(a)any other officer of equal grade functioning at his head-quarters or, if there be no officer of an equal grade there, any other officer of a superior grade so functioning or, if there be no such superior officer, any other officer of an inferior grade so functioning shall, without relinquishing his ordinary duty, assume charge of the office of the absentee officer and shall continue in charge thereof until the office is assumed by another officer duly appointed thereto and while in such charge, perform the routine duties of the absentee officer, and
