Complex sentence: His largest project was the system that supplied Manchester with much of its water during the 19th century; construction of what was in its day the largest chain of reservoirs in the world began in 1848 and was completed in 1877.
Split into simpler sentences: His largest project was the system that supplied Manchester with much of its water during the 19th century. The construction of what was in its day the largest chain of reservoirs in the world began in 1848 and was completed in 1877.