Comctary Bodies and Saturn? & Rings. 747 



their centres in contrary directions at opposite extremities 

 of their diameters is an axiomatic truth which finds its 

 concrete expression in the diagram referred to. This geo- 

 metrical relation is also practically illustrated in the reaction 

 steam engine of Hero of Alexandria, in which a hollow globe 

 is made to revolve by two jets of steam issuing in contrary 

 directions from opposite extremities of its diameter. Other 

 instances of direct and retrograde motion may also be seon 

 in the Catherine wheels of ordinary firework displays, and 

 in hydraulic turbines with multiple jets around the circum- 

 ference?. 



Halley's original conception of concentric spheres rotating 

 within the earth, with a differential motion, is fruitful in 

 leading to the further idea that the ejection of comets from 

 a planet may be periodic from causes within 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 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 



* Repeated attempts have been made to establish a causal connexion 

 between the periodic times of Jupiter and sun-spot frequency. The 

 absurdity of this notion is at once apparent from the fact that the 

 sun rotates on his axis about 173 times during 1 one revolution of the 

 planet and, consequently, every part of the solar surface is presented in 

 succession through nearly every two degrees of the Jovian orbit. The 

 like demonstration is equally applicable to other members of the solar 

 system.— H. W. 



