Trof. Hennessy — On the Flotation of Sand. 317 



On July 26, when approacliing 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. As 

 it was my intention to bathe, I was somewhat dissatisfied with the 

 appearance of the water, until I stood on the edge of the strand, and 

 I then perceived that what was apparently scum, seen from a dis- 

 tance, 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, and it is probable that the flakes of floating 

 matter seen farther off contained also a considerable proportion. The 

 air during the whole morning was perfectly calm, and the sky cloud- 

 less, so that, although it was only half-past nine, the sun had been 

 shining brightly for some hours 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. I collected a 

 few specimens of these little objects, but I regret that they have been 

 since mislaid. This phenomenon, it is scarcely necessary to say, is 

 due to molecular action, such as accompanies the familiar experiment 

 of floating needles on the surface of a basin of water. Although the 

 specific gravity of the floating objects exceeds that of the fluid on 

 which they rest, the principle of Archimedes still holds good, because 

 the displacement of liquid produced by the body is considerably 

 greater than the volume of the body itself. In the case of a floating 

 needle, the repulsion of the liquid from the polished surface of the 

 metal presents a groove, whose magnitude is obviously many times 

 greater than the needle ; but in the case of the floating pebbles this 

 was not so manifest. The specific gravity of needles made of fine 

 hard steel may be taken at 7-9 nearly, while that of the little pebbles 

 scarcely exceeds 2'6, so that other things being equal, the latter 

 would require one-third of the displacement required by the former 

 for perfect flotation. But, moreover, the small pebbles which I 

 saw floating were always flat and thin, and rested with their broadest 

 surface on the water. The attraction of the molecules of water for 

 one another produces, as is well established, a tension at the surface 

 of the liquid, which, although extremely feeble, and generally noticed 

 only in connexion with capillary phenomena, yet interposes some 

 resistance to the intrusion of foreign substances. This is seen in the 

 experiment of floating broad spangles or sheets of dry gold-leaf on a 



