In order to commemorate the campaign and siege and defense of Petersburg, Virginia, in 1864 and 1865 and to preserve for historical purposes the breastworks, earthworks, walls, or other defenses or shelters used by the armies therein, the battle fields at Petersburg, in the State of Virginia, are declared a national battlefield whenever the title to the same shall have been acquired by the United States by donation and the usual jurisdiction over the lands and roads of the same shall have been granted to the United States by the State of Virginia—that is to say, one hundred and eighty-five acres or so much thereof as the Secretary of the Interior may deem necessary