The power vested in Congress "to establish post-offices and post-roads" has been practically construed, since the foundation of the government, to authorize not merely the designation of the routes over which the mail shall be carried, and the offices where letters and other documents shall be received to be distributed or forwarded, but the carriage of the mail, and all measures necessary to secure its safe and speedy transit, and the prompt delivery of its contents. The validity of legislation prescribing what should be carried, and its weight and form, and the charges to which it should be subjected, has never been questioned. What should be mailable has varied at different times, changing with the facility of transportation over the post-roads. At one time, only letters, newspapers, magazines, pamphlets, and other printed matter, not exceeding eight ounces in weight, were carried; afterwards books were added to the list; and now small packages of merchandise, not exceeding a prescribed weight, as well as books and printed matter of all kinds, are transported in the mail. The power possessed by Congress embraces the regulation of the entire postal system of the country. The right to designate what shall be carried necessarily involves the right to determine what shall be excluded. The difficulty attending the subject arises, not from the want of power in Congress to prescribe regulations as to what shall constitute mail matter, but from the necessity of enforcing them consistently with rights reserved to the people, of far greater importance than the transportation of the mail. In their enforcement, *733 a distinction is to be made between different kinds of mail matter, — between what is intended to be kept free from inspection, such as letters, and sealed packages subject to letter postage; and what is open to inspection, such as newspapers, magazines, pamphlets, and other printed matter, purposely left in a condition to be examined. Letters and sealed packages of this kind in the mail are as fully guarded from examination and inspection, except as to their outward form and weight, as if they were retained by the parties forwarding them in their own domiciles. The constitutional guaranty of the right of the people to be secure in their papers against unreasonable searches and seizures extends to their papers, thus closed against inspection, wherever they may be. Whilst in the mail, they can only be opened and examined under like warrant, issued upon similar oath or affirmation, particularly describing the thing to be seized, as is required when papers are subjected to search in one's own household. No law of Congress can place in the hands of officials connected with the postal service any authority to invade the secrecy of letters and such sealed packages in the mail; and all regulations adopted as to mail matter of this kind must be in subordination to the great principle embodied in the fourth amendment of the Constitution.