July 11 marks the 150th anniversary of the first ringing of Big Ben's Great Bell. Big Ben is the name of the clock inside the famous Victorian Gothic St Stephen's clock tower, which forms part of the Houses of Parliament. Despite a common misconception, the name 'Big Ben' does not refer to the famous tower, nor to the four huge clock faces of this London landmark; instead, it refers to the largest of the five bells inside the tower. This impressive mechanism weighs in at over 13.5 tons, and the pendulum, which beats once every two seconds, is 13 feet long and weighs 690 pounds.