22 Prof. J. Joiy on Scientific 



communication of these data is a simple matter. It forms a 

 familiar part of the preliminary correspondence of a ship 

 station with a coast station after the latter has been called. 

 Both are numbers. The course is given in degrees from 0° 

 to 360° reckoned from North round by E. S. and W. ; i. e., 

 clockwise. The speed is signalled in nautical miles per 

 hour. The signals are emitted at the rate of some 20 words 

 per minute, or a word of five letters in 3 seconds. Thus, to 

 fix our ideas, we may suppose that ship A learns that ship 

 B is proceeding South (180°) at 16-J- knots, and B learns 

 that A is holding a course NE. f E. (say 53°) at a speed of 

 11 knots. 



Additional to these means of dealing with the problems 

 presented by methods of averting collision, it must be recalled 

 that in the radio-goniometer or wireless compass and in 

 submarine signals the mariner possesses a means of finding 

 the bearing of another ship with approximate accuracy. 



Now there are four criteria which enable the sailor to say 

 in advance whether a particular ship in his locality, but 

 assumed to be quite invisible to him, is moving so as to 

 collide with his ship or whether she is not. Let us first 

 write down what these criteria are. 



If two ships, A and B, are moving so as to collide : — 



(1) The mutual bearings of the ships are determined by and 



deducible from the courses and speeds of the vessels. 



(2) The rate of mutual approach of A and B — i. e., the 

 relative velocity — is fixed and determined by the courses 

 and speeds and is the maximum possible for these 

 courses aad speeds. 



(3) The bearing of ship from ship is constant and invariable 



up to the moment of collision. 



(4) The rate of mutual approach remains constant up to 



the moment of collision. 



It is convenient to first consider those two criteria which 

 are dependent upon and deducible from the prevailing courses 

 and speeds ; i. e., the bearing which indicates threatened 

 collision and the relative velocity which indicates threatened 

 collision. How may these two criteria be used by the 

 mariner ? The matter may be stated thus : — 



With the knowledge in his possession of the course and 

 speed of each ship the navigator, by simple methods to be 

 presently described, determines what the bearing of the 

 ships from each other must be if collision is threatened and 

 what the relative velocity or rate of mutual approach of the 

 vessels must be if collision is threatened. These criteria are 



