54 Mr. G. Johnstone Stoney, 



elsewhere. If this is done after the surface has been for a long 

 time exposed to the sun— 



" The sand rolls down with a sluggish viscous motion and the sound 

 begins, at first a low vibrating moan but gradually swelling out into a 

 roar like thunder, and as gradually dying a Way." (Palmer's rt Desert of 

 the Exodus," vol. 1, p. 218). 



That heat contributes largely to the effect was proved by the 

 valuable observations made by Captain Palmer, for it was found — 



" That the heated surface was much more sensitive to sound than the 

 cooler layers beneath, and that those parts of the slope which had lain 

 long undisturbed produced a much louder and more lasting sound than 

 those which had recently been set in motion." 



Moreover, when the experiments were repeated on the other 

 talus, which faced towards the North, and part of which was in 

 perpetual shade, it was found — 



" That the sand on the cool shaded portion, at a temperature of 17° C, 

 produced but a very faint sound when set in motion, while that on the 

 more exposed parts, at a temperature of 40°, gave forth a loud and even 

 startling noise." 



These observations were made in winter. They clearly indicate 

 that heat renders the surface of the slope more mobile by polariz- 

 ing the air between the hotter and cooler particles of the sand. 



The more intense the sunshine, the more powerful must the 

 Crookes's layers be, and the more widespread will be the effect of 

 any accidental disturbance. And if under the fierce glare of the 

 tropical sun the strength of the Crookes's layers becomes sufficient 

 to lift the uppermost grains of sand, the sliding motion, with its 

 humming, booming, and thundering noise, will spring up without 

 visible cause — a phenomenon that sometimes occurs and has 

 naturally occasioned much speculation. 



Mr. Howard Grubb has directed my attention to another natural 

 phenomenon which admits of being explained by the mechanical 

 properties of polarized layers of gas. In certain states of the 

 weather large grains of sand, flat pieces of shell, and even 

 flakes of stone of quite a considerable size may be seen floating on 

 the tide as it flows in. I saw this phenomenon myself when a 

 boy, but unfortunately did not make a careful examination of the 

 attendant circumstances. It is, however, easy to see the conditions 

 which would be most favourable to its production. They are— a 



