"Variation and deviation are used in setting courses and estimating dead reckoning." All of which was Greek to Simon Nishikanta, who would promptly take the Ancient Mariner's side of the discussion.
But the Ancient Mariner was fair-minded.

What advantage he gave the Jew one moment, he balanced the next moment with an advantage to the skipper.
"It's a pity," he would suggest to Captain Doane, "that you have only one chronometer.

The entire fault may be with the chronometer.

Why did you sail with only one chronometer ?" "But I _was_ willing for two," the Jew would defend.

"You know that, Grimshaw ?" The wheat-farmer would nod reluctantly and Captain would snap: "But not for three chronometers." "But if two was no better than one, as you said so yourself and as Grimshaw will bear witness, then three was no better than two except for an expense." "But if you only have two chronometers, how can you tell which has gone wrong ?" Captain Doane would demand.
"Search me," would come the pawnbroker's retort, accompanied by an incredulous shrug of the shoulders.