METHOD OF SAFE NAVIGATION IN FOG. 119 



ceiving system should, then, be circles. The bell probably fulfils this condition, due 

 to the fact that when struck it divides into four vibrating segments separated by 

 four nodal lines. These vibrations are illustrated in Fig. 3, Plate 45. 



The receiving apparatus shown in Fig. i is defective in respect to its arc of 

 receiving sounds being restricted. The greatest range is obtained when the sound 

 wave is normal to the plate. As the angle of incidence becomes more acute the 

 range diminishes probably nearly in proportion to the projected area. Complete 

 data on this point have apparently not been obtained. This deficiency is utilized in 

 the commercial application of the receiving tank. By the use of two tanks, one on 

 either bow of a vessel, and by listening to the microphone on either bow succes- 

 sively, it is possible to ascertain on which side of the observing vessel the signal is 

 heard, or appears more intense. The direction of the bell or sending device can 

 then be approximately ascertained by altering the course of the vessel until the sig- 

 nals appear equally intense. The signaling station should then bear ahead if the 

 microphones are equally sensitive and the observer sufficiently skilful in estima- 

 ting the relative intensity of the sound. Fig. 4, Plate 45, illustrates the audibility 

 curve of an average vessel. The actual range is greater when stopped than when 

 under way, and is naturally better in a calm sea, due to absence of rolling and 

 pitching. 



The commercial adoption of the submarine receiving tanks and the installa- 

 tion of submarine bells upon lightships have unquestionably been of immense value 

 in saving life and property, and one of the greatest steps ever taken in advancing 

 safety at sea. At the present time nearly 100 bells are in operation on many coasts, 

 and over 1,000 vessels are equipped to receive their signals. 



While the value of this aid to navigation in fogs cannot be overestimated, it 

 has unfortunately covered only a part of the field necessary in safe fog naviga- 

 tion. Vessels equipped only with receivers are as helpless to avoid collision as be- 

 fore their installation. A vessel crossing the North Atlantic is exposed to col- 

 lision in case of fog during any part of her 3,000 miles cruise, while the receivers 

 would only be used during the few miles in passing the lightships. 



The problem of safety in fogs involves at least, as one feature, communica- 

 tion between ships. The bell for many reasons has never been adopted on sea- 

 going vessels. 



The next great step in advance in the art of submarine signaling was made 

 by an Austro-Hungarian physicist, Mr. H. Christian Berger, who realized the 

 necessity for an efficient means of submarine signaling between moving vessels to 

 secure greater safety in fogs, and devoted his talents to the solution of the problem. 



The problem was a difficult one, as great power is required to compress water 

 sufficiently to set up the intense pressure waves requisite for radiating the required 

 distance. His investigations led him to adopt the principle of longitudinal vibra- 

 tions set up in a steel wire or strip set into vibration by frictional means. 



His first test was made in the river Danube near Budapest in 19 10. 



In August, 191 1, by using a piano wire of only 2 millimeters diameter set into 



