mast ponds by a mechanical timber transport system, and underground canals. Later the basement was converted into a steam laundry. Lower Boat House circa 1820 built as a storehouse for squared timber, and later to store ship's boats. Dry docks and covered slips The covered slips 1838–1855. It was on slipways that ships were built. The slipways were covered, to prevent ships rotting before they had been launched. The earliest covered slips no longer exist (Nos 1 and 2 Slips, which stood either side of the Assistant Queen's Harbourmaster's Office, were given wooden covers in 1817). By 1838 the use of cast and wrought iron in buildings had become feasible. Of those that survive, the oldest slip has a wooden roof, three have cast iron roofing and the latest uses wrought iron; together, they are of unique importance in showing the development of wide span structures such as were later used by the railways. No 3 Slip 1838. This had a linked timber roof truss structure and was originally covered in tarred paper, which was quickly replaced with a zinc roof. The slip was backfilled around 1900 and a steel mezzanine floor was added. It became a store house for ship's boats. No 4, 5 and 6 Slips 1848. These were designed by Captain Thomas Mould, Royal Engineers, and erected by Bakers and Sons of Lambeth. Similar structures were erected at Portsmouth but these are no longer extant. They predate the London train sheds of Paddington and King's Cross which were often cited as the country's first wide span metal structures. No 7 Slip is one of the earliest examples of a modern metal trussed roof. It was designed in 1852 by Colonel Godfrey T. Green, Royal Engineers. It was used for shipbuilding until 1966; was launched from there on 5 May 1962. Dry Dock. The docks are filled by sluice gates set into the caissons, and emptied by a series of underground culverts connected to the pumping station. No 1 Dry Dock 1824 (built on the site of "The Old Double Dock") no longer exists; it was filled in and converted into a covered workshop (No 1 Machine Shop) prior to the pioneering construction of in the adjacent No 2 Dock. No 2 Dry Dock 1856 was built on the site of "The Old Single Dock" where HMS Victory was constructed. In 1863, this dock constructed , the first