6i2 THE POPULAR SCIENCE MONTHLY. 



The reverse would be the case if this lathe-work was printed from 

 the plate as it is engraved by the machine. This reversal (mak- 

 ing that sunk on the plate which is left raised by the lathe, and 

 vice versa) is effected by a process that is one of the carefully 

 guarded secrets of the trade, and therefore can not be described 

 here. Its effect, however, is evident. 



We may suppose, for instance, that a very careful engraver 

 might possibly cut upon a plate a very fair imitation of the lines 

 forming the figure in our last diagram, but these cut lines, remem- 

 ber, would print black and would give an entirely different effect ; 

 therefore the black diamonds between the white lines must be cut 

 and the line left standing ; and what hand, be it ever so skillful, 

 could cut these black interspaces and leave the white lines in 

 their purity and regularity ? Yet this is just what the engraver 

 must do who would reproduce on steel this figure ; and, we repeat, 

 this is far less elaborate than those in actual use on bank notes. 



The tool which cuts these delicate lines is made of the finest 

 steel ; made very hard and very carefully tempered. It passes 

 through each line of the cutting about twenty times, cutting 

 down about one three-thousandth part of an inch each time, then 

 continues going over the line about fifty times more to clean out 

 the burr and polish the work. The machine must, therefore, 

 work with perfect precision, for a deviation of one tenth part of a 

 hair's breadth would destroy the whole cutting. 



On account of this required precision, these lathes are made 

 adjustable in every part that any loss-motion caused by wear 

 may be taken up. They are so sensitive as to be affected by a 

 sudden change of temperature in the room ; and if a partially 

 finished line should be entered by the tool after such sudden 

 change, the result would be a defective piece of work. 



We may watch one of these machines for hours and each mo- 

 ment discover some new movement. In the hands of a skillful 

 operator (for, after all, the machine itself, to produce the required 

 effects, must be under the direction of human intelligence) it 

 will produce almost any form desired. 



There are only three or four first-class operators of these 

 machines in the world, and they are all Americans. Only one of 

 these is able to make a cutting from a pencil sketch and figure 

 out the required combination of wheels, set the lathe, and know 

 it is properly set and adjusted before turning a wheel. The turn 

 of a screw, the substitution of one wlieel for another with the 

 variation of a single cog the shifting of the axis of an eccentric, 

 will produce an entirely new effect ; it may give distortion where 

 perfect regularity is demanded; therefore a perfect and long 

 familiarity with the machine is necessary to the successful han- 

 dling of it. 



