Read it through once

'My boy, you've got to remember it. You've got to remember the exact spot and the exact marks the boat lay in when we had the shoalest water, in everyone of the five hundred shoal places between St. Louis and New Orleans; and you mustn't get the shoal soundings and marks of one trip mixed up with the shoal soundings and marks of another, either, for they're not often twice alike. You must keep them separate.'