The electors at special town meetings, when properly convened, shall have full power to fill any vacancies in any of the town offices when the same shall not already have been filled by appointment; to provide for raising money for repairing highways or buildings, or repairing bridges in case of emergency, and to direct the repairing or building thereof; to act upon any subject within the power of the electors at any annual town meeting which was postponed at the preceding annual town meeting for want of time, to be considered at a future special town meeting; but special town meetings shall have no power to act upon any subject not embraced in the statement of the notice calling the same.