Our progress was very slow, for, as you will easily understand, and must have noted scores of times in connection with some wreck, a ship is of immense weight, and, even if moving ever so slowly, touching a rock at the bottom means a tremendous grinding crash, and either the vessel fixed, perhaps without the possibility of removal, or a hole made which will soon cause it to sink.  Navigation, then, is beset with dangers for a captain.  If he is in well-known waters, matters are simple enough; every rock will be marked upon his chart, every mile near shore will have been sounded, and he will know to a foot or two how much water is beneath his keel.  But as soon as he ventures up some strange creek or river, paradoxically speaking, "he is at sea."  In other words, he would be journeying haphazard, if the greatest precautions were not taken.