AN ARMED POLICE. – A very important regulation has just been adopted by the Police Department, which may possibly lead to some beneficial results in the arrest of nocturnal marauders. One quarter of the Detective Force are put on patrol duty after midnight, in citizens’ dress, armed with revolvers and clubs, and are kept out until daylight. Their first round of duty on Wednesday morning ended in a serious farce, as will be seen by our Police Report; but they may be more successful hereafter. They were arrested themselves on suspicion of being a party of garroters, and if they are not cautious they may be shot by some timid head of a family on the look-out for burglars.