458 Mr. G. J. Stoney on Orookes's Layers at 



mobility that may be imparted to a light powder by heating 

 it in a metal capsule. It is shown that in this case the powder 

 floats on a stratum of air which it compresses by its weight, 

 at the same time that it maintains the requisite polarized con- 

 dition of the layer by radiating away its own heat so freely as 

 to keep itself cooler than the capsule. 



2. In exactly the same way we may explain a very curious 

 phenomenon which has been recorded by travellers in Arabia, 

 and to which Professor Barrett has directed my attention. 

 There is in Arabia a mountain called Jebel Nagus, or Gong 

 Mountain, which produces sounds resembling the booming of 

 the Nagus, or wooden gong, used in Eastern churches instead 

 of bells. The mountain consists of a white friable sandstone, 

 which produces to the south-westward a great slope of very 

 fine drift sand, and another smaller one to the north. The 

 large one is 115 metres high, 70 metres wide at the base, and 

 tapers towards the top. It is so steep, being inclined to the 

 horizon at an angle of nearly 30°, and consists of such fine 

 sand, that its surface can be easily set in motion by scraping 

 away a portion from its base, or by disturbing it elsewhere. 

 If this is done after the surface has been for a long time ex- 

 posed 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 gra- 

 dually dying away " (Palmer's ' Desert of the Exodus,' vol. i. 

 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 star- 

 tling noise." 



positions are fixed, is greater than the flow of heat by conduction ; whereas 

 experiment shows that it is always feebler. Accordingly penetration is 

 not be sought, as I supposed, in the whole of the shaded portions of the 

 diagrams accompanying that paper, but only where the outline of them 

 slopes downwards, i. e. between and n. But this error does not affect the 

 mechanical part of the paper, of which the present paper is a continuation. 



