On or before the thirty-first day of August 1861 the lands required to be taken absolutely for the construction of the said works and depot and arsenal, and the lands in the vicinity of the fortifications and works aforesaid, which are required to be kept free from buildings and other obstructions shall be ascertained by a declaration or declarations to be at any time, or from time to time, made and signed by Her Majesty’s Principal Secretary of State for the War Department.]