ordinary Atmospheric Tensions, 459 



These observations were made in winter. They clearly in- 

 dicate that heat renders the surface of the slope more mobile 

 by polarizing 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 stress becomes 

 sufficient to lift the uppermost grains of sand, the sliding mo- 

 tion, with its humming, booming, and thundering noise, will 

 spring up without visible cause — a phenomenon that sometimes 

 occurs, and has naturally occasioned much speculation. 



3. Mr. Howard Grubb has directed my attention to another 

 natural phenomenon which admits of being explained by the 

 mechanical properties of polarized 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 exami- 

 nation of the attendant circumstances. It is, however, easy 

 to see the conditions which would be most favourable to its 

 production. They are : — a very powerful sun, to heat the stones 

 and to maintain their temperature sufficiently high after they 

 are set floating ; calm air, that no breeze may cool them ; a 

 cold sea, to increase as much as possible the difference in tem- 

 perature between the flakes of stone and the water ; and the 

 absence of waves, that the heavy little barges may escape ship- 

 wreck. 



I think it fortunate that I had written out the foregoing 

 statement of the conditions indicated by the theory before I 

 saw the following record of observations upon this phenomenon 

 made by Professor Hennessy (see l Proceedings of the Royal 

 Irish Academy,' vol. i. series 2): — 



" On the 26th of July, 1868, when approaching the strand 

 at the river below the village of Newport, county Mayo, I 

 noticed what appeared to be extensive streaks of scum floating 



on the surface of the water until I stood on the edge 



of the strand ; and I then perceived that what was apparently 

 scum seen from a distance, consisted of innumerable particles 

 of sand, flat flakes of broken shells, and the other small debris 

 which formed the surface of the gently sloping shore of the 

 river. The sand varied from the smallest size visible to the 

 eye, up to little pebbles nearly as broad and a little thicker 

 than a fourpenny piece. Hundreds of such little pebbles were 

 afloat around me. The air during the whole morning was 

 perfectly calm, and the sky cloudless ; so that although it was 



