364 Royal Society. 



the author refers to his paper on the Torpedo, which was pubUshed 

 in the Philosophical Transactions for 1834; and also to Miiller's 

 work " De Glandularum secernentium structura penitiori," whose 

 descriptions and views are not in accordance with those given in 

 that paper. In the present memoir he adduces evidence of the ac- 

 curacj'' of his former statement, chiefly founded on microscopical ob- 

 servations, and offers some conjectures respecting the functions of 

 several organs found in cartilaginous fishes ; but does not pretend 

 to attach undue importance to his speculations. 



A paper was also read, entitled, "Researches in Physical Geo- 

 logy. — Third Series. On the Phenomena of Precession and Nuta- 

 tion, assuming the interior of the earth to be a heterogeneous fluid." 

 By W. Hopkins, Esq., M.A., F.R.S., &c. 



HaAong, in his last memoir, completed the investigation of the 

 amount of precession and nutation, on the hypothesis of the earth's 

 consisting of a homogeneous fluid mass, contained in a homogeneous 

 solid shell, the author here extends the inquiry to the case in which 

 both the interior fluid and external shell are considered as hetero- 

 geneous. After giving the details of his analytical investigation, he 

 remarks, that he commenced the inquiry in the expectation that the 

 solution of this problem would lead to results diff"erent from those 

 pre\'iously obtained on the hypothesis of the earth's entire solidity. 

 This expectation was founded on the great difference existing be- 

 tween the direct action of a force on a solid, and that on a fluid mass, 

 in its tendency to produce a rotatory motion ; for, in fact, the dis- 

 turbing forces of the sun and moon do not tend to produce directly 

 any motion in the interior fluid, in which the rotatory motion causing 

 precession and nutation is produced indirectly by the effect of the 

 same forces on the position of the solid shell. A modification is thus 

 produced in the effects of the centrifugal force, which exactly com- 

 pensates for the want of any direct effect from the action of the dis- 

 turbing forces; acompensation which the author considers as scarcely 

 less curious than many others already recognized in the solar system, 

 and by which, amidst many conflicting causes, its harmony and per- 

 manence are so beautifuU)^ and wonderfully preserved. 



The solution of the problem obtained by the author destroys the 

 force of an argument, which might have been urged against the hy- 

 pothesis of central fluidity, founded on the presumed improbability 

 of our being able to account for the phenomena of precession and 

 nutation on this hypothesis, as satisfactorily as on that of internal 

 solidity. The object, however, of physical researches of this kind 

 is not merely to determine the actual state of the globe, but also to 

 trace its past history through that succession of ages, in which the 

 matter composing it has probably passed gradually through all the 

 stages between a simple elementary state and that in which it has 

 become adapted to the habitation of man. In this point of view the 

 author conceives the problem he proposes is not without value, as 

 demonstrating an important fact in the history of the earth, pre- 

 suming its solidification to have begun at the surface ; namely, the 

 permanence of the inclination of its axis of rotation, from the epoch 



