Intelligence an J Miscellaneous Ao tides. 395 



coral, with some basalt pebbles and shells ; 3. Compact limestone, 

 with thin layers of basalt at base. The author described the 

 supposed fossil trees noticed in this island by Messrs. Ayres and 

 Clarke (Q. J. G. S. xxiii. p. 185) as composed simply of hard 

 portions of coral rock left outstanding by the weathering of the 

 softer intervening parts ; they show the same stratification as the 

 rock below. The islet known as Gunner's Quoin consists of co- 

 lumnar basaltic lava, capping volcanic sand, below which is a 

 browner volcanic sand with seams of coral fragments. Flat Island 

 is in pait the remains of a volcanic crater, and the rest consists of 

 volcanic sand strewn with coral blocks. There are basaltic dykes in 

 the hill, the top of which appears to show traces of one or more 

 plugs. The author concludes that Mauritius was once an active 

 volcano, now elevated with the old reef. The islets also formed 

 part of a volcano or volcanoes, and have also been elevated with 

 reef-material. 



LVI. Intelligence and Miscellaneous Articles. 



ON THE PITCH OF A TUNING-FORK IN AN INCOMPRESSIBLE FLUID. 

 BY FELIX AUERBACH. 



WHEN a tuning-fork is struck and then quickly immersed in 

 a vessel containing water, there is heard, especially if the qar 

 be applied to the resonant table, a tone the height of which does 

 not accord with that of the tuning-fork in air. 



This phenomenon, which seems to have hitherto excited but little 

 notice (I have found a remark in reference to it only in Chi acini's 

 AJcustik), follows as a necessary consequence from the fact that the 

 dissipation of the kinetic energy takes place in the so-called incom- 

 pressible fluids under other circumstances than those which subsist 

 in gases. For into all formulas for the velocity of propagation and 

 for the number of vibrations enters the square-root of a quantity 

 to which a determinate signification cannot at once be assigned. 

 In fact the coefficient of elasticity is usually defined merely as the 

 ratio of an infinitesimal increment of pressure to the corresponding 

 infinitesimal compression of volume ; yet its value essentially de- 

 pends on what are the circumstances under which the change of 

 state in question proceeds. Of special importance are two cases — 

 namely, that in which the entropy of the system, and that in which 

 the temperature remains constant. With the sonorous vibrations 

 of elastic solid bodies in gases the first of these cases is approxi- 

 mately realized. For the alternate condensations and rarefactions 

 undergone by the gas have, it is true, for their consequence changes 

 of the temperature ; hence, in the equalization of this, kinetic 

 energy is given up to the environment ; but the conductivity of 

 gases for heat is so inconsiderable that the dissipation of energy 

 can be neglected for rapid vibrations like those of sound : for these, 

 according to Clausius, it may even be said that the entropy remains 



