404 Cambridge Philosophical Society. 



poles. This must be attended by an enormous monthly inequality, 

 amounting to nearly 60° C, supposing the matter of which her su- 

 perficial crust is composed to have the same conductivitj% specific 

 heat, and radiating power as the crust of the Earth. If these be 

 much greater for the Moon, this inequality might be considerably 

 diminished. At the poles it must be comparatively small. 



The lunar temperatures here spoken of are those of the matter 

 forming her external crust. The temperature which would be indi- 

 cated by a thermometer placed in her immediate vicinity would be 

 affected by the Moon (in the assumed absence of an atmosphere) 

 only by her direct radiation . We have not the means of determining 

 what this temperature may be. 



Nov. 12. — A paper was read by the Master of Trinity on the 

 Intellectual Powers according to Plato. 



Also, Prof. Sedgwick gave a lecture on the Classification and 

 Nomenclature of the Palaeozoic Rocks. 



Dec. 10. — A paper was read by Mr. Maxwell on Faraday's Lines 

 of Force. 



The method pursued in this paper is a modification of that mode 

 of viewing electrical phaenomena in relation to the theory of the uni- 

 form conduction of heat, which was first pointed out by Professor W. 

 Thomson in the Cambridge and Dublin Mathematical Journal, vol. iii. 

 Instead of using the analogy of heat, a fluid, the properties of which 

 are entirely at our disposal, is assumed as the vehicle of mathematical 

 reasoning. A method is given by which two series of surfaces may 

 be drawn in the fluid so as to define its motion completely. The 

 uniform motion of an imponderable and incompressible fluid permea- 

 ting a medium, whose resistance is directly as the velocity, is then 

 discussed, and it is shown how a system of surfaces of equal pressure 

 may be drawn, which, with the two former systems of surfaces, 

 divides the medium into cells, in each of which the same amount of 

 work is done in overcoming resistance. It is then shown that if the 

 fluid be supposed to emanate from certain centres, and to be absorbed 

 at others, the position of these centres can be found when the pres- 

 sure at any point is known ; and that when the centres are known, 

 the distribution of pressures may be found. Methods are then given 

 by which the motion of the fluid out of one medium into another, 

 the resistance of which is difll^erent, may be conceived and calculated, 

 and the theory of motion in a medium in which the resistance is 

 different in different directions is stated. 



The mathematical ideas obtained from the fluid are then applied 

 to various parts of electrical science. It is shown that the expres- 

 sion for the electrical potential at any point is identical with that of 

 the pressure in the fluid, provided that " sources " of fluid are put 

 instead of positive electrical " matter," and centres of absorption or 

 " sinks " for negative " matter." 



The theory of Faraday with respect to the effect of dielectrics in 

 modifying electric induction, is illustrated by the case of different 

 media having different conducting power; and it is shown, that, in 

 order to calculate the effects by the ordinary formulae of attractions. 



