One of Her Majesty's Principal Secretaries, of, State shall, in the Month of April in every Year, by Order under his Hand, appoint some fit Person to be the Auditor of the Accounts of the said Metropolitan Board for the preceding Year; and every such Auditor shall be paid by such Board not exceeding Five Guineas for every Day he is fully employed on such Audit, and all Expenses he is put to in the auditing of such Accounts ; provided always, that such Payment shall not exceed Fifty Guineas.