24 TRANSACTIONS OF THE [OCT. 21^ 



zation; and reverberation within subterranean cavities. Charles 

 Didier, when travelling in Arabia, heard of the acoustic pheno- 

 mena at Jebel Nagous, and, although he did not visit the place, 

 promptly offers to exphiin it by attributing the sounds to ''inte- 

 rior cataracts or a subterranean volcano." 



Mr. Cams- Wilson, of Bournemouth, England, explains the 

 sonorousness of sand on a beach as follows: — "The music from 

 sand is simply the result of the rubbing together of the surfaces 

 of millions of perfectly clean grains of quartz free from angu- 

 larities, roughness, or adherent matter in the form of clinging 

 fragments investing the grains, and these microlithic emissions 

 of sound, though individually inaudible, might in combination 

 produce a note sufficiently powerful to be sensible to us" (Lec- 

 ture of November 2d, 1888). 



Without attempting to analyze or criticise the above theories, 

 all of wliich we believe to be wliolly inadequate, we briefly repeat 

 our own. 



Dr. Julien and I believe that the true cause of sonorousness in 

 the sands of singing beaches and of deserts is connected with thin 

 pellicles or films of air, or of gases thence derived, deposited and 

 condensed upon the surface of the sand -grains during gradual 

 evaporation after wetting by seas and lakes or by rains. By 

 virtue of these films, the sand-grains become separated by elastic 

 cushions of condensed gases, capable of considerable vibration, 

 and whose thickness we have approximately determined. The 

 extent of the vibration and the volume and pitch of the sound 

 thereby produced, after any quick disturbance of the sand, we 

 also find to be largely dependent upon the forms, structures, and 

 surfaces of the sand-grains, and especially upon their purity or 

 freedom from fine silt or dust. 



Though the environment of the sand on beaches and in the 

 desert differs greatly as respects moisture, we believe that the 

 above theory is applicable to both. Statistics of rainfall in the 

 desert are wanting ; but the experiences of travellers, and my 

 own observations, show that rain falls in the winter months 

 abundantly in many parts of the peninsula. In the vicinity of 

 Mount Sinai, heavy snows and rain are precipitated in Decem- 

 ber and January, occasionally causing veritable floods in narrow 

 valleys, as experienced by Capt. Palmer; I noted a ten-hours* 

 shower in Wadi Feiran in March (1889), and I saw raindrop im- 

 pressions on mud-flakes at several points on the coast. Water, 

 therefore, is not wholly lacking to aid in the cliemical cleansing 

 of the sand-grains, and the i)owerful winds accomplish tiie work 

 of sorting and winnowing done by the waves elsewhere. The 

 fineness of the grains at the Bell Slopes makes the displacement 

 of a very large amount of sand necessary for the production of 



