He discovers that the article has been left out of his own cargo altogether.

[shootingstar]

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.
(Life On The Mississippi by MarkTwain, chapter 13, A Pilot's Needs)

What does the underlined sentence mean; chiefly, what does "article" and "cargo" mean in this context? "Article" could mean something like "product" or "commodity" I fancy, figuratively used though - a mental or intellectual skill. Thus, I take "cargo" figuratively to mean something like an assortment of mental goods a person commands - Mark Twain is drawing on steamboatmen's parlance here in my opinion.

 

[SoothingDave]

When the young pilot first sets out on his own, in complete control of the boat, he discovers that his courage is missing. It is not an item (article) that was brought on board.

 

[shootingstar]

However, Mark Twain says "that the article has been left out of his own cargo altogether". Mark Twain does not refer to the ship's cargo, actually. He refers to a person's cargo, right? In the sentence above he also says "but the first time the pilot steps out and leaves him to his own devices he finds out it was the other man's". In the sentence in question Mark Twain merely uses the rivermen's terminology or parlance and transfers it to mental terms in my opinion.

 

[5jj]

Yes.