Signalling and Safety at Sea. 33 



being watched is a serious objection to the use of submarine 

 signalling for the purpose now in view. refer to these 

 difficulties because prima facie one might think that by 

 merely determining the constancy of bearings by wireless 

 goniometer or submarine signals the danger of collision can 

 be always foretold and averted. 



The principle involved in this method is familiar to every 

 sailor. If the bearing of B from A is steady, collision is 

 threatened. Represent the courses of A and B by inclined 

 lines, intersecting at 0. Now suppose that A runs the dis- 

 tance A in the time that B runs the distance B : then 

 collision must occur. And it follows that when A has run 

 half the distance A 0, B must have run half the distance B 0. 

 If we join the simultaneous positions of the two ships we get, 

 therefore, for the bearings lines which are parallel. That 

 is, the bearing of ship from ship remains constant. 



Even when used in clear daylight it is often objected to 

 this method : — " While you are looking for a change of 

 bearing the ships may collide." And the answer sometimes 

 given is "there is no other method." Now, whatever may 

 be said against this objection when the sailor can guess 

 his distance from the other vessel by visual observation, it 

 must be remembered that the proposed use in this way of 

 the wireless goniometer leaves the sailor without any reliable 

 estimate of distance. For the strength of the wireless 

 signals must afford but a doubtful estimate of distance. 

 They suffer not alone from instrumental sources of variation 

 but from variations, due to atmospheric causes, quite out of 

 the control of the operator to alter or to predict. See on 

 this subject, more especially, the observations of Admiral 

 Sir Henry Jackson which showed a capricious reduction 

 of carrying power from 65 to 22 miles ; or again, those of 

 Captain Wildman who obtained frequent successive varia- 

 tions in audibility of I to 5 and rising to 1 to 10 or even 

 more. The cause of these variations may most probably be 

 traced to changes in the conductivity of the atmosphere due 

 to mist, spray, etc., whereby variations in the absorption of 

 the energy of the electric waves are brought about. Judging 

 from these observations any dependence on the strength of 

 wireless signals as giving an estimate of distance must be 

 attended with danger ; just as much as judging the distance 

 of a light by its brightness. 



The wireless compass may indeed under certain conditions, 

 in addition to other inestimable benefits conferred by it 

 upon the sailor, be a valuable means of discriminating 

 between safety and danger. For if there is a rapid 



Phil. Mag. S. 6. Vol. 36. No. 211. July \ 918. D 



