Persons who are not bound by law, but excused from the performance of duties imposed upon others. 2. By the Act of Congress of May 8, 1792, 1 Story, L. U. S. 252, it is provided, Section 2. That the vice-president of the United States the officers, judicial and executive, of the government of the United States; the members of both houses of congress, and their respective officers; all custom-house officers, with their clerks; all post officers, and stage drivers, who are employed in the care and conveyance of the mail of the post office of the United States; all ferrymen employed at any ferry on the post road; all inspectors of exports; all pilots; all mariners, actually employed in the sea service of any citizen or merchant within the United States; and all persons who now are, or may hereafter be, exempted by the laws of the respective states, Shall be, and are hereby, exempted from militia duty, notwithstanding their being above the age of eighteen, and under the age of forty-five years.