What does the count teach jacopo?

From the text:

Then in the long days on board ship, when the vessel, gliding on with security over the azure sea, required no care but the hand of the helmsman, thanks to the favorable winds that swelled her sails, Edmond, with a chart in his hand, became the instructor of Jacopo, as the poor Abbe Faria had been his tutor. He pointed out to him the bearings of the coast, explained to him the variations of the compass, and taught him to read in that vast book opened over our heads which they call heaven, and where God writes in azure with letters of diamonds. And when Jacopo inquired of him, "What is the use of teaching all these things to a poor sailor like me?" Edmond replied, "Who knows? You may one day be the captain of a vessel. Your fellow-countryman, Bonaparte, became emperor." We had forgotten to say that Jacopo was a Corsican.