Opportunity was then taken of the whole Fleet being in company with the Commodore (T) to carry out a battle exercise. At dusk the Commodore (T) was detached to Harwich, the 3rd Battle Squadron and 3rd Cruiser Squadron to Rosyth, the 2nd Battle Squadron and 2nd Flotilla to Scapa, and the _Marlborough_ to Rosyth, for the purpose of allowing Sir Lewis Bayly and Sir Cecil Burney to exchange commands in accordance with orders received from the Admiralty by wireless telegraphy when at sea. The light cruiser _Bellona_ and the flotilla leader _Broke_ collided during the battle exercises, and were seriously damaged, being sent to Rosyth under the escort of the _Devonshire_. During the 17th wireless messages intercepted in the _Iron Duke_ showed that the enemy raiding force had laid a large number of mines off the Yorkshire coast and that some British and neutral merchant ships had been sunk as a result. During December 18th the 1st and 4th Battle Squadrons, the 1st Battle Cruiser Squadron and the 1st Light Cruiser Squadron remained at sea to the eastward of the Orkneys, and in the evening the battleships shaped course for Scapa, arriving on the 19th, the 1st Battle Cruiser Squadron and 1st Light Cruiser Squadron reaching Cromarty on the same day. The 1st and 2nd Cruiser Squadrons also proceeded to Cromarty, and the 6th Cruiser Squadron remained out on patrol. During the 18th a submarine was reported inside Scapa Flow, and the 2nd Battle Squadron raised steam; but investigation showed that the report was not well founded. The strength of the 10th Cruiser Squadron had now risen to 18 ships, and directions were given by me to the Rear-Admiral commanding to establish patrols as follows: Patrol A.--North of the Faroes and to westward of Long. 5.30 W. Patrol B.--North of the Shetlands and to westward of Long. 1.0 W. Patrol C.--South of the Faroes and to westward of a line joining Sydero and Sule Skerry lighthouses. Patrol D.--West of the Hebrides and a line N.½W. from St. Kilda. The _Sappho_ and the seven armed boarding-steamers, now available, were based on Loch Ewe, and completed the blockade line between Patrol D and the Hebrides. They were, however, shifted later on to Scapa, where they were employed on patrol and boarding duties in the approaches to the Pentland Firth, or at sea with cruiser squadrons.