repair). Extensively rebuilt in 1769, the Great Stone Dock is now known as No.5 dock. Along with the new dock, Dummer proposed that two wet docks (non-tidal basins) be built: the first ("Lower") Wet Dock was entered directly from the harbour and provided access to the Great Stone Dock; since much expanded, it remains in place (now known as "No. 1 Basin"). The second ("Upper") Wet Dock was entered by way of a channel. To empty the dry dock, Dummer designed a unique system which used water from the Upper Wet Dock to drive a water-wheel on the ebb tide, which in turn powered a set of pumps. (At high tide, an auxiliary set of pumps was used, powered by a horse gin.) In 1699 Dummer adapted the channel leading to the Upper Wet Dock, enabling it to be closed off at each end by a set of gates, thus forming a second dry dock (called the "North Stone Dock" after it was rebuilt with stone altars in 1737, and known today as No 6 dock). Severed from the harbour, the Upper Wet Dock became a reservoir into which water from various nearby dry docks could be drained; it was vaulted and covered over at the end of the eighteenth century, but still exists today underground. By 1700 a shipbuilding slip had been constructed off the (Lower) Wet Dock, parallel with the dry dock (roughly where No 4 dry dock is today). Eighteenth century Between 1704 and 1712 a brick wall was built around the Dockyard, following the line of the town's 17th-century fortifications; together with a contemporary (though altered) gate and lodge, much of the wall still stands, serving its original purpose. A terrace of houses for the senior officers of the yard was built at around this time (Long Row, 1715–19); later in the century it was joined by a further terrace (Short Row, 1787). In 1733 a Royal Naval Academy for officer cadets was established within the Dockyard, the Navy's first shore-based training facility and a forerunner of Britannia Royal Naval College in Dartmouth. The 'Great Rebuilding' The second half of the eighteenth century was a key period in the development of Portsmouth (and indeed of the other Royal Dockyards). A substantial planned programme of expansion and modernisation was undertaken from 1761 onwards, driven (as would be future periods of expansion) by increases both in the size