The Prison Commissioners shall he the department of judicial statistics under the [32 & 33 Vict. c. 33.] Judicial Statistics (Scotland) Act, 1869, and shall discharge the duties of the said department, and all statutory returns relating to such statistics shall he made to them accordingly. The Secretary of State shall from time to time appoint a superintendent of the said department at such salary as shall he determined with the sanction of the Treasury, and. such superintendent may, with the consent of the Treasury, fill any other office under the said Prison Commissioners.