steam hammers designed by James Nasmyth. Three new dry docks were constructed over the next 20 years, opening off the new basin, and another was built on reclaimed land west of the basin alongside the shipbuilding slips, which were now five in number (Slips 3-5 were covered by interlinking metal roofs, believed to have comprised the widest iron span in Britain when built in 1845). Meanwhile, facilities for building wooden hulls continued to be updated with a large sawmill having been built behind the slips in 1843. It transpired, however, that developments in shipbuilding technology led to several of the new amenities having to be rebuilt and expanded almost as soon as they were finished. Technological change affected not only ships' means of propulsion, but the materials from which they were built. By 1860 wooden warships, vulnerable as they were to modern armaments, had been rendered largely obsolescent. The changeover to metal hulls not only required new building techniques, but also heralded a dramatic and ongoing increase in the potential size of new vessels. The Dockyards found themselves having to expand in kind. At Portsmouth, plans were drawn up in the late 1850s for further land reclamation north and east of the new Steam Basin, and from 1867 work was begun on a complex of three new interconnected basins, each of 14–22 acres. Each basin served a different purpose: ships would proceed from the repairing basin, to the rigging basin, to the fitting-out basin, and exit from there into a new tidal basin, ready to take on fuel alongside the sizeable coaling wharf there. Three dry docks were also constructed as part of the plan, as well as parallel pair of sizeable locks for entry into the basin complex; the contemporary pumping station which stands nearby not only served to drain these docks and locks, but also delivered compressed air to power cranes, caissons and capstans. This "Great Extension" of Portsmouth Dockyard was largely completed by 1881. In 1860 policing of the dockyard was also transferred to the new No. 2 Division of the Metropolitan Police, a role it fulfilled until 1933. Before the end of the century, however, it was recognised that there would have to be still further expansion across all the Royal Dockyards in order to keep pace with the increasing likely size of future naval vessels. At Portsmouth two more dry docks, Nos 14 & 15,