All plates must be rolled very accurately to gauge, allowance of variation often being not over one or two hundredths of an inch. The roller must be a man of experience and of very good judgment for slabs for almost any plate may come of any one of several thicknesses and lengths. He must know his temperatures, speeds of rolling and the amount of reduction given with each pass, and, particularly in case of thin, wide plates, the condition of his rolls, which after two or three days’ wear will produce plates thicker in the middle than at the edges. As the “screw-down” man on top screws the rolls together a little with each successive pass and the “hookers” under the roller’s direction keep the plate entering the rolls properly, he must with his very accurate gauge measure the thickness of the plate as it nears completion. Especially when plates are ordered and paid for by average weight per square foot must he judge accurately the thickness of the center of the plate where he cannot measure, and pull down the edges enough that the finished plate when sheared will average right. It is fortunate for the steel mill men of this country which does not know the advantages of the metric system that a steel plate one inch thick weighs very close to 40.8 pounds per square foot. This is an easy figure and the clerk, roller, hot bed foreman, weighers and all concerned “think” in terms of a plate one foot square and one inch thick. One-half inch plate, therefore, weighs 20.4 lbs.; ¼″, 10.2 lbs.; and ³⁄₁₆″, 7.66 lbs. per square foot. As will be seen when we consider wire drawing and cold-drawn seamless tubes, the strength and other physical properties of steel depend first, upon composition, and, secondly, upon temperature at which they are hammered, rolled, or otherwise “worked.” Therefore, plates can be much modified in physical properties by finishing at chosen temperatures. A steel containing .19% of carbon and .45% of manganese, for instance, which in one inch plate should give a tensile strength of around 55,000 pounds per square inch, 58,000 pounds in ½″ or 62,000 pounds in ¼″ when finished at usual temperatures, by slightly “colder rolling” can be made to show a considerably greater strength. Of course, the ductility is somewhat reduced, but, with a moderate amount of cold rolling, it will not be enough to do harm. All of these and many other details must be not only kept in mind but become second nature to the plate worker.