Read it through once

'That's the very main virtue of the thing. If the shapes didn't change every three seconds they wouldn't be of any use. Take this place where we are now, for instance. As long as that hill over yonder is only one hill, I can boom right along the way I'm going; but the moment it splits at the top and forms a V, I know I've got to scratch to starboard in a hurry, or I'll bang this boat's brains out against a rock; and then the moment one of the prongs of the V swings behind the other, I've got to waltz to larboard again, or I'll have a misunderstanding with a snag that would snatch the keelson out of this steamboat as neatly as if it were a sliver in your hand. If that hill didn't change its shape on bad nights there would be an awful steamboat grave-yard around here inside of a year.'