Read the excerpt from chapter 6 of The Travels of Marco Polo. This, then, I would have you know, is how they are made. They are built of the wood called spruce and of fir. They have one deck; and above this deck, in most ships, are at least sixty cabins, each of which can comfortably accommodate one merchant. They have one steering-oar and four masts. Often they add another two masts, which are hoisted and lowered at pleasure. The entire hull is of double thickness: that is to say, one plank is fastened over the top of another, and this double planking extends all the way round. It is caulked outside and in, and the fastening is done with iron nails. What is the author's purpose for writing this text?