24 Prof. J. Joly on Scientific 



before collision can occur." The second determination of 

 distance compared with the first tells him whether the actual 

 rate of approach approximates to the danger rate of approach. 

 If it does, and continues to do so on a few more observations 

 by synchronous signal, one of the ships must give way 

 in good time. 



In short, the procedure whereby collision may be averted. 

 in all weather involves : — (a) exchange of signals, wireless 

 or submarine, giving courses and speeds : (6) finding from 

 these data the "danger bearing" and "danger rate of 

 approach " : (c) ascertainment of the actual bearing and rate 

 of approach. 



These operations, even if called for in their completeness, 

 are simple and easily carried out : characteristics of value 

 under conditions which may involve hurry and anxiety. 

 It is necessary now to consider the successive steps more in 

 detail and to enter briefly on the principles upon which the 

 operations are based. 



What are the conditions determining collision? Suppose 

 ships A and B are moving on paths which intersect. Then 

 the conditions for collision involve that A and B are, at a 

 given instant, at distances A and BO from the point 

 of intersection, 0, such that their speeds will carry each ship 

 over the respective distances AO and B in the same 

 interval of time. In other words courses, speeds, and 

 positions are involved. When these three factors are such 

 as to lead to collision then is the following important 

 condition fulfilled : — the direct distance between the ships 

 will decrease at the maximum rate possible for the given 

 courses and speeds. In other words, the relative velocity is 

 a maximum for the courses and speeds. This is evident, for 

 its entire velocity is then carrying each ship directly towards 

 the other ship at the only point where they can meet : that 

 is, the point of intersection of their courses. 



We may reverse the steps of our reasoning and say, if the 

 relative velocity of the vessels is the maximum for the 

 courses and speeds, then is collision sure to occur, and if it 

 is not the maximum, collision cannot occur : the ships will 

 pass clear. 



In order to find the maximum relative velocity or " danger 

 rate of approach " of the ships, knowing the courses and 

 speeds we may construct a triangle of velocities. Two of 

 the sides of the triangle are parallel with the courses ; and 

 the lengths of these sides are proportional to the speeds of 

 the ships. The third side, completing the triangle, gives us, 

 now, by its direction the bearing of A from B and of B from 



