Sentence:  For several days things went along in about the same course. I took our position every morning with my crude <WSD>sextant</WSD>; but the results were always most unsatisfactory. They always showed a considerable westing when I knew that we had been sailing due north. I blamed my crude instrument, and kept on. 
Meaning of the word marked with <WSD> tags:  A navigational device for deriving angular distances between objects so as to determine latitude and longitude.