89. If the office of Chief Justice of the Supreme Court becomes vacant, or if the Chief Justice is, by reason of absence or for any other reason, unable to perform the duties of his office, those duties shall, until some person appointed by the President to the vacant office has entered on the duties thereof or until the Chief Justice has resumed his duties, as the case may be, be performed by such one of the other judges of the court as the President may appoint for the purpose.