314 Prof, Thomson, Experiments on Electric Discharge. [Mar. 7, 



Monday, March 7, 1892. 

 Professor G. H. Darwin, President, in the Chair. 



The following communications were made to the Society : 



(1) Some Experiments on Electric Discharge. By Professor 

 Thomson. 



A series of experiments were shown in which the electric 

 discharge took place in bulbs without electrodes. It was shown 

 that the colour of the discharge through the same gas varied 

 very greatly with the density of the gas and the intensity of 

 the discharge. This was illustrated by two bulbs each con- 

 taining air; the discharge through one was a bright blue and 

 through the other an apple-green. Another experiment showed 

 that gas at a very low pressure could not act as an electromagnetic 

 screen, though it did so at a high pressure. The laws governing 

 the absorption of energy by conductors placed near very rapidly 

 alternating currents were illustrated by experiments which showed 

 that there was much greater absorption of energy by small pieces 

 of tin-foil than large masses of brass or copper. 



(2) On the perturbation of a comet in the neighbourhood of a 

 planet. By G. H. Darwin, F.R.S., Plumian Professor and Fellow 

 of Trinity College. 



In Chapter II. of Book ix. of the Mecanique Celeste, Laplace 

 considers the transformation of the orbit of a comet when it 

 passes a large planet. His object is to show that the action of 

 Jupiter suffices to account for the disappearance of Lexell's comet 

 after 1779. 



He remarks that if a comet passes very near to Jupiter, it 

 will throughout a small portion of its orbit move round the 

 planet almost as though it were unperturbed by the Sun, and 

 that both before its approach to and after its recession from 

 the planet it will move round the Sun almost as though it were 

 unperturbed by the planet. The nature of the orbit of the 

 comet will usually be much transformed by its encounter with 

 the planet. It is clear then that there must be some surface 

 surrounding the planet which separates the region, inside of 

 which the comet moves nearly round the planet, from the region 

 in which it moves nearly round the Sun. Such a surface is to 

 be found by the comparison of the ratio of the perturbing force 



