I 
Set to 1200 Yds 
along the line GP will differ but slightly from the true speed, and may 
be taken as such, as shown in the following table :— 
TABLE II. 
Value of v. 
Time for range to 
change by 100 yds. 
Value of v. 
Time for range to 
change by 100 yds. 
14 
6-9" to 7-4" 
10 
9'5" to 10 # 5" 
13 
7-4" to 8" 
9 
10-5" to 11*7" 
12 
8" to 8-7" 
8 
11-7" to 13-3" 
11 
8-7" to 9-5" 
7 
13-3" to 15-4" 
It is convenient to have a copy of this table on the outside cylinder. 
In many cases the distance G A will not vary considerably from the 
perpendicular distance to the “ fairway,” as marked on the Admiralty 
charts, and if there is no means of determining the exact point the 
ship is making for, this value might, in the first instance, be selected : 
small inaccuracies in evaluating G A are of no great moment when the 
ship is at a distance, as is shown by the appearance of the line /*//, as 
compared with a v a/, in Fig. 3, and as she approaches, the eye is able to 
determine, the part of the channel, she is in. 
If the position-finder is properly installed, with a chart beneath it, 
or if certain modifications, proposed for the depression range-finder, 
are adopted, the distance of G A (Fig. 1) can be obtained, with much 
accuracy, by either instrument; in many cases, however, a good know¬ 
ledge of the water way will render this unnecessary. 
The drum is set as follows : when G A and v are known, the inner 
cylinder is turned until the column marked with the proper value of v 
