30 Prof. J. Joly on Scientific 



setting of c, the gradual mutual approach of the ships is 

 traced out on the scale carried by the arm c at its intersection 

 with the scale on b. Thus, suppose when the second reading 

 is made we move the cursor along a a total distance cor- 

 responding to the run of A in 4 minutes, then the arm c 

 shifts on the limb b to the corresponding run of B (as it 

 moves parallel to itself) and twice the danger distance is 

 read on c *. Now if we have a mark on the arm c at the 

 reading of the initial distance separating the vessels — i. e. 

 5 miles — then as we shift forward the cursor at each 

 synchronous signal, that is to say at intervals of two 

 minutes, we can by simply looking at the reading on c at its 

 intersection with b, observe upon it the total distance which 

 has so far been run and the distance which remains to be 

 run if collision is really threatened. For example, if collision 

 is actually approaching, in the first position of c we read on 

 it 0'8 knots as run and 4*2 knots still to run. In the second 

 position 1*6 knots run and 3*4 knots to run, and so on. The 

 navigator compares these readings one by one with the 

 actual readings of distance coming in. If they show a ten- 

 dency to sustained agreement he knows for certain there is 

 danger, and by a glance at the scale on c he infers how 

 much time remains before collision can occur. He has, in 

 the still remaining length of the scale on c, a visible and 

 tangible indication of the time left to him for action ; and 

 with each reading a simple and definite mechanical opera- 

 tion has to be effected which necessitates his attention being- 

 fixed on the lie of the two ships relatively to each other. 

 // lie holds the Collision Predictor in its true compass position 

 he sees at once the direction in which the other ship is 

 approaching, this being the direction in which the arm c is 

 pointing, and at the same time he has indicated on the arm c 

 at once the distance separating the ships and the time taken 

 to cover this distance. It is like as if he followed the approach 

 of the vessels upon a chart. 



Incidentally we may observe here that the Collision 

 Predictor finds a use even in clear weather for averting 

 collision. For suppose two ships sighting each other and, 

 uncertain as to the risk, exchange courses and speeds. Then 

 the Predictor tells immediately the danger bearing. If the 

 ships show that bearing towards one another then is collision 

 threatened. Otherwise there is safety. 



The foregoing description, embodying as it does an account 



* This is the position of e as shown in the figure. 



