The Continental Army, further diminished by expiring short-term enlistments, and by January 1776 reduced by half to 9,600 men, had to be supplemented with the militia, and was joined by Knox with heavy artillery captured from Fort Ticonderoga. When the Charles River froze over, Washington was eager to cross and storm Boston, but General Gates and others were opposed to untrained militia striking well-garrisoned fortifications. Washington reluctantly agreed to secure the Dorchester Heights, 100 feet above Boston, in an attempt to force the British out of the city. On March 9, under cover of darkness, Washington's troops brought up Knox's big guns and bombarded British ships in Boston harbor. On March 17, 9,000 British troops and Loyalists began a chaotic ten-day evacuation of Boston aboard 120 ships. Soon after, Washington entered the city with 500 men, with explicit orders not to plunder the city. He ordered vaccinations against smallpox to great effect, as he did later in Morristown, New Jersey. He refrained from exerting military authority in Boston, leaving civilian matters in the hands of local authorities.