... Between these latter, which varied from one to six miles in length, sixteen locks, like huge steps, were built, overcoming a rise of one hundred and four feet from tide water at Charlestown, to the Concord at Billerica, and a descent of twenty-six feet to the Merrimack at Chelmsford. Five others provided entrance into these rivers, and also into the Mystic at Medford, while suitable waste weirs were ...