Sir George Callaghan, under these conditions, on the arrival of the Fleet at Scapa Flow at the end of July, took immediate steps, with the resources at his disposal in the Fleet, to improvise defences for the Base against destroyer attack. All that he could do was to land some 12-pounder guns from the ships of the Fleet and mount them at the entrances. No searchlights could, however, be provided, so that the guns were not of much value at night. Arrangements were also made for placing light cruisers and destroyers at the various entrances to assist these defences. The further step, of course, was taken when the Fleet was present at the Base of placing patrols to the eastward of the Pentland Firth. It was not felt in the Fleet, however, that these measures gave much security even against destroyer attack on a dark night, and, it gave no security whatever against submarine attack. Nothing but _obstructions_ of some kind could give that security. The matter was frequently discussed. Although many brains had been at work, no satisfactory anti-submarine obstruction had been devised. Under the urgent pressure of war the solution was found. The conclusion generally held by experienced submarine officers was that, whilst the least important entrances, such as the Hoy, the Switha and the Holm Sound Channels, would be extremely difficult for the passage of a submarine, entry by the Hoxa Sound Channel was quite practicable by a determined submarine officer. So much for the Base at Scapa Flow. At Cromarty the conditions were somewhat better. The only entrance to this Base is comparatively narrow, and was defended against the entry of destroyers and larger vessels by guns, which had been mounted by the Admiralty before war broke out. There was, however, no boom protection against the entry of destroyers, and the conditions in regard to submarine attack were the same as at Scapa Flow, there being no obstructions. At Rosyth the same conditions prevailed as at Cromarty, namely, the harbour was defended by guns only against attack by destroyers. In this case, the guns were manned by the military, and not by marines, as at Cromarty; again the harbour was quite open to submarine attack. [Illustration: DESTROYERS RAISING STEAM ON THE FIRST OCCASION OF A SUBMARINE BEING REPORTED INSIDE SCAPA FLOW ] [Illustration: THE FIRST OBSTRUCTION AT SCAPA FLOW ]