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 
Piofessor Hennessy. (See Proceedings of the oa 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 débris 
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 
frcm the theory, are all recorded in these observations, 
This rare phenomenon must not be confounded with the familiar 
one in which patches of jfime 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 asswning 2 5 as the specific gravity of the 
F 2 
