154 
Proceedings of the Royal Irish Academy. 
morning was perfectly calm, and the sky cloudless, so tliat, 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 
filightly depressed in curved liollows 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 dis- 
placement of liquid produced by the body is considerably greater than the 
volume of the body itself. In the case of a floating needle, the repul- 
sion 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 floatation. Eut, 
moreover, the small pebbles which I saw floating were always flat and 
thin, and rested with their broadest surface on the water. The at- 
traction 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 vessel of water. When a 
piece of gold-leaf is held edgeways it sinks, and it also sinks if wetted. 
In fluids more viscid than water, such as lava or melted metals, flat 
pieces of the stone or solid metal are known to swim on their broad 
surfaces, while they sink when turned on their edges. I have recently 
made a few experiments on the floatation in water of small bodies of 
greater density than the liquid ; and I find that needles have remained 
for days together floating. I have also easily floated sand, flat pieces 
of shells, and small pebbles for several days, and whenever they sank, 
it was due to some disturbance of the liquid sufficient to produce a 
wave on its surface. Mr. Alphonse Gages placed twenty-four needles 
