74 THE NEW WORLD OF SCIENCE 



will have traveled more rapidly than was assumed in the 

 preparation of the plotting board and the intervals will thus 

 be too small for the scale adopted in drawing the board. The 

 amounts by which the intervals must be augmented or dimin- 

 ished if the temperature of the air be known are easily cal- 

 culated and the strings may be set for the corrected intervals 

 and the intersection then determines the true position of the 

 gun as before. If a wind be blowing with' a known velocity 

 in a given direction it is only the components of this velocity 

 which lie along the directions G M 1? G M 2 , and G M 3 which 

 will affect the times of arrival of the sound at the listening in- 

 struments; the observed intervals may therefore be easily cor- 

 rected to what they would have been were there no wind and 

 the plotting strings set accordingly. 



The theory of the application of the wind and temperature 

 corrections is an extremely simple matter and the application 

 itself is easy and rapid because of the graphical method of 

 calculation employed in the construction of the string plotting 

 board. The real difficulty lies in an uncertainty as to what the 

 true temperature and wind are, since the sound comes by a 

 path inaccessible to observation. More than half of the path 

 lies behind the enemy lines and the remainder lies in a region 

 in which it is not permissible to attract the attention of the 

 enemy by carrying on any unnecessary activities. 



A very valuable study of the wind and temperature cor- 

 rections to be applied to the observed data of sound-ranging 

 was made by the British before the entry of the United States 

 into the war and an empirical rule was found to hold that 

 these corrections should be based on observations of wind and 

 temperature made as near the front as convenient and at a 

 height of fifty meters above the ground. In the American 

 service the meteorological data were not available from army 

 sources when the first sound-ranging sections went into the 

 field in March 1918, so each section was equipped to obtain 

 its own data. 



There are other wind and temperature effects which are of 



