102 SCIENCE PROGRESS 



an angle less than the possible error of the method in practice, 

 the officer knows that there is risk of a collision, and can also 

 predict the instant of its occurrence if speeds and courses are 

 unchanged. Of course it is assumed that both ships are utilis- 

 ing the method, and that both are forewarned of the probability 

 of a collision, and have ample time to take measures against 

 its occurrence. Prof. Joly claims that the degree of skill or 

 intelligence called for in timing the receipt of the signal is 

 not so great as to limit the timing of the arrival of the signals 

 to the more highly educated seaman ; and indeed most people 

 are aware that a very little practice with a stop watch will 

 give all the requisite skill. If the times of the signals and the 

 communications by wireless are then given to the navigating 

 officer, the graphical constructions which he has to carry out 

 are not in any way more troublesome than many with which 

 he is familiar in laying the course of his ship. Specially ruled 

 and prepared paper will considerably simplify the operations, 

 and enable him to reach a conclusion in a few seconds. Indeed 

 Prof. Joly describes a little instrument consisting of a protractor 

 and three suitably graduated scales, which renders all the 

 operations exceedingly simple, and enables a judgment of the 

 risk to be formed, and the time of a possible disaster predicted, 

 with great ease and promptitude without recourse to drawing 

 instruments at all. The limits of error will involve a certain 

 latitude in each determination of distance apart ; but the 

 possible error in this observation will be known, and at all 

 events each observation is independent of its predecessor as a 

 determination of distance, and so the errors will not accumulate. 

 Experience with the instrument would reveal how far its pre- 

 dictions that another vessel is going to pass clear of his own 

 boat could be trusted by an officer. The possibility of making 

 five or six independent judgments in the space of ten minutes 

 or so would obviously narrow the errors to close limits. The 

 speed of the operations involved is such as to permit the 

 navigator to deal with several vessels in his vicinity. 



In the Phil. Mag. for March Prof. Barkla and Miss J. G. 

 Dunlop describe some experiments recently carried out by 

 them on the scattering of X rays, and certain conclusions con- 

 cerning atomic structure which they seem to warrant. Experi- 

 ments carried out by Prof. Barkla some twelve or thirteen years 

 ago showed that when Rontgen radiation is incident on a 



