Cavendish here pointed to a chart which he had open on a table before him, together with a pair of compasses and a ruler. "Here we are, you see," resumed the captain, pointing to a spot on the chart. "Here is the island of Cuba, and here"--pointing to a little indentation in the coast-line--"are we in latitude 20 north, and longitude 75 west. "Now you had better remember that bearing, my boy, in case you should ever wish to return here when you get command of a ship of your own. We sailors would call this bearing `20 north, 75 west', leaving out the word degrees. You, Roger, if you will take my advice, my lad, and your friend Harry as well for that matter, will start in as soon as you can and thoroughly perfect yourselves in the science of navigation, for you never can tell, lad, when you may want it; and if you intend--as I suppose you do--to follow the sea as a business you will not be able to do anything without it. "I will tell you all that you want to know about it if you will come to me from time to time when I am not busy; and I have here a book which you may study at your leisure. You will find it very interesting." Roger thanked the commander both for himself and on Harry's account, and promised to take up the study as soon as they were again at sea, as he and his friend would be much too busy for anything of that kind while the vessels were in harbour undergoing repairs and overhaul. He was then on the point of informing the captain of what Martin and he had seen, when Cavendish interrupted him. "Roger," said he, "I have been much exercised in my mind lately as to what shall be done with these Spanish prisoners we have on board. There are nearly three hundred of them, and although many of them are in irons, and all are imprisoned below, I do not altogether like the idea of carrying them about with me too long, for they are a dangerous cargo; and not only that, they are also a great drain upon our stock of provisions. When we leave this island we shall probably be at sea for a very long time, as I intend to cruise in the Caribbean Sea, out of sight of land for the most part, on the lookout for the plate and bullion galleons from Mexico; and when we finally sail from here I wish to take on board as much fresh meat, fruit, and vegetables as I can, to help eke out the ships' stores. Now I do not want to carry about with me nearly three hundred men who will be of no use to me, and who will only help us to eat up our provisions faster than I wish. Moreover, these men are a constant menace to us while they are on board.