CHAP. XVII.] DELINEATION, SYMBOLS, COLOURING 383 



In the case of extended surveys, or when there is no regular 

 triangulation, the scale will depend upon the distance obtained 

 between two stations by astronomical observations. Tliis dis- 

 tance being calculated, the scale will be obtained as before. 



The soundings in the chart sent home should be as thick as Soundings 

 possible, without sacrificing legibility. There is always a great "^^^c^- 

 temptation to thin them out, so as to look better ; but that is 

 the work of the Hydrographic Office, and will probably have to 

 be done again there in any case, as the scale on vv hich the chart is 

 published is usually smaller than that on which it is constructed, 

 and if so, will not permit all soundings in the original to appear. 



The natural scale, or the proportion which our chart lineally natural 

 bears to the actual size of the portion of the globe it represents, ° * 

 is obtained by dividing the number of inches corresponding to 

 one mile on our chart, obtained as above, by the number of 

 inches in the nautical mile at the latitude. It is given in the 

 form of a fraction, whose numerator is one. 



Thus, supposing our scale is found to be 1-8 inches to a mile, 

 in latitude 3°, we divide 72,552 (the number of inches in a mile) 

 by 1-8. 



1 



This gives as the natural scale. 



40,306 



This natural scale should be noted on all sheets that are not 

 graduated. 



When the chart includes a considerable extent of coast-line 

 that is intended to form part of a navigational sheet, it will have 

 eventually to be redi^awn on Mercator's projection, as it is on 

 that projection all charts are published. 



To do this, the sheet must be graduated — i.e., have the 

 meridians and parallels placed upon it, as it is by means of 

 them that a chart on one projection is redraw n on another. 



GRADUATION OF THE SHEET. 



We have before said that a chart constructed by drawing Gnomonic 

 right lines from one object to another, when graduated, has to ^^q^^^' 

 be considered as being on the gnomonic projection, and the 

 general features of this projection have been explained.* It 

 now remains to consider how to graduate such a chart. 



* Taee 98. 



