it sloped upwards to meet the bases of the main-gun barbettes, but was otherwise 1 inch thick forward of the citadel. Aft it ranged in thickness from 2 inches on the flat and 3 inches on the slope to protect the steering gear. The forward conning tower was protected by 12 inches of armour on its sides and it had a 3-inch roof. The aft conning tower had 3-inch armour plates all around. The Lord Nelsons were the first British ships fitted with unpierced watertight bulkheads for all main compartments with access gained by using lifts. In service the inconvenience of this feature for the crew, especially in the engine and boiler rooms, led to its abandonment in the next generation of battleships. Naval historian R. A. Burt assessed the greatest weaknesses of their armour scheme as the waterline belt being submerged at deep load and the reduction in the thickness of the barbette armour below the upper deck. He believed that this made the ships' magazines vulnerable to plunging fire from long range. Modifications Modifications to the sisters before 1920 were relatively minor. In 1909 the number of 3-pounders was reduced to four in Agamemnon and two in Lord Nelson. In 1910–1911 a rangefinder was installed of the roof of the forward turret in both ships and another was added to the spotting top in Agamemnon. The following year Lord Nelson had her spotting top modified to accommodate one as well. In 1913–1914 the ship had an additional rangefinder added to her bridge. The remaining 3-pounders were removed from the ships in 1914–1915 as were the rooftop and bridge rangefinders. A pair of 12-pounders were removed from the after superstructure in exchange for a pair of 3-pounder Hotchkiss guns on high-angle mounts. In 1916–1917, four 12-pounders were removed from the forward superstructure in Agamemnon while Lord Nelson only lost two. That ship lost two more from her aft superstructure in 1918. Early in 1919 the Admiralty decided that the Navy needed a radio-controlled target ship to properly train gunnery officers. It conducted tests to evaluate the effectiveness of shells on armour plates as thick as the typical pre-dreadnought deck armour. At an equivalent range of , the plates were completely destroyed and the Admiralty realized that 15-inch shells would do much the same to any of the surplus early dreadnoughts. It then limited all gunnery practice against the target