8 Wilde, Origin of Covietmy Bodies and Saturn's Rii/gs. 



To those who are not familiar with the problems of 

 experimental mechanics, it may be of some advantage to 

 demonstrate more fully the direct and retrograde motions 

 of cometary bodies by further illustrations than those 

 shown in my Halley lecture. 



It is common knowledge, based on well-established 

 observations, that the axial and orbital rotations of all the 

 planets are in the same direction, the sun also revolving 

 on its axis in the same direction as the planets. 



As a consequence of the common direction of the 

 axial rotations, the adjoining circumferential parts revolve 

 in opposite directions to each other, as will be seen in the 

 annexed diagram of the Sun and Jupiter. Hence, while 

 the circumferential parts of the planets next to the sun 

 revolve from west to east, the sun apparently revolves from 

 east to west, as is manifest from the motion of the dark 

 spots across the solar disk. 



That the circumferences of moving circles rotate about 

 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 

 geometrical 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 seen in the Catharine wheels of 

 ordinary firework displays, and in hydraulic turbines with 

 multiple jets around their circumferences. 



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 



