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The advantage [enabled by the Roman highway system] of receiving the earliest intelligence, and of conveying their orders with celerity, induced the emperors to establish, throughout their extensive dominions, the regular institution of posts. Houses were every where erected at the distance only of five or six miles; each of them was constantly provided with forty horses, and by the help of these relays, it was easy to travel an hundred miles in a day along the Roman roads. The use of posts was allowed to those who claimed it by an Imperial mandate; but though originally intended for the public service, it was sometimes indulged to the business or conveniency of private citizens. DF, i, 77-78.