Page 421 equipment and instruments 4813 



Hence, a distance spanned by the points of one set of legs has the same relation to the 

 distance spanned by the points of the other set as the scale of one sheet has to the 

 scale of the other sheet. 



A thumb nut mores the pivot in a rack-and-gear arrangement. When the desired 

 setting is reached, a thumb-nut clamp on the opposite side of the instrument locks 

 the pivot in place. A scale and vernier are provided on one leg to facilitate accurate 

 setting. The dividers may bo set by reference to the table of settings which is furnished 

 with each pair; they will accommodate varying ranges of scales up to 1 to 11.5. 



For use on hydrographic sheets the dividers must be set by test to the exact ratio . 

 The table of settings may be used for an approximate setting but further adjustments 

 will usually be found necessary to take into account distortion in the sheets. The cor- 

 rect adjustment is usually reached by trial and error between corresponding projection 

 lines on the two sheets. When the correct setting has been obtained it should be read 

 from the scale and recorded for future use. 



The points of the dividers are of hardened steel and if handled carefully will retain 

 their sharpness during long use. If damaged they may be sharpened and adjusted 

 without affecting the accuracy of the instrument, but the table of settings will no 

 longer be exact. 



4813. Spacing Dividers 



Spacing dividei-s (E in fig. 91) are specially constructed with multiple legs arranged 

 so as to subdivide the total distance spanned into a number of equal parts and are used 

 by the Coast and Geodetic Survey particularly for spacing soundings between positions 

 on the smooth sheet. The size most frequently us3d is about 6 inches long and contams 

 11 points numbered consecutively from to 10. With these dividers distances up to 9 

 inches may be divided into 10 eq«al intervals. 



Spacing dividers should not be handled roughly and particular care must be taken 

 to ensure that the points are not injured or bent. Should the pivot become too loose 

 or too tight the mstrument should be returned to the Washington Office for repairs. 



To plot equally spaced soundings on the hydrographic sheet, the space between 

 successive positions is subdivided by placing the zero point of the dividers on the 

 first plotted position and the numbered point corresponding to the number of 

 intervals desired on the next position. The sheet is touched lightly with pencil at 

 each intermediate point to indicate the position of each sounding. 



If the smallest interval obtainable with the dividers is greater than the distance 

 between adjacent soundings, the positions of every second or third sounding may be 

 located with the dividers and those of the intermediate soundings estimated. 



Where soundings are not equally spaced or where positions are not obtained exactly 

 on the minute, the dividers should be set to account for the elapsed time between posi- 

 tions rather than for the number of soundings. The soundings may then be plotted 

 by the time interval from the first position. 



For example, if a position was taken at 9:01 and the next at 9:05i-^, the elapsed time of m minutes should be set on the dividers 

 by placing the point numbered zero on the first plotted position and adjusting the dividers until the second plotted position falls half- 

 way between points 4 and 5. If soundings were taken at 9:02, 9:03, and 9:04, the divider points numbered 1, 2, and 3 will indicate their 

 positions. If a sounding was at 9:04:15, a dot placed at one-fourth the space between points 3 and 4 will indicate the correct location 

 of the sounding. If a position was taken on a half-minute and soundings were taken on whole minutes thereafter, the dividers are 

 first spaced as above to account for the elapsed time between positions, then moved back one-half space so that point number 1 on the 

 dividers will indicate the position ofthe first sounding. If the scale of the sheet is large and a 1-minute interval is too long to be spanned 

 by adjacent points, the dividers may be set to intervals of 30 seconds or 15 seconds and the same general method used. 



