the hemicycle have been suggested to be libraries because of their similar set-up to those in the Baths of Caracalla. Historians, to support this theory, have demonstrated that these halls with their niches could properly house scrolls and/or codices. (The author of the Life of Probus mentions that part of the Bibliotheca Ulpia, was located in the Forum of Trajan, and part within the Baths of Trajan, although he later contradicts that statement when referring to the Bibliotheca Ulpia). The presence of similar spaces in the Baths of Caracalla and the Baths of Trajan therefore makes it not unreasonable to assume that the baths of Diocletian contained a library. Architectural styles Within the frigidarium, the use of external buttresses for the cross vaults were considered by some to be the first example of the scientific system of thrusts and counter-thrusts in architecture. Concerning the baths as a whole, it has been described as evoking the Imperial style, or a "Classical" image, which is the style of "manipulation of space". To manipulate the space within this style, the forms of the building were simple and give the impression of a vast amount of open space. The builders of the baths used different techniques to create this effect. The exterior walls of the bath were encrusted with stucco to give the impression of stonework. This technique was quite common within the structures built during the Imperial style of Roman architecture, e.g., the baths of Constantine, the Basilica Nova, and parts of the Sessorian bridge. The interior parts of the bath were supported by vaulting ceilings and arches to create curvilinear lines. The structure of the roof is a typical example of Classical design. Architects used sloped forms to cover curved extrados (the outer surface of the arch) of the vaulted halls. Legacy From the central structure were derived the plans for the Basilica of Constantine. The Smithsonian's National Museum of Natural History building in Washington, D.C., was partially based on design elements from these baths, including its Diocletian windows. Present day Parts of the structure were converted to ecclesiastical or other use, including: Basilica of Santa Maria degli Angeli e dei Martiri Church of San Bernardo alle Terme Sections of the National Roman Museum A part was for many years, starting in the 1870s, the studio of the sculptor Moses Jacob Ezekiel. The octagonal hall served as a planetarium from 1928 until