Signalling and Safety at Sea. 29 



And there is another reason why this difference between 

 danger entries and safety entries in- the table will increase. 

 We must anticipate here the reasons for the 4th criterion 

 denoting collision. It is easy to see that only in the case of 

 coming collision is the velocity of approach constant. If we 

 place A in the centre of a series of concentric circles described, 

 say, at radial distances of one knot ; then if collision is 

 coming B is traversing these circles radially in its advance 

 towards A and her velocity of approach is constant. But if 

 B is not aiming for A she is not advancing radially. Her 

 velocity of approach towards A cannot be constant. When 

 she is far from A her rate of approach to A is greater than 

 when she draws near. When she gets abeam of A there is 

 a moment when there is no further diminution of distance. 

 The relative velocity is then zero. After this B begins to 

 recede from A. 



Now this must come out in the successive observations of 

 distance between A and B and will tend to further accen- 

 tuate the difference between danger and safety readings of 

 distance. This cumulative and increasing distinction in the 

 character of the two sets of figures— those which have been 

 written down on the assumption that collision is threatened 

 and those which come in if it is not threatened — is a feature 

 of much value. It tends to redress the want of sensitiveness 

 of the readings and to distinguish true from fictitious safetv. 

 Danger readings cannot be confounded with safety readings 

 as observations multiply. 



There are advantages in keeping the observations on paper 

 as described above. There must be ample time for several 

 observations if a good look-out has been kept. The obser- 

 vations are actually made by the wireless operator. The 

 Officer of the Watch has only to write them down. His own 

 ship's course and speed are already entered. The fact that 

 the siguals are sent out and read by one individual, who has 

 no responsibility beyond reporting them, and that they are 

 interpreted by another, is an element of safety, for it leaves 

 each operator free to give his attention to one matter only. 



The Collision Predictor is intended to give the navigator 

 the means of following the approximation of the two ships 

 step by step as the readings of distance come in, and per- 

 mitting him to appreciate the imminence of danger in 

 case of threatened collision by merely looking at the 

 instrument. 



These functions it accomplishes in virtue of the fact that 

 if the cursor is slipped along the limb a, keeping the arm c 

 clamped on the cursor at the angle determined by the first 



