4. (1) The Government may, by notification in the official Gazette,- (a) extend this Act to any port in which this Act is not in force or to any part of any navigable river or channel which leads to a port and in which this Act is not in force; (b) specially extend the provisions of section 31 or section 32 to any port to which they have not been so extended; (c) withdraw this Act or section 31 or section 32 from any port or any part thereof in which it is for the time being in force. (2) A notification under clause (a) or clause (b) of sub-section (1) shall define the limits of the area to which it refers. (3) Limits defined under sub-section (2) may include any piers, jetties, landing-places, wharves, quays, docks and other works made on behalf of the public for convenience of traffic, for safety of vessels, or for the improvement, maintenance or good government of the port and its approaches, whether within or without high-water-mark, and, subject to any rights of private property therein, any portion of the shore or bank within fifty yards of high-water-mark. (4) In sub-section (3) the expression “high-water-mark” means the highest point reached by ordinary spring tides at any season of the year.