HAYES— DETECTION OF SUBMARINES. 3 



the presence of some field of force surrounding the body. The first 

 method promises detection at a greater range than the second since 

 the intensity of radiant energy varies as the inverse square of the 

 distance from the source while the strength of a field of force sur- 

 rounding a polarized body varies as the inverse cube of the distance. 

 Both methods are applicable to the case of a moving submarine. 

 The steel shell of the submarine must be surrounded by a magnetic 

 field, due to polarization induced by the earth's magnetic field and 

 also due to such permanent polarization as it may have taken on dur- 

 ing construction. Also a certain amount of sound must radiate from 

 the motors and propellers and other moving parts of the submarine. 



Magnetic Methods Unsatisfactory Because of Limited 



Range. 



The intensity of polarization which a submarine takes on through 

 the action of the earth's magnetic field can be predicted with some 

 accuracy and, as a result, the range at which it can be detected by 

 magnetic methods foretold. Both theory and practice show this 

 range to be about Yz the length of the submarine. This range, which 

 is too slight to be helpful in searching or avoiding an operating sub- 

 marine, is hardly sufficient for detecting any but the largest sub- 

 marine when lying at rest on the bottom at maximum depth. No 

 satisfactory method for determining the presence of a submarine 

 lying at rest at considerable depth, say from lOO to 150 feet, has yet 

 been perfected, but one or two promising methods are in the process 

 of development. 



Sound Methods Promise Greater Range. 



Water is an excellent medium for transmitting sound. Its homo- 

 geneity and low viscosity makes the dissipation due to reflection, re- 

 fraction, and transformation into heat comparatively slight. 



The relation between intensity and distance is more favorable 

 than that given by the inverse square law, because of the fact that 

 the surface and bottom reflect the sounds and tend to keep them 

 within two dimensional motion, much as the speaking tube confines 

 sound to motion of one dimension. Because of this fact, sounds can 

 be heard farther than they could if they were not confined. 



