“A piece of wood was thrown from the ship into the sea, with a thin rope attached to it. As the ship sailed on, the wood remained bobbing in the water at about the same place; the rope unwound as the ship sailed by and away from the wood. By leaving the wood, or “log” as English sailors called it, in the water for a minute . . . the seaman would know how far the ship had traveled in that time, and so work out how far it had traveled in an hour. . . . [i]f a knot were tied in the rope every one-sixtieth of a mile, all a sailor had to do was to count the knots as he pulled in the rope, and he would know how many miles per hour his ship was traveling.”

What do you think was a drawback to this method of calculating distance?