new Houses of Parliament: his plan called for a great central chimney through which what he called "vitiated air" would be drawn out of the building with the heat and smoke of about four hundred fires around the Palace. To accommodate the tower, Barry was forced to lower the lofty ceiling he had planned for the Central Lobby and reduce the height of its windows; however, the tower itself proved to be an opportunity to improve the Palace's exterior design, and Barry chose for it the form of a spire in order to balance the effect of the more massive lateral towers. In the end, the Central Tower failed completely to fulfill its stated purpose, but it is notable as "the first occasion when mechanical services had a real influence on architectural design". Apart from the pinnacles which rise from between the window bays along the fronts of the Palace, numerous turrets enliven the building's skyline. Like the Central Tower, these have been added for practical reasons, and mask ventilation shafts. There are some other features of the Palace of Westminster which are also known as towers. St Stephen's Tower is positioned in the middle of the west front of the Palace, between Westminster Hall and Old Palace Yard, and houses the public entrance to the Houses of Parliament, known as St Stephen's Entrance. The pavilions at the northern and southern ends of the river front are called Speaker's Tower and Chancellor's Tower respectively, after the presiding officers of the two Houses at the time of the Palace's reconstruction—the Speaker of the House of Commons and the Lord Chancellor. Speaker's Tower contains Speaker's House, the official residence of the Speaker of the Commons. Grounds There are a number of small gardens surrounding the Palace of Westminster. Victoria Tower Gardens is open as a public park along the side of the river south of the palace. Black Rod's Garden (named after the office of Gentleman Usher of the Black Rod) is closed to the public and is used as a private entrance. Old Palace Yard, in front of the Palace, is paved over and covered in concrete security blocks (see security below). Cromwell Green (also on the frontage, and in 2006 enclosed by hoardings for the construction of a new visitor centre), New Palace Yard (on the north side) and Speaker's Green (directly north of the Palace) are all private and