208 Royal Society. 



A further discussion is entered into in order to prove that the pres- 

 sures propagated from the surfaces into the interior parts balance and 

 destroy one another, which completely establishes the permanence of 

 the figure of the fluid. It is also shown that the mass of fluid, under 

 these circumstances, has no tendency to turn upon an axis. 



To illustrate the foregoing problem, the author applies it to the de- 

 termination of the figure of equilibrium of a homogeneous mass of 

 fluid entirely at liberty, of which the particles attract one another with 

 a force directly proportional to the distance, at the same time that 

 they are urged by a centrifugal force caused by rotation about an 

 axis. 



He then enters upon the investigation of the second problem, in 

 which the law of attraction of the particles is that of the inverse du- 

 plicate ratio of the distance j and finally arrives at the conclusion, 

 that the form of the fluid in equilibrium is, exclusively of all other 

 figures, an oblate elliptical spheroid of revolution, and that its axis of 

 rotation is the lesser axis of the spheroid. He also shows that within 

 the spheroid there are no more than two sets of surfaces equally 

 pressed by the action of the exterior fluid ; arid no more than two diffe- 

 rent spheroids of equilibrium answering to the same rotatory motion. 

 If the whole spheroid be one of small oblateness, the greatest of the 

 interior surfaces of equable pressure, which is not a level surface, 

 stands upon the equator ; and the rest are within this, and are simi- 

 lar to it, and similarly posited. When it is very oblate, the greatest 

 of these surfaces is described about the lesser axis -, and the rest are 

 within it, and are similar to it, and similarly posited. The existence 

 of two sets of interior surfaces, that are equally pressed at all their 

 points by the action of the exterior fluid, is inconsistent with Clairaut's 

 theory, and is a proof of its insufficiency for determining the figure of 

 a homogeneous planet. 



Jan. 27. A paper was read, On the probable electric origin of 

 all the phenomena of Terrestrial Magnetism, with an illustrative ex- 

 periment ; by Peter Barlow, Esq. F.R.S. Corr. Mem. Inst. France, 

 and of the Imp. Acad. St. Petersburg!!. 



The author begins his paper by a retrospect of the several discove- 

 ries on terrestrial magnetism made since the commencement of the 

 present century. Humboldt, by his numerous and accurate observa- 

 tions on this subject, laid the foundation of all the scientific know- 

 ledge relating to it, which we hitherto possessed. The task of reducing 

 these observations to definite principles, by subjecting them to calcu- 

 lation, was undertaken by Biot ; and the conclusion which he drew 

 from them was, that, on the hypothesis of the earth's being a great 

 magnet, the facts would best accord with the supposition that its 

 two poles are coincident, or indefinitely near to each other, at the 

 centre of the globe. The same result was also obtained, though by a 

 different process of reasoning by M. Kraft of St. Petersburgh. It 

 followed as a necessary consequence that terrestrial magnetism ob- 

 serves a law different from that of a permanently magnetic body, but 

 identical with that of a body in which transient magnetism is excited 

 by induction. The law which obtains in the case of a sphere of iron 



rendered 



