Signalling and So frig at Sea. 25 



A, and by its length the relative velocity of A and B. The 

 conditions are now the same as if A was at rest and B 

 moving with this velocity towards A. Evidently the 

 distance between them will diminish at the maximum rate, 

 for B is moving straight towards A. But this procedure for 

 finding the maximum rate of approach is one which we 

 cannot expect the seaman to carry out in the urgent circum- 

 stances of his position. We suppose, instead, that lie is 

 provided with a simple instrument which may be named 

 a Collision Predictor. 



This instrument (PL I. fig. 4) consists of a circle upon 

 which compass bearings and angles measured from N, clock- 

 wise, are engraved. It carries two limbs, a and b, which 

 rotate independently about the centre : which are divided 

 to read speeds in knots per unit time ; and which can be 

 clamped in any position. An arm, c, is pivoted upon a sliding 

 piece or cursor, which can be slipped along the limb a. 

 This arm carries centrally a transparent divided scale, as 

 shown. 



When the sailor is given the courses and speeds he 

 proceeds as follows: — One limb, say a, he sets round to the 

 course of his own ship A. The other, b, he sets to the 

 course of the other ship B. He then slides the cursor along 

 a till it reads on a a number which is proportional to the 

 velocity of his own ship A. He next inflects the arm c, so 

 that it intersects the limb b at a distance from the centre 

 proportional to the speed of the otlier ship B. He has now 

 constructed his triangle of velocities, and he reads on the 

 transparent scale of the arm c the relative velocity he seeks: 

 that is, he reads on it the relative velocity when collision 

 is threatened : which, as we have seen, is the maximum for 

 the courses and speeds. 



It is convenient to read on c the rate of approach or 

 relative velocity in terms of the amount by which the direct 

 distance separating the vessels diminishes in two minutes; 

 or one minute, according to the interval separating the 

 observations of distance by synchronous signalling. 

 I assume that this is 2 minutes. Then, reverting to our 

 example, having set the limb a NE.fE., and the limb b due 

 South and slipping the cursor along a till it reads the speed 

 of A — i. e., 11 knots — and inflecting c to read on b 16| knots 

 (i. <?., the speed of B), the navigator finds that the scale on c 

 is cut by its intersection with b, at the reading 0'8. What 

 is this? It is the distance in knots by which the ships A 

 and B, in our example, must approach towards one another 

 in two minutes if collision is threatened, i. £., eight-tenths of 



