so remove all risk of fruitless stair-climbing, no letter beyond dd is used for the first storey. The press-mark denoting press and shelf was considered sufficient until 1875, when the numbers denoting sequence on the shelf, known in the Museum as "third-marks," began to be added. If accessions were placed upon an empty shelf, these marks could easily be determined; but if on a shelf half-full, the new books were placed on the left and "third-marked," those marked on the old system following them. Since the introduction of "third-marking" steady progress has been made with the task of adding this additional mark to the books arranged on the old system. (In 1896–97 "third-marks" were added to 25,736 books.) The portions of the library most frequently used were first subjected to this treatment, and at the present time a very small portion of the library, and that consisting of ancient jurisprudence and polemical theology for the most part, remains to be marked. The chief difficulty at present occurring is presented by "works in progress." Such of these as avowedly have no period fixed even approximately for their completion are not marked on the fixed-location system at all. Such are the publications usually called "periodicals" or "journals" (magazines, reviews, newspapers, Proceedings, &c., of societies), directories, almanacks, &c. Other works in progress, more properly to be called "books," do not receive a "third-mark" till completed, and until that time arrives are put on the right of all third-marked books on the shelves assigned to them.
