While the northern campaign was progressing thus unfavourably, the British were at length driven from Boston. Howe had unaccountably neglected to occupy Dorchester heights, which commanded the town; and Washington, after waiting till a sufficient number of heavy guns could be collected, advanced on the night of March 4 and occupied them with 2000 men. His position was secure. The British had no alternative but to carry it by storm or retire from Boston. Not caring to repeat the experiment of Bunker Hill, they embarked on the 17th of March and sailed to Halifax, where they busied themselves in preparations for an expedition against New York. Late in April Washington transferred his headquarters to New York, where he was able to muster about 8000 men for its defence. Thus the line of the Hudson river was now threatened with attack at both its upper and lower ends. [Sidenote: Lord Dunmore in Virginia.] This change in the seat of war marks the change that had come over the political situation. It was no longer merely a rebellious Massachusetts that must be subdued; it was a continental Union that must be broken up. During the winter and spring the sentiment in favour of a declaration of independence had rapidly grown in strength. In November, 1775, Lord Dunmore, royal governor of Virginia, sought to intimidate the revolutionary party by a proclamation offering freedom to such slaves as would enlist under the king's banner. This aroused the country against Dunmore, and in December he was driven from Norfolk and took refuge in a ship of war. On New Year's Day he bombarded the town and laid it in ashes from one end to the other. This violence rapidly made converts to the revolutionary party, and further lessons were learned from the experience of their neighbours in North Carolina. [Sidenote: North Carolina and Virginia.]