A compensation allowed by law to officers, for their trouble and expenses in travelling on public business. 2. The mileage allowed to members of congress, is eight dollars for every twenty miles of estimated distance, by the most usual roads, from his place of residence to the seat of congress, at the commencement and end of every session. Act of Jan. 22, 1818; 3 Story, Laws U. S. 1657. 3. In computing mileage the distance by the road usually travelled is that which must be allowed, whether in fact the officer travels a more or less distant way to suit his own convenience.