the use of Naval airships. A single large Zeppelin, L 5, played an unimportant part in the Battle of the Dogger Bank on 24 January 1915. L 5 was carrying out a routine patrol when it picked up Admiral Hipper's radio signal announcing that he was engaged with the British battle cruiser squadron. Heading towards the German fleet's position, the Zeppelin was forced to climb above the cloud cover by fire from the British fleet: its commander then decided that it was his duty to cover the retreating German fleet rather than observe British movements. In 1915, patrols were only carried out on 124 days and in other years the total was considerably less. They prevented British ships from approaching Germany, spotted when and where the British were laying mines and later aided in the destruction of those mines. Zeppelins would sometimes land on the sea next to a minesweeper, bring aboard an officer and show him the mines' locations. In 1917, the Royal Navy began to take effective countermeasures against airship patrols over the North Sea. In April, the first Curtiss H.12 Large America long-range flying boats were delivered to RNAS Felixstowe, and in July 1917, the aircraft carrier entered service and launching platforms for aeroplanes were fitted to the forward turrets of some light cruisers. On 14 May, L 22 was shot down near Terschelling Bank by an H.12 flown by Lt. Galpin and Sub-Lt. Leckie, which had been alerted following interception of its radio traffic. Two abortive interceptions were made by Galpin and Leckie on 24 May and 5 June. On 14 June, L 43 was brought down by an H.12 flown by Sub Lts. Hobbs and Dickie. On the same day Galpin and Leckie intercepted and attacked L 46. The Germans had believed that the previous unsuccessful attacks had been made by an aircraft operating from one of the Royal Navy's seaplane carriers; now realising that there was a new threat, Strasser ordered airships patrolling in the Terschilling area to maintain an altitude of at least 4,000 m (13,000 ft), considerably reducing their effectiveness. On 21 August, L 23, patrolling off the Danish coast, was spotted by the British 3rd Light Cruiser squadron which was in the area. launched its Sopwith Pup, and Sub-Lt. B. A. Smart succeeded in shooting the Zeppelin down in flames. The cause of the airship's loss was not discovered by the