METHOD OF SAFE NAVIGATION IN FOG. 127 



courses from 140° to 270° but with the same speed in each case, viz., 15 knots, and 

 5 miles apart when distance is first measured. "A" holds course east (90°); "B" 

 is shown on courses 140°, 160°, 200°, 220° and 270°. 



It should be noted that on course 270° the nearest approach is over 3^ miles, 

 and only between 160° and 200° or 40° of the compass would the vessels approach 

 within one mile of each other, or make any change in speed or course advisable. • 



As the vessels would constantly know their distance apart within a couple of 

 ships' lengths, as well as whether the bearing was changing, it is apparent that col- 

 lision is practically impossible. 



It is possible to locate a vessel by the instrument shown in Fig. 12, after a suf- 

 ficient run has been made, were this data necessary. The first distance measured 

 gives the large circle shown as a locus, and the course and speed of the two vessels 

 being known, a second circle is obtained. The intersection of this circle with the 

 first circle moved with the observed vessel gives the location required. Practically 

 the two graduated arms have the distances set on them and these points are brought 

 together. A double solution results as shown. In practical navigation this should 

 be unnecessary. In case the commercial type of submarine microphones is used, 

 the side on which the observed vessel bears would be ascertained. 



In the practical application of this method at sea, a standard wireless installa- 

 tion might be used, but such range and power are not required. A short wave- 

 length of not more than 200 meters, such as is reserved for the use of amateur 

 operators, would be sufficient for the purpose. An inexpensive set with a variable 

 frequency would be practical by which the changing pitch and single letter would 

 indicate an unmistakable fog signal. There would be no danger of interference 

 with submarine signals. If more than one vessel were within the field of audibil- 

 ity, the variations in speed or course would eliminate any probability of confusion. 



The recent simultaneous tests of the Berger and Fessenden submarine sys- 

 tems have shown an effective range of 10 miles for both. A range of 5 miles should 

 show four times this intensity, and therefore this element in the system appears to 

 be solved. 



The method above outlined appears therefore to furnish the information neces- 

 sary for safe fog navigation, namely, at a range of over 5 miles, speed and course 

 of other vessel, whether her bearing is changing, sea room to maneuver if neces- 

 sary, the condition that both vessels are able to hold their course and speed until 

 bearing change is ascertained, and in addition the distance separating the vessels. 

 Vessels having this knowledge will in nine cases out of ten find themselves passing 

 more than one mile apart without either changing her course or speed. 



The apparatus has been planned to operate with the greatest possible 

 simplicity. 



The sending apparatus (Fig. 10) is set for the course and speed and is there- 

 after automatic, operating until shut down after the fog lifts. 



As soon as a distinctive wireless fog letter of a vessel is heard the pivoted 

 scale in Fig. 12 is swung to the letter, the speed set as soon as the letter is again 



