Section 21. There shall be paid by the commonwealth to the chairman of the board a salary of twelve hundred and fifty dollars, to the secretary thereof eighteen hundred and seventy-five dollars, and to the other members thereof one thousand dollars each, and to each member thereof his necessary traveling expenses actually incurred in attending meetings of the board. The board shall keep a record of all money received and expenses incurred by it, and a duplicate thereof shall be open to public inspection in the office of the comptroller.