ABSTRACTS OF TECHNICAL ARTICLES 535 



equations fail only at substantially grazing incidence and then merge 

 into the Sommerfeld ground wave solution. The ground etfect is 

 always to discriminate against radiation or rece|)tion at very \o\v 

 angles. 



Two methods of determining the electrical constants of the ground 

 are described. One comprises measurements of the elliptical polariza- 

 tion of the ground wave, and is based on Sommerfeld's propagation 

 theory. The other is a method of measuring, at radio frequencies, 

 the conductivity and dielectric constant of samples of ground re- 

 moved from the natural state. Suitable agreement between the two 

 methods is found if the nonuniformity and stratification of natural 

 ground is considered. The sample method is also used to determine 

 the conductivity of ocean water. 



On Minimum Audible Sound Fields.* L. J. SiviAX and S. D. White. 

 The minimum audible field (M.A.F.) has been determined from data 

 taken on 14 ears over the frequency range from 100 to 15,000 c.p.s. 

 The observer is placed in a sound field which is substantially that 

 of a plane progressive wave, facing the source and listening monaurally. 

 The M.A.F. is expressed as the intensity of the free field, measured 

 prior to the insertion of the observer. Similar data are presented for 

 binaural hearing, over the range from 60 to 15,000 c.p.s., obtained 

 with 13 observers. At 1000 c.p.s. the average M.A.F. observed is 

 1.9 X iO-^^ watts per cm.-, corresponding to a pressure 71 db below 

 1 bar. Included are data showing how the M.A.F. varies with the 

 observer's azimuth relative to the wave front. Another type of 

 threshold data refers to minimum audible pressures (M.A.P.) as 

 measured at the observer's ear drum. The differences obviously to 

 be expected between M.A.F. and M.A.P. values are due to wave 

 motion in the ear canal and to diffraction caused by the head. The 

 M.A.F. data are discussed in relation to the M.A.P. determinations 

 from several sources. Some possible causes of difference between 

 the two, which are due to experimental procedure and may add to 

 the causes already mentioned, are pointed out. 



Naturally-Occurring Ash Constituents of Cotton.^ A. C. Walker and 

 M. H. Quell. Precise information on the inorganic ash constituents 

 which are deposited in cotton fibres during growth, and on the changes 

 which occur in these constituents when cotton is washed with distilled 

 water or aqueous solutions, is desirable as an aid in understanding 

 many of the properties of this important industrial fibre. In a 



■» Jour. Acous. Soc. Amer., April, 1933. 



5 Journal of the Textile Institute, March, 1933. 



