4811 HYDEOGRAPHIC MANUAL PaGE 420 



4811. Ordinary Dividers 



The type of ordinary dividers to be used in plotting hydrographic sheets depends 

 on the distance to be measured. 



For distances less than about 2 centimeters, the 3K-inch bow-spring dividers are 

 most convenient. This type is constructed from a single piece of spring steel, the ends 

 of which are tapered to form the points. The dividers are sprung so that unscrewing 

 a thumb nut on a threaded spindle connecting the legs increases the distance between 

 the points. The distance is shortened by taking up on the thumb nut. In making 

 large changes in setting, the legs should be compressed with the fingers, while the thumb 

 nut is being set. This ensures minimum wear on the threads and speeds the operation. 

 As the instrument is made of spring steel, the points are fragile and unless used carefully 

 are apt to be broken near the ends. 



For measuring distances greater than 2 centimeters, 6-inch hairspring dividers 

 (D in fig. 91) are generally used. These are made of two separate legs of stainless steel, 

 held together by a pivot-joint assembly which provides the proper friction for smooth 

 manipulation by means of a small screw and locknut on each side. One leg of the di- 

 viders is fitted with a spring and thumbscrew, providing a slow-motion adjustment 

 of about 9 millimeters in setting the distance between the points. 



Hairspring dividers are set for the correct friction by skilled instrument makers 

 and it is inadvisable for the field party to attempt adjustments. A special wrench is 

 needed which is not ordinarily available in the field. The use of other tools will usually 

 mar the pivot screws and will seldom accomplish the correct adjustment. When 

 dividers become loose or unserviceable they should be returned to the Washington 

 Office for repairs. 



Divider points that have become dull or uneven in length may be sharpened or 

 made even by grinding on a hard oilstone. The points should first be made of even 

 length by placing the legs together and grinding them lightly back and forth against 

 the oilstone, while holding the dividers vertically. Each point should then be sharpened 

 by whetting the outside of it back and forth, at the same time giving the leg a rotary 

 motion by turning it with the fingers. The inside of the leg should remain flat and should 

 not be ground. Neither should any part of the outside of the point be ground so that 

 a flat surface will result. When the shape of the point is nearly correct, great care is 

 necessary to avoid shortening the length of the leg. Dividers with points that are 

 bent or badly damaged should be returned to the Washington Office. 



The hydrographer should take special steps to protect his dividers. They should 

 never be used for such purposes as removing corks from bottles or for marking celluloid 

 or metal. Dividers should always be secured on a plotting table on board vessels, 

 especially launches, so they cannot roll off the table. The points of dividers may be 

 protected by sticking them into a Small piece of soft rubber eraser, or cork; this is 

 also advisable when carrying dividers in the pocket. 



4812. Proportional Dividers 



Proportional dividers {F in fig. 91) are instruments used chiefly to transfer details 

 between sheets of different scales. They consist of two legs of equal length, pointed 

 at each end and held together by a movable pivot. By varying the position of the 

 pivot, the lengths of the legs on opposite sides of the pivot may be adjusted so that 

 the ratio between them is equal to the ratio between the scales of the two sheets. 



