462 HYDROGRAPHICAL SURVEYING [chap. xx. 



Angle of depression for 10 miles — take the mile 



=6,080 feet: 

 Dist. for 10 miles, 



60,800 feet - - log. 4-784332 

 Perp. 500+dip 88 = 



588 feet - - „ 2.769377 



sin 7-985045=0 33 13 



- 50 Ref . = iVth 



distance. 



Angle of depression would be 



from observer - - - 32 23 



Worked out in a similar manner for distances of 9, 8, 7, 6, 

 5, 4, 3, 2, and 1 miles, the results will be found to be respectively 

 35' 10"; 38' 40"; 43' 18"; 49' 36"; 58' 38"; 1° 12' 20"; 1° 35' 30"; 

 2° 22' 13"; and 4° 43' 31". 



The results are plotted on curves, so that the distance for any 

 angle of depression up to 10 miles can be taken off. Two 

 curves on different scales are used, as a larger scale is required 

 for distances from 5 to 10 miles than for distances below 5 miles. 



GRADUATION OF A GNOMONIC CHART. 



By J. W. ATHERTON, Esq., Cartographer, Hydrograpuic 

 Department. 



The following method of graduating a gnomonic chart in- 

 volves, perhaps, a little more labour than that given on pp. 383, 

 et seq., but it has certain features which make it worthy of 

 consideration — viz., there are no very small distances to mea- 

 sure; each point of the projection, or each meridian, can be 

 obtained independently of the others; the longitude scales are 

 obtained indirectly, and thus are easily checked; the main 

 points are plotted and measured directly from the original 

 starting-point. 



The necessary data required are one geodetic position, the 

 meridian passing through that position and the scale of the 

 chart. 



