Curiosities of Science. 117 



its present solidity by the radiation of heat into surrounding 

 space, and its consequent refrigeration. 



The mathematical solution of this problem of Central Heat, 

 assuming such heat to exist, tells us that though the central 

 portion of the earth may consist of a mass of molten matter, 

 the temperature of its surface is not thereby increased by more 

 than the small fraction of a degree, Poisson has calculated 

 that it would require a thousand millions of centuries to reduce 

 this fraction to a degree by half its present amount, supposing 

 always the external conditions to remain unaltered. In such 

 cases, the superficial temperature of the earth may, in fact, be 

 considered to have approximated so near to its ultimate limit 

 that it can be subject to no further sensible change. 



DISAPPEAEANCE OF VOLCANIC ISLANDS. 



Many of the Volcanic Islands thrown up above the sea-level 

 soon disappear, because the lavas and conglomerates of which 

 they are formed spread over flatter surfaces, through the weight 

 of the incumbent fluid ; and the constant levelling process goes 

 on below the sea by the action of tides and currents. Such 

 islands as have effectually resisted this action are found to 

 possess a solid framework of lava, supporting or defending the 

 loose fragmentary materials. 



Among the most celebrated of these phenomena in our times may be 

 mentioned the Isle of Sabrina, which rose off the coast of St. Michael's 

 in 1811, attained a circumference of one mile and a height of 300 feet, 

 and disappeared in less than eight months ; in the following year there 

 were eighty fathoms of water in its place. In July 1831 appeared Gra- 

 ham's Island off the coast of Sicily, which, attained a mile in circum- 

 ference and 150 or 160 feet in height ; its formation much resembled 

 that of Sabrina. 



The line of ancient subterranean fire which we trace on the 

 Mediterranean coasts has had a strange attestation in Graham's 

 Island, which is also described as a volcano suddenly bursting 

 forth in the mid sea between Sicily and Africa ; burning for 

 several weeks, and throwing up an isle, or crater-cone of scorise 

 and ashes, which had scarcely been named before it was again 

 lost by subsidence beneath the sea, leaving only a shoal-bank 

 to attest this strange submarine breach in the earth's crust, 

 which thus mingled fire and water in one common action. 



Floating islands are not very rare : in 1827, one was seen 

 twenty leagues to the east of the Azores ; it was three leagues 

 in width, and covered with volcanic products, sugar-canes, 

 straw, and pieces of wood. 



PERPETUAL FIRE. 



Not far from the Deliktash, on the side of a mountain in 

 Lycia, is the Perpetual Fire described some forty years since 



