 The spotting tops , a modern equivalent of the sailing ship 's fighting top , at the head of the tripod fore and main masts controlled the fire of the Indefatigable 's main guns . Data from a 9 @-@ foot ( 2 @.@ 7 m ) Barr and Stroud rangefinder was input into a Dumaresq mechanical computer and electrically transmitted to Vickers range clocks located in the Transmitting Station ( TS ) located beneath each spotting top where it was converted into range and deflection data for use by the guns . The target 's data was also graphically recorded on a plotting table to assist the gunnery officer in predicting the movement of the target . Each gun turret had its own transmission equipment and the turrets , Transmitting Stations , and spotting tops could be connected in almost any combination . Firing trials against Hero in 1907 revealed this system 's vulnerability to gunfire as the spotting top was hit twice and a large splinter severed the voice pipe and all wiring running along the mast . To guard against this possibility ' A ' turret of Indefatigable was fitted with a 9 @-@ foot rangefinder at the rear of the turret roof and it was equipped to control the entire main armament during a refit between 1911 and 1914 . 
