Why did General Howe give up on Boston as a headquarters?

General Howe left Boston after Henry Knox brought cannons from Ticonderoga and George Washington deployed them on Dorchester Heights. From that position Howes ships wer sitting ducks in Boston Harbor. Howe also felt that Boston was a very difficult place to defend because of the hiily terain and many estuaries etc. Howe told Washington he would not burn the city if he were allowed to leave the city with his army unharmed. Howe did not know that Washington had no gun powder! Washington allowed Howe to leave Boston. General Howe evacuated Boston and sailed for Nova Scotia in early 1776. There he waited for the return of spring and the proper weather for campaigning (his main objective would be New York) and more importantly reinforcements from England. This link gives a good background on this event: http://www.earlyamerica.com/review/2001_winter_spring/howe_1.html