Section 36. The trustees of the state library may expend such sums annually as the general court may appropriate for permanent assistants and clerks, for books, maps, papers, periodicals and other material for the library and for binding the same and for incidental expenses including binding their report. All accounts for the maintenance of the state library shall be approved by the trustees thereof or by such person as may be designated for the purpose in a vote of said trustees who shall, nevertheless, remain responsible for such approval.