It took 10 years to build the Pontcysyllte Aqueduct, which towers over the River Dee valley in Wrexham, Wales. A feat of civil engineering, it is still in use more than two centuries later, allowing boats to cross the Llangollen Canal about 120 feet (35 m) above the valley. The aqueduct consists of a narrow cast iron trough supported by stone columns and bordered by a railed path that was built so horses could tow canal boats. The span occasionally closes for maintenance. How is it emptied?
Discuss 
