the carilloneur's studio and the twenty-second floor is devoted to mechanical space. Carillon The 23rd floor of the tower contains a three-level belfry that houses a carillon whose final complement of 74 bronze bells, which at the time of its construction the largest carillon of bells in the world, includes the 20-ton, bourdon, the world's largest tuned bell. Though other carillons with more bells have been commissioned, Riverside Church's carillon is still the largest in the world by aggregate weight: the bells and associated mechanisms weigh a combined . The bells themselves are cited as weighing approximately or over 100 short tons. Of the carillon's bells, 53 were made for the original Park Avenue church by English founders Gillett & Johnston and another 19 were made for Riverside Church when it opened. Two bells were added in 1955 and 58 treble bells were replaced by bell founders Van Bergen. The bells were replaced again by Whitechapel Bell Foundry in 2004. The bells can reportedly be heard from up to from the tower. The bourdon sounds one octave lower than other bourdons. A mechanical power room and control room are in the belfry, with the clavier cabin at the top, above the carillon. Due to the weight of the carillon, the heaviest steel beams used in the construction of Riverside Church were used in the tower. The north facade, which overhangs the nave, is supported by a single cross truss that weighs . Outside the carillon, the tower's facade has ornate Neo-Gothic detailing that includes features such as gargoyles. On top of the carillon is a public observation deck; the deck was closed after the September 11, 2001, attacks due to security concerns but the church resumed tours in January 2020. Cloister passageway The cloister passageway leads from the southern portion of the nave to Claremont Avenue in the east. It has four pointed-arch bays, each with a Corinthian-style colonette topped by a grisaille window opening on the south wall. The north and south walls also contain stained-glass windows; the northern wall's windows are artificially illuminated. Inside the cloister passageway are five vaults, which are illuminated by six lanterns. The entrance to the passageway is a small, two-story structure with two arched doorways facing Claremont Avenue and a set of double doors facing a short wheelchair ramp to the south. The top of the cloister entrance's eastern facade contains three niches