It will be recalled from the chapter on Paper-Making (No. VI) how plastic paper is in its moist stage, and how tenacious of water are the cellulose fibers. It will also be recalled that there is considerable shrinkage across the web of the paper from the time it leaves the wire to the moment it is reeled. In fact, the very thing which makes paper-making a possibility is the shrinking of each individual fiber, occasioned by the expulsion and evaporation of the water, which has served as a carrier from the machine chest to the wet end of the machine. This propensity of each individual fiber does not cease when the paper is made, but persists forever. A cellulose fiber will absorb moisture from the air in proportion to the relative humidity, just as the hair in a barometer is continually shrinking or expanding as the weather changes. A definite percentage of moisture is normal to a cellulose fiber in proportion to the moisture in the air. The fiber swells as it absorbs, and shrinks as it gives off water. Herzberg gives as the results of investigation with a good writing-paper made of rags, sized with rosin, the following report of the percentage of moisture retained under various degrees of relative humidity: Relative humidity Moisture contained of the air, in the paper, percentage percentage 100 21.5 90 13.5 80 8.9 70 8.4 60 6.5 50 5.6 40 3.4 30 2.3 In a sheet of paper, where thousands of fibers lie side by side, the combined expansion is distinctly noticeable in the changing dimensions of the sheet. This gives rise to difficulties in securing accurate register in color-printing, owing to atmospheric changes. The manufacturer may minimize this difficulty by a careful formation of the paper and the regulation of the drying, so as to turn out the paper as nearly as possible containing an average normal percentage of moisture. The same conditions are responsible for wavy edges, which occur principally along the cross-grain dimension of the sheets. The ends of the fibers, being exposed, easily absorb moisture as paper lies in a pile, but the moisture seldom permeates more than a few inches into the pile. Therefore, the larger part of each sheet is unaffected, but the fibers exposed to the air expand when absorbing moisture increasing the area of the exposed end and, consequently, causing it to assume a wavy formation which is suggestive of a ruffle.