connected icith Astronomical Physics. 267 



" The interesting property o£ rotating bodies, illustrated 

 above in the case of the gyroscope and fully explained by its 

 theory, now at once makes the matter perfectly clear. In 

 the case of the planetary bodies, the force rotating the stand 

 of the gyroscope is supplied by the annual tide raised on the 

 planets by the sun. In former times, when the planets were 

 large diffuse bodies, the tidal force was of considerable im- 

 portance. Neptune, however, is so remote from the sun that 

 the tidal influence upon it has always been small. The plane 

 of its rotation, therefore, has been but slightly shifted from 

 that of its orbit — about 35°. Uranus being nearer the sun, 

 has had its plane shifted nearly half-way over, or through 

 &2°*. The plane of rotation of Saturn has been shifted through 

 153°, while that of Jupiter has suffered a nearly complete 

 reversal, and the planet now revolves approximately in the 

 plane of its orbit. The deviation amounts to but 3°, and the 

 plane of its rotation has therefore shifted through 177°. 



" The explanation of the retrograde motion of Phoebe is 

 now also clear. Phoebe, the first-born of Saturn's numerous 

 retinue, came into being while the planet itself still retained 

 its original plane of rotation, that is, while it was still re- 

 volving in a retrograde direction. Before Japetus, Saturn's 

 second satellite, reckoning from without inwards, was created, 

 the mighty tides acting upon the planet in its then diffuse 

 condition had shifted its plane of rotation more than 90°. " 

 (' Nature, 5 April 27, 1905, pp. 608-609.) 



The case of our earth, — where, from its greater proximity 

 the sun, the reasoning would be more cogent than in the 

 instance of the planet Jupiter, — is not alluded to by Prof. 

 Pickering, who takes as a basis the Nebular Hypothesis, 

 involving a fiery origin to our planet, but a birth subsequent 

 to Jupiter. The far less time available for past influence of 



* More correctly in the case of the planet Uranus, the axis has shifted 

 through 100° approximately, rather than 82° [query ?]. 



Contrary to the preconceived ideas of some, a planet's axis can be in- 

 verted [i. e. turned through 180°) without the performance of work (in 

 the mere deviation of the axis). So in the experiment above described, 

 the gyroscope inverts itself without resistance encountered. For we may 

 observe that merely to change the direction of an axial line in space, 

 by itself entails no work. It may be superfluous to add that the work 

 concomitant on tidal-action in producing deviation of an axis, is the 

 equivalent of the heat inevitably attendant on that particular physical 

 process. 



Distinguishing nomenclature between " resistance " where work is done, 

 and " constraint " where there is mere guidance, might be advantageous : 

 and Prof. Poynting agrees that " The distinction would certainly add to 

 clearness " — as I may sav. 



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