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 away.” (Palmer's “ 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 madein 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 
