Mane lie ster Memoirs, Vol. Iv. (1910), No. \. 9 



itself, in like manner to the eleven years maximum 

 sun-spot ejections of elementary gaseous substances. 

 For it is only necessary to assume that, after the ejection 

 of cometary matter through the double thickness of two 

 concentric shells, the differential motion would retard, or 

 wholly prevent, the further discharge until the vents were 

 again coincident. 



The planet Jupiter, from his vast dimensions, is the 

 most interesting member of the solar system for the study 

 of planetary and cometary evolution. The great red spot 

 on his surface is generally considered to be caused by 

 luminous vapours at great depths within the globe, if not 

 by the actual incandescent crust of that part of the planet. 

 The great extent and permanency of this spot indicate it 

 as the locus of one of the vents through which comets and 

 cometary satellites have been ejected at different periods 

 of the history of the planet. 



It is now generally recognized that certain groups of 

 periodic comets are associated in some way unknown 

 with the larger planets respectively ; the comets of short 

 period belonging to Jupiter, as nearest to the sun, and 

 the long period comets (of which Halley's is the most 

 notable member) to Neptune and intermediate planets. 



All the motions of periodic comets are well explained 

 on the assumption of their moving in elliptical orbits 

 more or less elongated, but the vast tabulated periodic 

 times of comets supposed to move in parabolic and 

 hyperbolic curves are necessarily ultra-speculative. 



As the attraction of solar gravitation extends far 

 beyond the orbit of Neptune, the motion of a body on the 

 line of an open curve would ultimately be arrested and a 

 comet would necessarily return over the same track, 

 approximately, with a retrograde motion as an unknown 



