4 Prof. J. Joly on Scientific 



signals propagated through the atmosphere. They are 

 difficult to pick up in stormy weather ; more especially when 

 the wind is blowing from the ship towards the source of 

 sound. The sound appears to be weakened by these 

 conditions, and if the wind is making much noise on the 

 ship it may not be heard at a sufficient distance. It has to 

 be listened for in the open. A more serious objection is the 

 strange and, fortunately, not very common phenomenon of 

 silent areas as referred to in the Admiralty Sailing Directions 

 quoted above. A ship may be well within the range of 

 audition ; the weather may be calm, even windless, and the 

 sound may be mute over certain areas. The phenomenon is 

 a remarkable one, and a full explanation cannot be said to 

 exist. It has been investigated by Tyndall and by Lord 

 Rayleigh. The nature of the sound seems without influence. 

 Even the very beautiful elliptic trumpet of Lord Rayleigh, 

 whereby the sound is caused to spread laterally and its 

 vertical dissipation prevented, cannot counteract the evil 

 when the necessary conditions prevail. There appears to be 

 -such a deviation of the sound as causes it to rise and arch 

 over certain areas. It may be heard ten miles from the 

 source and be entirely mute close to it. Or, again, when 

 approaching the source we may find more than one silent 

 area as if the sound waves followed a sinuous path, rising 

 and again sinking to the surface of the sea. In such cases 

 the value of the synchronization may be lessened in another 

 way. The sound which is heard outside a mute area will 

 not have travelled directly from its source. The question 

 is : how much has its journey been lengthened ? Probably 

 the increase of distance is not much. Nevertheless there 

 may be appreciable error here. Again^ the use of light- 

 flash has, of course, the drawback of being invisible in fog 

 or thick weather. Hence only under certain conditions 

 and at certain times is the combination of synchronized 

 light and sound signals of value. 



Notwithstanding these limitations such a system would 

 undoubtedly prove very useful. To condemn it in advance 

 is as senseless as to condemn all our lighthouses and fog-signal 

 stations because conditions arise when they are useless. And 

 it should be considered by all responsible authorities if, for 

 the general use of small craft — fishing boats, small coasters,, 

 and the like— a system of buoyage based on light-flash and 

 bell -stroke would not be valuable! We may profitably 

 consider here, before going further, how such a system may 

 be worked so as to meet the requirements of untrained 

 observers. 



