No Police Officer shall be at liberty to resign his office, or withdraw himself from the duties thereof, unless expressly allowed to do so in writing by the District Superintendent; or unless he shall have given to his superior Officer two months’ notice in writing of his intention to do so. Nor shall any such Police Officer engage in any employment or office whatever, other than his duties under this Act, unless expressly permitted to do so in writing under the seal of the Inspector-General.