The chancellor of the Smithsonian Institution may, by an instrument in writing filed in the office of the Secretary thereof, designate and appoint a suitable person to act as Secretary of the Institution when there shall be a vacancy in said office, and whenever the Secretary shall be unable from illness, absence, or other cause to perform the duties of his office; and in such case the person so appointed may perform all the duties imposed on the Secretary by law until the vacancy shall be filled or such inability shall cease. The said chancellor may change such designation and appointment from time to time as the interests of the Institution may in his judgment require.