice in my rubbish; but it laughed at the captain. It
laughed at a man to whom such a thing was new and strange and dreadful. I
did
not know then, though I do now, that there is no suffering comparable with
that which a private person feels when he is for the first time pilloried in
print.

Captain Sellers did me the honor to profoundly detest me from that day
forth. When I say he did me the honor, I am not using empty words. It was a
very real honor to be in the thoughts of so great a man as Captain Sellers,
and I had wit enough to appreciate it and be proud of it. It was distinction
to be loved by such a man; but it was a much greater distinction to be hated
by him, because he loved scores of people; but he didn't sit up nights to
hate
anybody but me.

He never printed another paragraph while he lived, and he never again signed
'Mark Twain' to anything. At the time that the telegraph brought the news of
his death, I was on the Pacific coast. I was a fresh new journalist, and
needed a nom de guerre; so I confiscated the ancient mariner's discarded
one,
and have done my best to make it remain what it was in his hands-- a sign
and
symbol and warrant that whatever is found in its company may be gambled on
as
being the petrified truth; how I have succeeded, it would not be modest in
me
to say.

 Alan Eliasen