He was "mud" clerk.

Mud clerks received no salary, but they were in the line of promotion.

They could become, presently, third clerk and second clerk, then chief clerk--that is to say, purser.

The dream begins when Henry had been mud clerk about three months.

We were lying in port at St.Louis.Pilots and steersmen had nothing to do during the three days that the boat lay in port in St.Louis and New Orleans, but the mud clerk had to begin his labors at dawn and continue them into the night, by the light of pine-knot torches.