4852 HYDROGRAPHIC MANUAL PaGE 434 



4852. Erasing Machine 



An electric erasing machine is of particular value in making small erasures on 

 hydrographic sheets, especially where it is necessary to remove inked detail from small 

 confined areas, since a single soundmg or number may be erased without disturbing 

 adjacent detail. It may also be used, if necessary, to remove extensive inking from 

 sheets. 



Erasing machines are manufactured by a number of commercial firms and vary 

 in type from an elaborate assembly of electric motor and stand with foot rheostat 

 speed control and flexible cable to which the eraser is attached, to a small inexpensive 

 electric motor with a screw chuck attached directly to one end of the shaft to hold 

 the eraser. The small-type erasing machine is held in the hand and is provided with 

 a starting switch on the motor. The machines are made for 110- and 220-volt a-c 

 or d-c current. Erasers are made in several different grades: a soft pliable grade for 

 erasures of pencil lines and for use on tracing paper, a medium soft grade for pencil 

 or light ink lines on thin drawing paper, and a hard abrasive grade for heavy ink lines 

 on heavy paper. 



The effectiveness of an erasing machine is proportional to the speed of rotation of 

 the eraser and does not depend so much on the pressure applied to the paper. Pre- 

 caution is necessary to avoid wearing a hole through the paper. The correct technique 

 is to move the end of the rotatmg eraser lightly back and forth or arovmd the surface 

 to be erased. The abrasive eraser should be used with extreme caution and only as a 

 last resort after a softer grade has failed to remove the ink. 



4853. Pantograph 



A pantograph is an instrument designed to reduce or enlarge maps or drawings to 

 any desired scale. Its construction is based on the principle of similar parallelograms, 

 the essential condition of its design being that three points, one a fixed pivot and the 

 other two movable tracing points, must be in perfect alinement, and each point must 

 be on one of three different sides (or sides produced) of a jointed parallelogram. 

 The ratio of the reduction or enlargement from the original is then equal to the ratio 

 of the distances of the movable points from the fixed pivot. 



Pantographs var}^ from an expensive suspended precision type of all-metal con- 

 struction to an inexpensive instrument of four wooden bars fitted with metal joints. 

 Its principal use in the Co.ast and Geodetic Survey is in the reduction of topographic 

 and hydrographic surveys. The instruments used iii the Washington Office are of the 

 precision type with three of the four metal bars graduated. Different scale settings 

 are made by moving the graduated cross bar identical amounts on its supporting arms 

 with a corresponding movement of the tracing point support on that bar. 



To make reductions, the original is traced with the point farther from the pivot, 

 the nearer point following its movement to draw a reduced replica of the original. For 

 use in chart construction the reductions made in the Washington Office are carbon 

 impressions on transparent paper. A carbon paper is placed, carbon side up, beneath 

 the transparent paper and as the metal point on the cross bar moves, the reduced 

 detail is impressed on the underside of the transparent paper. The detail is then 



