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, that you ought to be able to take the boat there and put her in that same spot again yourself! The cry of 'quarter twain' did not really take his mind from his talk, but his trained faculties instantly photographed the bearings, noted the change of depth, and laid up the important details for future reference without requiring any assistance from him in the matter.

If you were walking and talking with a friend, and another friend at your side kept up a monotonous repetition of the vowel sound A, for a couple of blocks, and then in the midst interjected an R, thus, A, A, A, A, A, R, A, A, A, etc., and gave the R no emphasis, you would not be able to state, two or three weeks afterward, that the R had been put in, nor be able to tell what objects you were passing at the moment it was done.

But you could if your memory had been patiently and laboriously trained to do that sort of thing mechanically.
Give a man a tolerably fair memory to start with, and piloting will develop it into a very colossus of capability.

But ONLY IN THE MATTERS IT IS DAILY DRILLED IN.

A time would come when the man's faculties could not help noticing landmarks and soundings, and his memory could not help holding on to them with the grip of a vise; but if you asked that same man at noon what he had had for breakfast, it would be ten chances to one that he could not tell you.