CHAP. XX.] PLOTTING WORK 461 



some attempt at adjustment may be made, but in plotting the 

 intermediate point C, whether by traverse from either end or 

 by the introduction of an arbitrary point, the discrepancy is 

 bound to show itself ; it cannot be got rid of, and the only thing 

 to be said is that, in plotting the points on either side of C, the 

 error, whatever it may be, is a diminishing quantity. 



PLOTTING WORK. 



Captain T. H. Tizard, R.N., C.B., in the Hydrographic 

 Annual, makes the following remarks, which may be useful 

 on occasion : 



One of the difficulties of j)lotting work is for the plotter to 

 interpret the sketch-books of all the officers employed in the 

 survey, more especially when their stations are of different 

 heights and there are many islands visible between the stations. 



Such a case occurred when the Challenger was surveying 

 Kerguelcn Island, and the manner in which the difficulty was 

 surmounted was by the officer plotting the work taldng a 

 station on the summit of Hog Island, in Royal Sound, and care- 

 fully obtaining his own height fu'st, from a base line measured 

 by sound between his station and a low-water rock on which the 

 gun was placed, and then by constructing a curve or curves of 

 angles of depression, and by taking angles of depression to every 

 low-water point seen (the rise and fall of tide bemg insignificant) 

 he was able to plot a great number of points, roughly, from his 

 own angles alone. His height was about 400 or 500 feet, and 

 was got by the mean result of an angle of depression to the 

 base rock and by an angle of elevation from the base rock. 



In calculating for the curve it is, of course, necessary to add 

 the dip for the distance of the point in the curve, and to sub- 

 tract from the result the approximate refraction to obtain the 

 correct angles of depression which would be obtained by the 

 observer. 



The following examples wiU show how it is done practically : 



Supposing the height of the observer to be 500 feet — 



The dip for distances of 10, 9, 8, 7, 6, 5, 4, 3, 2, and 1 miles 

 is respectively 88-25 feet; 72-48 feet; 5648 feet; 43-24 feet; 

 31-77 feet; 22 feet; 14 feet; 8 feet; 3-5 feet; and 1 foot. 



