The board may prescribe such duties of assessors and others appointed by it, in addition to the duties prescribed in this act, as the board shall find necessary in order to carry out the provisions of this act. The board may prescribe such qualifications for assessors and other appointees of the board, as it shall find desirable. So far as possible the board shall, in making appointments, advance existing appointees or employes of the board.
72 P.S. § 5341.19