Of course, when any of these symptoms appear it is the duty of the men on the machines to correct them, and in the continuous course of paper-making it is inevitable that felts become filled up and require washing or changing, or that the variations of consistency in the stuff should call for some form of regulation. Stuff which runs too moist on the wire will often “crush” under the couch roll, producing a curdled appearance. Stuff run with insufficient water will not form evenly. The skilful machine-tender avoids these extremes. TRADE CUSTOMS.--In recognition of the many variable elements in paper-making, trade customs have been established, such as allowances for a normal variation in the weight of paper above or below the nominal ream-weight, and reasonable allowance should be made for normal variations in other characteristics. Eternal vigilance and alert judgment are certainly required for setting high standards in the manufacture of paper. It is a matter of common observation that mills using practically the same raw materials vary widely in their reputation for uniformity and excellence of product. The reason for this is to be found in the human element. CALENDER DEFECTS.--A number of difficulties may arise from much less excusable causes than those mentioned. For example, the paper sometimes may run slack through the calenders, with the result that it wrinkles and cuts in diagonal jags called “calender cuts.” Sheets containing such defects sometimes elude the finishers, and on the printing-press such a sheet may crack and go around one of the ink rollers. On a web press the trouble from such a defect would be even worse, causing breaks and necessitating delays on the press. It is more difficult to exclude calender cuts from roll paper, as it is not always easy to see them in the fast-running paper, so that an occasional cut is not an unforgivable sin. Among other defects arising on the calenders are little scarlike depressions in the paper, made by small scraps of paper which have become lodged on a calender roll and are embossed into the web at each revolution of the roll. HOLES, DIRT, ETC.--In very light papers, holes are sometimes found, the most likely cause of which may be picking under the dandy roll or grease spots on the wire cloth. Of less frequent difficulty are the so-called pinholes, caused by sand or grit, while slime spots, or spots caused by slight bundles of fibers, are also occasionally noticed. Dirt and specks originate from careless handling of rags or paper stock, and are also derived from shives of undigested wood in the wood-pulp. Streaks in the paper may originate from a crease in the wire, and mottled effects denote some fault in the handling of the paper in the wet stages of making.