The chief of police shall be the keeper of the city prison and shall keep the same in a clean condition. He shall safely keep all prisoners consigned to such jail and shall appoint a member of the police force to guard all prisoners sentenced to hard labor on the streets and public works of the city while at work. When, by their labor, such prisoners have paid their fines and costs, he shall discharge them. For the keeping of such persons safely he may use the city jail. (Code 1941, Art. 107-8)