ii6 Bxiyal Society. 



of the veins in the Sheep and in Man, of the vestiges of tlie left pri-' 

 niitive vein ordinarily found in the adult human subject, and of the' 

 fresh example of double vena cava superior in Man met with by the 

 author. 



" A Mathematical Theory of Magnetism." By William Thomson^ 

 M.A., F.R.S.E , Fellow of St. Peter's College, Cambridge, and Pro- 

 fessor of Natural Philosophy in the University of Glasgow. 



The theory of magnetism was first mathematically treated in a 

 complete form by Poisson, Brief sketches of his theory, with some 

 simplifications, have been given by Green and Murphy in their works 

 on Electricity and Magnetism. In all these writings a hypothesis 

 of tM'O magnetic fluids has been adopted, and strictly adhered to 

 throughout. No physical evidence can be adduced in support of 

 such a hypothesis ; but on the contrary, recent discoveries, especially 

 in electro-magnetism, render it extremely improbable. Hence it is 

 of importance that all reasoning with reference to magnetism should 

 be conducted without assuming the existence of those hypothetical 

 fluids. 



The writer of the present paper endeavours to show that a com- 

 plete mathematical theory of magnetism may be established upon 

 the sole foundation of facts generally known, and Coulomb's special 

 experimental researches. The positive parts of this theory agree 

 with those of Poisson's mathematical theory, and consequently the 

 elementary mathematical formulae coincide with those which have 

 been previously given by Poisson. 



The paper at present laid before the Royal Society is restricted to 

 the elements of the mathematical theory, exclusively of those parts 

 in which the phenomena of magnetic induction are considered. 



The author hopes to have the honour of laying before the Society 

 a continuation, containing some original mathematical investigations 

 on magnetic distributions, and a theory of induction, in ferro- 

 magnetic or diamagnetic substances. 



" On the Structure of the Dental Tissues of Marsupial Ani- 

 mals, and more especially of the Enamel." By John Tomes, Esq. 

 Communicated by Dr. Grant, F.R.S. 



The author of the communication bearing the above title, after 

 examining microscopically the teeth of many marsupial animals taken 

 from the majority of the families that make up the order Marsupialia, 

 finds that they possess a structural character by which they may be 

 distinguished from other mammalian teeth, subject only to one. or 

 two exceptions; in which exceptions, however, the teeth are small, 

 and may readily be distinguished from marsupial by their external 

 character. They are the teeth of the Hyrax Capensis, the British 

 Shrews, and the molar teeth of the Jerboa. 



The author states, that so far as he has had opportunities of ex- 

 amination, the teeth of the various species may also be distinguished, 

 the one from the other. He points out, for instance, that, on com- 

 parison, the teeth of Dasyurus ursinus may be distinguished from 

 the D, macrourus. 



The peculiar characteristic of marsupial teeth exists iu the con- 



