That the said city council shall have power, and it is hereby made their duty to make and publish from time to time, all such ordinances as shall be necessary to secure said city and the inhabitants thereof . . . to impose fines, forfeitures and penalties on all persons offending against the laws and ordinances of said city, and provide for the prosecution, recovery and collection thereof, and shall have power to regulate by ordinance the keeping and sale of gun-powder within the city.