318 Notices of Memoirs. 



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 flotation 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 sufScient to produce a 

 wave on its surface. Mr. Alphonse Gages placed twenty-four needles 

 on the surface of a large basin of water, and after a few hours they 

 were found grouped in parallel parcels, varying in their contents 

 from two to seven needles. They continued to float for more than 

 five days, and their sinking was evidently due to the progress of 

 oxidation, which destroyed their polish, together with their repulsive 

 action on the liquid. I have floated small flat pebbles, similar in 

 size and appearance to the largest of those observed floating on 

 Newport river, for more than six days, while fragments of shells, 

 and thin pieces of slate as broad as a sixpenny piece, have continued 

 to float much longer. These little bodies occasionally sank from the 

 gradual absorption of water, but much more frequently from some 

 accidental motion of the vessel containing the liquid. 



It is manifest that the flotation of sand in a tidal estuary, as in 

 the instance I have seen, can occur only under favourable conditions. 

 The shores must be very gently inclined, the air perfectly calm, and 

 the weather dry and warm. Under these circumstances thin cakes or 

 sheets of sand may not only be uplifted by the water, but if the tide 

 flows rapidly they may continue afloat sufficiently long to allow many 

 of them to be drifted far from their original place up to the higher 

 limit of the brackish water. In this way fragments of marine shells 

 and exuviae might become mingled with those belonging to fresh 

 water. The conditions favourable for sand flotation must exist 

 during calm weather in a veiy high degree of perfection on the 

 sandy shores of tidal rivers in tropical and subtropical districts of 

 the earth. As this phenomenon can take place only with the rising 

 tide, and never with the falling tide, the result must generally be 

 favourable to the transport of sand and marine debris in the direction 

 of the flow of flood tide ; and this may sometimes hold good along a 

 coast as well as on the shores of a tidal estuary. Geologists, as far 

 as I am aware, have not hitherto noticed this phenomenon in con- 

 nexion with the formation of stratified deposits by the agency of 

 tides and rivers, although they have paid great attention to the in- 

 fluence of the molecular resistance of water to the sinking of very 

 minute solid substances, with the view of explaining the wide sur- 

 face over which matter held in suspension by water may be spread 

 when ultimately deposited over the sea-bottom.^ 



1 Since this paper was written, I have been informed by a lady, that she observed 

 similar phenomena durinff a former summer, close to the sandy seashore at Youghal ; 

 and Dr. E. Percival Wright has stated that he has witnessed the realization of the 

 results which are alluded to as likely to occur within the tropics. 



