The materials upon which literature has been embodied, and the changes and improvements which these materials have undergone from age to age, opens up one of the most interesting chapters of bibliographical science and of the world's history. A knowledge of the materials successively used in the book-making industry, and of the improvements through which these have continually passed, together with the various kinds of the completed products, the style of writing (there is a "gait" of hand as well as of foot), and certain distinguishable characteristics of the literature of the different periods, all assist in fixing with approximate certainty the date at which a manuscript was produced. In considering the materials of books it needs to be held in mind that the time of a manuscript's production was seldom affixed to it until a late date; that must be determined or inferred from collateral data. We would instance the "water marks" of manufactured paper as an example of these collateral data helping to determine the age of a manuscript. It is a well known fact that every paper manufactory has its own individual mark of identification for its output. This is its protective "water mark" and is impressed in the texture or fiber of every sheet made, and at regular intervals in the sheet. This is by no means an exclusively modern device of authentication, for these were known as early as the thirteenth century. In the fifteenth century, when the quality of the paper was improved, the "water marks" became more elaborate and, as early as the sixteenth century, the name of the maker of the paper was inserted. These marks of identification greatly aid the antiquarian student in fixing the date of any writing. They are often, too, of legal significance, inasmuch as important cases in courts of law in our times--and earlier times--have been known to turn upon such facts of evidence as the "water marks" of the paper used in documents, as other cases have turned upon the kind or quality of the ink or the "hand" in which the documents at issue were written. An incident narrated in a book by Dr. N. D. Hillis may not be historical though it does illustrate what has often actually occurred: "In looking at the thick white paper, upon a sheet of which the guide said that the deed had been written, John noticed that it was the usual parchment paper of the time--a paper strong, and made of linen, so that it might survive the rough usage of the settler's cabin. Holding it up between his eyes and the sun he noticed this water-mark and stamp--'C. Saur, Philadelphia, 1787.' The purported deed was dated 1740."[23] The press dispatches some time ago reported a case before the Senate in one of our states in which the conviction or the acquittal of the defendant turned, largely, upon the quality of the ink which had been used in signing a certain check, given in payment of a claim. It was admitted by experts on both sides that the ink employed in signing the check was