28 



Prof. J. Joly on Scientific 



of the signals is automatic. On receiving the third signal B 

 says " our distance is 3-J miles." This also is evidently quite 

 in keeping with the danger rate. There is now 8 minutes to 

 go. On the 4th signal the distance falls, we will suppose, to 

 2*5 miles with 6 minutes to go. The danger may now be 

 regarded as established. But there is no reason why further 

 signals should not be exchanged, before B gives way to A. 



The successive observations of distance may be recorded 

 on paper as they come in, and be compared with the successive 

 danger distances written down upon the finding of the 

 initial distance. Or they may be observed and followed 

 one by one on the Collision Predictor. Taking the former 

 method first we may suppose a ruled sheet with columns 

 set out to take the figures thus : — 



Course A 53°. Course B 180°. 



Speed A 11. Speed B 16J. 



Danger Bearing NNE. tObs, Bearing NNE. 

 Danger Rate 0'8. 





Danger 



Observed 





Sig. 



Distance. 



Distance. 



T. 



1 





.. 5 



12 



2 



... 4-2 



.. 4 



10 



3 



... 34 



.. 3*5 ... 



8 



4 



... 2-6 



.. 2-5 ... 



6 



5 



... 1-8 



.. 2-0 ... 



4 



6 



... 1-0 



.. l'O ... 



2 



7 



... 0-2 









8 









9 









10 









In the two minutes interval between the 1st and 2nd 

 observation of distance the Officer of the Watch fills in 

 columns 2 and 4. 



We assume here readings typical of threatened collision. 

 If collision is not £oinor to occur the observed distances will 

 disagree with the danger distances already written down. 

 In what way will they differ ? We have seen that the 

 danger rate is the maximum for the courses and speeds. If 

 then collision is not threatened the vessels will be approaching 

 more slowly than if collision is threatened. The difference 

 between the rates will give rise to a cumulative increase in 

 the distance separating the vessels. As the observations 

 progress it will be found that danger distances and safety 

 distances differ more and more wit 



