76 THE NEW WORLD OF SCIENCE 



from air strata of unequal density at various heights above the 

 ground. 



Three listening instruments electrically connected with a 

 " central " or calculating station are theoretically sufficient to 

 permit of a survey of the enemy batteries by sound ; however, 

 it is in practice advisable to employ more than three instru- 

 ments for the following reasons : The electrical connections 

 between the instruments and the time-recording mechanism at 

 the " central " are unavoidably subject to considerable cutting 

 by the enemy's shell-fire; it is out of the question to bury the 

 lines to a sufficient depth to avoid all cutting and to bury them 

 in a shallow trench renders the location and repair of the breaks 

 which do occur, extremely difficult ; the practice has been to lay 

 the lines exposed on the ground and to provide a sufficient force 

 of linesmen to ensure quick repairs. In order that the section 

 may continue its work while such repairs are being made, six 

 listeners instead of three are provided in the expectation that at 

 least three will always be in working condition. If only the 

 minimum number (three) of listening instruments are employed 

 no estimate of the accuracy of a location can be formed whereas 

 if six instruments be used the location is determined by the 

 intersection of five strings on the plotting board; if these all 

 intersect in a point the location is probably accurate whereas 

 if they intersect in a large cat's cradle the location is probably 

 badly in error. A study was made of the errors corresponding 

 to certain typical cat's cradles and a general plan of reporting 

 the probable accuracy of a location from the character of the 

 intersection was adopted. The officer in command of a sound- 

 ranging section thus reported to the artillery not only the loca- 

 tion, target and probable caliber of an active enemy battery 

 but also whether the location was probably accurate to fifty, 

 one hundred or one hundred and fifty meters; these estimates 

 were formed in a scientific manner and all of the various sec- 

 tions on the front employed the same method. After a con- 

 siderable number of the enemy's battery positions had been 

 captured it was possible to check up the errors of the in- 



