NB.Often overlooked,the Roman soldiers who actually constructed The Wall and the garrison forts along the Wall,were excellent engineers.From the fort in shot,has been located a Roman aqueduct,almost 6 miles long.It is laid out to a steady, regular descent,from a spring north of the Wall.It was dug into the ground,rather than on columns.It was lined with wood.
see http://en.wikipedia.org/wiki/Aesica