And similar reasoning applies to the assumption of any such power as is involved in that to establish post-roads and to regulate commerce. If the particular improvement, whether by land or sea, be necessary to the execution of the enumerated powers, then, but not otherwise, it falls within the jurisdiction of Congress. To this extent only can the power be claimed as the incident of any express grant to the Federal Government. But there is one clause of the Constitution in which it has been suggested that express authority to construct works of internal improvement has been conferred on Congress, namely, that which empowers it "to exercise exclusive legislation in all cases whatsoever over such district (not exceeding 10 miles square) as may by cession of particular States and the acceptance of Congress become the seat of the Government of the United States, and to exercise like authority over all places purchased by the consent of the legislature of the State in which the same shall be for the erection of forts, magazines, arsenals, dockyards, and _other needful buildings_..." But any such supposition will be seen to be groundless when this provision is carefully examined and compared with other parts of the Constitution. It is undoubtedly true that "like authority" refers back to "exclusive legislation in all cases whatsoever" as applied to the District of Columbia, and there is in the District no division of powers as between the General and the State Governments. In those places which the United States has purchased or retains within any of the States--sites for dockyards or forts, for example--legal process of the given State is still permitted to run for some purposes, and therefore the jurisdiction of the United States is not absolutely perfect. But let us assume for the argument's sake that the jurisdiction of the United States in a tract of land ceded to it for the purpose of a dockyard or fort by Virginia or Maryland is as complete as in that ceded by them for the seat of Government, and then proceed to analyze this clause of the Constitution. It provides that Congress shall have certain legislative authority over all places purchased by the United States for certain purposes. It implies that Congress has otherwise the power to purchase. But where does Congress get the power to purchase? Manifestly it must be from some other clause of the Constitution, for it is not conferred by this one. Now, as it is a fundamental principle that the Constitution is one of limited powers, the authority to purchase must be conferred in one of the enumerations of legislative power; so that the power to purchase is itself not an unlimited one, but is limited by the objects in regard to which legislative authority is directly conferred.