The intention of the Society's members was that the structure should be of ‘such magnitude and character, as might comprise the whole of the collection in one room, to hold meetings at which scientific communications will be received and to establish a library’.
The architect, RH Sharpe of York, produced designs for the central Rotunda following a plan suggested by William Smith, "the Father of English Geology", who then resided in Scarborough. Its circular plan was intended to interpret Smith’s great discovery of the stratification of rocks; round the gallery John Phillips painted a section showing the geological formation of the East Coast from the Humber to the Tees, below which were to be displayed appropriate rocks and fossils. A similar building had been erected to house the Leverian Museum of fossils near Blackfriars Bridge and it is probable that Smith had been influenced by the design of the structure which had closed in 1806.
The Rotunda, standing fifty two feet high, is built of Kelloways Rock donated by Sir John VB Johnstone of Hackness Hall, President of the Society. A rusticated ground floor on a moulded plinth supports an upper floor articulated by pilasters. The cornice and windows were said to be based on those of the Theatre Marcellus and The Temple of Ridiculo at Rome. The structure is roofed by a coffered dome and stone lantern.
Sharpe had anticipated the need for future expansion of the museum by including optional wings in his plans; these were redesigned and built in 1860 when the collection of antiquities demanded more display space. The low, one storey wings, each of three bays and with segmented bowed ends, were constructed using similar material to the rotunda. The east wing was used to house the burgeoning geological items and the west wing was further extended to its present size in the 1880s to provide a meeting room and a library.
The original plans, specifications and accounts have survived. From these it is known that the beautiful curved mahogany showcases which line the upper galleries were designed by John Barry and constructed by the joiner George Crosby in 1836 for the sum of £128.12.0d.
The Scarborough Philosophical Society amalgamated with the Scarborough Archaeological Society in 1853 and continued to run the museum until l937 when the responsibility of maintenance of collections and building was assumed by the Scarborough Corporation. It was decided that the natural history items should be moved to separate accommodation when suitable premises could be found (now the Wood End Museum of Natural History). The East Wing was modernised in 1948 and similar treatment to the West Wing followed in 1950.
