As this question of sizes concerns publishers and booksellers, as well as librarians, and the metric system, though established in continental Europe, is in little use in the United States and England, it remains doubtful if any general adherence to this system of notation can be reached--or, indeed, to any other. The Publishers' Weekly (N. Y.) the organ of the book trade, has adopted it for the titles of new books actually in hand, but follows the publishers' descriptions of sizes as to others. Librarian J. Winter Jones, of the British Museum, recommended classing all books above twelve inches in height as folios, those between ten and twelve inches as quartos, those from seven to ten inches as octavos, and all measuring seven inches or under as 12mos. Mr. H. B. Wheatley, in his work, "How to Catalogue a Library," 1889, proposed to call all books small octavos which measure below the ordinary octavo size. As all sizes "run into each other," and the former classification by the fold of the sheets is quite obsolete, people appear to be left to their own devices in describing the sizes of books. While the metric notation would be exact, if the size of every book were expressed in centimetres, the size-notation in the table given is wholly wanting in precision, and has no more claim to be adopted than any other arbitrary plan. Still, it will serve ordinary wants, and the fact that we cannot reach an exact standard is no reason for refusing to be as nearly exact as we can. And while we are upon the subject of notation may be added a brief explanation of the method adopted in earlier ages, (and especially the years reckoned from the Christian era) to express numbers by Roman numerals. The one simple principle was, that each letter placed after a figure of greater equal value adds to it just the value which itself has; and, on the other hand, a letter of less value placed before (or on the left of) a larger figure, diminishes the value of that figure in the same proportion. For example: These letters--VI represent six; which is the same as saying V+I. On the contrary, these same letters reversed represent four; thus--IV: that is V-I=4. Nine is represented by IX, _i. e._, X-I, ten minus one. On the same principle, LX represents 60--or L+N: whereas XL means 40--being L-X. Proceeding on the same basis, we find that LXX=L+XX=70; and LXXX or L+XXX is 80. But when we come to ninety, instead of adding four X's to the L, they took a shorter method, and expressed it in two figures instead of five, thus, XC, _i. e._ 100 or C-X=90. The remarkable thing about this Roman notation is that only six letters sufficed to express all numbers up to one thousand, and even beyond, by skilful and simple combinations: namely the I, the V, the X, the L, the C, and the M, and by adding or subtracting some of these letters, when placed before or after another letter, they had a whole succession of numbers done to their hand--thus: