CHAP. XVII.] REDUCING PLANS 375 



sextant has also been employed in triangulation, should cer- 

 tainly be so. The " points " are necessary to join one chart 

 to another, and also, in case of future revision of the chart, 

 they afford means to the reviser of measuring the accuracy 

 of his predecessor's groundwork. 



Plans sent home by officers in general service ships often 

 lose much of their value from neglect of this. The existence 

 of the " points," and their proper position, will at once give 

 a confidence in the detail of the plan, that it is impossible to 

 accord to the work of an officer, however zealous, of whom 

 nothing is known as to his hydrographical capability, and Mho 

 fails to give any indication in his chart of how it has been 

 constructed. 



REDUCING PLANS. 



In a survey of an extensive nature, bays, harbours, etc., ^\■ill 

 often be done on a larger scale than the rest of the sheet. These 

 must be either left blank on the coast sheet, or else reduced 

 from the large-scale plans. 



It may sometimes happen that a portion of an anchorage is 

 surveyed on the small scale before it is decided to make a large 

 plan of it, on discovering it to be worth while to do so. This 

 must not appear, however, on the completed chart ; it must be 

 all reduced from the larger scale. 



Instruments for reducing — e.gf.jeidographs — are not supplied, Reduction 

 and the reduction is accomplished by " squaring." ing. 



This consists of ruling similar lines on both sheets, forming 

 squares and diagonals all over the part to be reduced. 



The two stations farthest apart on the plan, which raust 

 also be plotted on a small-scale chart, are joined by a line on 

 both sheets, the " directing line." Then, taking the smaller 

 first, divide this line into as many equal parts as is thought 

 necessary. These parts will be from j to g inch long. Set 

 off lines at right angles to the directing line from each point 

 measured, and then lines parallel to the directing line, at the 

 same distance apart as the others. The portion of the sheet 

 required is now covered with squares. Rule also the diagonals. 

 These will check the correctness of the squares, as they should, 

 of course, pass exactly through each corner. 



