The Act of Congress of March 3, 1815, 2 Story L. U. S. 1530, authorizes and directs the district attorneys of the United States to appoint by warrant, an attorney as their substitute or deputy in all cases when necessary to sue or prosecute for the United States, in any of the state or county courts, by that act invested with certain jurisdiction, within the sphere of whose jurisdiction the said district attorneys do not themselves reside or practice; and the said substitute or deputy shall be sworn or affirmed to the faithful execution of his duty.