RECENT ADVANCES IN SCIENCE 101 



It is remarkable that these relations, which are surprisingly 

 accurate, have so long escaped notice. It seems hardly possible 

 that they can also be merely chance arrangements valid only 

 at the present time, yet there seems no room for them in 

 planetary theory. It remains to be seen what further light 

 investigation can throw upon these relations. 



PHYSICS. By J. Rice, Lecturer in Physics, Liverpool University. 



In the Proc. Roy. Soc. for January and February, Prof. Joly 

 describes a very simple and elegant method by which distances 

 of ships from shore or from each other may be estimated in 

 fog or thick weather, and so the danger of collision considerably 

 minimised. The method depends on the kinematic fact that 

 if the bearing of one ship to another has the same direction as 

 the velocity of the second ship relative to the first, then there 

 will be a collision ultimately if the two ships maintain their 

 present courses and speeds unchanged. Dr. Joly proposes that 

 each ship should be fitted with wireless, by which it could in 

 foggy weather periodically send out code signals giving its 

 course and speed. In the second place it should also have 

 some means of emitting crisp, and sufficiently loud, sounds in 

 air, which can be timed to coincide in moment of emission with 

 wireless signals, and as long distance wireless signals are not 

 required, the wireless installation need not be of great power 

 or excessively costly. An officer on a ship would receive from 

 another ship in his neighbourhood two signals, one practically 

 received at the moment of emission and the other received 

 some few seconds after the first ; this interval and the known 

 velocity of sound in air determines the distance apart of the 

 boats at the moment. With a more elaborate installation, one 

 might use submarine signals as well. Suppose the signals are 

 repeated at a stated interval, say two minutes, later, the officer 

 can again determine the distance away, course and speed of 

 the ship he wishes to avoid. It is now a comparatively simple 

 exercise in the drawing of " vector diagrams " to determine 

 from the double set of signals and from the course and speed 

 of his own ship, both the bearing of the second ship to his own 

 and also his relative velocity to the other vessel. If the direc- 

 tions of these two vector quantities are not the same, there 

 will be no risk of collision ; but if they are alike or differ by 



