On Crookes's Layers, at Atmospheric Tensions. 55 



very powerful sun to heat the stones and to maintain their tem- 

 perature 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 temperature between the flakes of stone 

 and the water, and the absence of waves that the heavy little 

 barges may escape shipwreck. 



I think it fortunate that I had written out the foregoing state- 

 ment of the conditions indicated by the theory, before I saw the 

 following record of observations upon this phenomenon made by 

 Professor Hennessy. (See Proceedings of the Royal Irish Academy, 

 Vol. L, Series 2) :— 



"On the 26th 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 only half-past nine, the sun had been shining brightly on the exposed 

 beach. The upper surface of each of the little pebbles was perfectly dry, 

 and the groups which they formed were slightly depressed in curved 

 hollows of the liquid. The tide was rapidly rising, and owing to the 

 narrowness of the channel at the point where I made my observations 

 the sheets of floating sand were swiftly drifting farther up the river into 

 brackish and fresh water. On closely watching the rising tide at the 

 edge of the strand, I noticed that the particles of sand, shells, and small 

 flat pebbles, which had become perfectly dry and sensibly warm under 

 the rays of the sun, were gently uplifted by the calm steadily rising 

 water, and then floated as readily as chips or straws." 



The calm air, tranquil water, hot sun, and warm stones, predicted 

 from the theory, are all recorded in these observations. 



This rare phenomenon must not be confounded with the familiar 

 one in which patches of fine sand float upon water in con- 

 sequence of its surface tension. The surface tension of water in 

 contact with air will not support flakes of stone of above a certain 

 size, and those described by Professor Hennessy are at or beyond 

 the limit of size* that could even if separate be floated by surface 



* Taking the surface tension of water in contact with air as 8*25 grammes per metre 

 as determined at 20° C. by M. Quincke, and assuming 2*5 as the specific gravity of the 



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