In general, paper is ordered with the grain running the length of the sheets for all purposes. In the folder, booklet, or bound book the grain should run parallel to the fold or binding. This gives a smoother folded edge and the pages, being more flexible, lie flatter.

B. F Perkins 6t Son, Inc., Holyoke, Mati.

Perkins Pressure Bulker to Measure the Bulk of Sheets of Paper.

1. The diameter of the pressure foot is

three square inches and the pressure is figured in pounds per square inch of paper.

2. There is no fixed standard for the

amount of pressure. The amount is intentionally flexible to meet current requirements.

3. The pressure bulker is used chiefly

to measure a specified number of sheets of paper to ascertain how thick a book with that many pages would be. The number of inches is recorded on the scale on the left.

SELECTION OF PAPER

4. Physical durability

The physical strength of paper may best be tested by tearing it with and against the grain.

5. Permanence

Book papers are generally made of rag, chemical wood pulp, mechanical wood pulp, or a combination of these. Chemical wood pulp is wood cellulose extracted by chemicals from the wood. In the process, gums, resin, and lignin are eliminated. In the better grades such fiber has much of the characteristics and permanence of rag paper. On the other hand, mechanical pulp is merely the crushing of wood into pulp with nothing eliminated. These fibers deteriorate in strength and color just as wood does under exposure. Mechanical pulp is used only in the cheapest grades of Book paper, which are classified as Groundwood papers whether they contained a large amount, as in news paper, or a small amount. All Book papers free from Groundwood are classified as free sheets, indicating that they contain only chemical wood pulp or rag, or both.