6o8 THE POPULAR SCIENCE MONTHLY. 



be printed. And not only does an impression from plate excel iu 

 delicacy but also in force and deptb of color. 



There never was and never will be a woodcut line having the 

 power of a deep line in a plate, for, in an impression from wood, 

 the print is only a blackened surface of paper, whereas that from 

 plate is a cast with an additional thickness of ink, for the damp 

 paper is forced into the line and brings out the ink upon its em- 

 bossed surface. 



Plate ink is soft and thin as compared with that used for sur- 

 face prints, and the body of it is ground carbon mixed with oil 

 for black ; and colored ink is made from white lead mixed with 

 dry colors and very finely ground in oil. Some inks are much 

 more wearing on the plates than others, green being about the 

 worst in this respect. 



Retouching, or "re-entering," as it is called in the trade, is 

 done by re-entering the roll upon the lines of the transferred work 

 and putting the pressure on, as in the original transfer. This 

 sharpens and restores the line, making it print as good as when 

 new. 



The very best linen fiber paper is used in printing bank notes, 

 bonds, stock certificates, etc., and it is dampened before printing 

 to make it more pliable, and it also takes the ink better in this 

 condition. After printing the sheets are placed on racks in a 

 drying room heated by steam. When they are thoroughly dry 

 they are found wrinkled and curled, so they are placed between 

 Bristol boards and put under a hydraulic pressure of several hun- 

 dred tons and kept there for a few hours, then taken out ready 

 for trimming, numbering, and shipping. Each time the impres- 

 sions are handled they are counted and kept track of, good or 

 bad, so there shall be no possibility of loss or theft. 



There are some very delicate machines used in bank-note work, 

 known as ruling machines and cycloidal and geometric lathes. 

 The straight and curved line ruling machine is used in making 

 the background of portraits and vignettes, shade and ruled faces 

 of letters, background of panels, and is capable of ruling three 

 thousand six hundred lines to the inch with great perfection and 

 regularity, but is seldom set as fine as this. 



The cycloidal ruling machine is more complicated than the 

 plain line ruler, having from one to four "eccentrics," or "cams," 

 in connection with the forward and back movement of the bed, 

 and is used for producing fine tints to print over other colors 

 and work. The principle of its operation is readily understood. 

 A diamond point is arranged in the machine and given a circular 

 motion by the action of the " cams." Now, while the point is re- 

 volving, let a forward movement be given to the plate and the 

 line traced by the point will assume a form like this (Fig. 7), 



