To secure the permanence of the records would be more difficult. Ordinary books and papers would clearly be unsuitable for long keeping; though for comparatively limited periods they might answer if securely packed in airtight waterproof cases. Nothing liable to spontaneous decay should be admitted. Stereotype plates of metal would be even more open to objection than printed sheets. The noble metals would be too costly, the baser would corrode; and with either the value of the plates as metal would be a standing danger to the deposit. The material basis of the library must be, as nearly as possible, worthless for other uses (to insure them against the natural greed of man), yet such as will hold the records sharply and faithfully under all circumstances. The _terra cotta_ tablets of ancient Assyria are instructive in this connection. Possibly plates of artificial stone, or sheets of a _papier-mache_-like preparation of asbestos, might be less bulky and equally durable. Having determined this point, and dug from the solid rock a chamber for the reception of our legacy, the next step would be the selection of its contents. Obviously the books to be preserved should embrace first of all lexicons and grammars of every known form of speech, since it is impossible to tell which of the dialects of to-day will be the parents of the dominant tongue of any distant future time; while we may be practically certain that some one or more of the languages of to-day will furnish a key to any language that men will ever use. Next in order would come encyclopaedias, the most comprehensive and complete that there might be room for. The sacred books of all nations might come next; then the works of the great poets, historians and novelists; after them, the best obtainable records of art, science, the various industries, and so on, with specimens of the best and most typical of our works of art, manufacture, and the like. The spaces between the various articles should be filled in with some insoluble and neutral substance, to prevent corrosion, or the infiltration of water and consequent damage to the plates. Then, the entrance to the chamber being securely sealed, permanent records should be made in many places and in various ways, setting forth the purpose of the deposit, its exact location, and the nature of its contents. Among such records not the least valuable would be deeply cut polyglot inscriptions on natural cliffs in different parts of the world, observation having shown that such records may remain to challenge human curiosity for ages after all other records of their time have disappeared.