Sentence:  The mules of Piedmont are exceeding strong and hardy. ... They are the only carriage that can be used in crossing the mountains, being very <WSD>sure-footed</WSD>: and it is observed that in choosing their steps, they always march upon the brink of the precipice. 
Meaning of the word marked with <WSD> tags:  Walking steadily, without stumbling; capable of finding good footing.