Royal Institution. 395 



Thus it would follow, that this extraordinary instance of savage 

 invention, which long ago puzzled inquirers, is simply a case (like 

 the last) of "the composition of rotatory motion." 



It should, however, be mentioned, that some experimentalists have 

 entertained a different view of the cause of deviation in this instance. 



Besides the results abo/e stated. Professor Magnus (in the same 

 memoir) mentions several other highly curious cases produced by 

 certain modifications oi the apparatus, but all referable to the same 

 principles. 



M. Fessel has also invented an apparatus (since called the gyro- 

 scope), an account of which is given with some remarks by Professor 

 Pliicker, and the editor in Poggendorff's Annalen (1853, Nos. 9 and 

 10), which, though apparently invented Vv'ithout any knowledge of 

 Bohnenberger's apparatus, is a modification of it, referring to phse- 

 nomena (f the same kind as those of the equilibrium experiment 

 mentioned at first. 



This apparatus has been greatl)' improved upon by Professor 

 Wheatstone, who has introduced other movements to include the 

 conditions of rotation in different planes. One of these instruments 

 was exhibited. 



From these singular applications of a very simple mechanical 

 truth, we may now turn to what is but another exemplification of 

 the same thing, however apparently remote from those we have con- 

 sidered, and upon a far grander scale. 



The phsenomenon of the precession of equinoxes was known to 

 Hipparchus, but no explanation of the fact was for ages imagined. 

 Even Kepler, in the multiplicity of his hypothetical resources, could 

 not succeed in devising anything plausible. The axis of the earth 

 is slowly shifting its position, so that its pole points continually to a 

 new part of the heavens — a new pole star — at the rate of about 50" 

 a year, and of course carries with it the point of intersection of the 

 earth's equator with the ecliptic or plane of its orbit, at the same 

 rate and in a direction opposite to that of its motion, or the order of 

 the signs. 



These phaenomena remained wholly without explanation till 

 Newton, led by the analogy of those disturbing forces on the orbit 

 of a planet which cause its nodea to regress, showed that the same 

 would occur in a satellite to the earth — in a ring of such satellites — 

 in such a ring adhering to the equator, or the protuberant part of 

 the terrestrial sphere, and thus that the equinoctial points would 

 slowly regress. ( ee Principia, vol. i. p. 66, corr. 11-22.) 



The more exact determination of quantitative results was reserved 

 for Newton's successors, when a more powerful analysis had been 

 applied by Euler, D'AIembert and others, to the full exposition of 

 the theory, founded on general equations of motion ; as since given 

 in tlie writings of Laplace (^M^c. Cd. liv. xiv. ch. 1), and Pontecou- 

 lant {Tlu'orie du Syste'me du Monde, liv. iv. ch. 5), which are neces- 

 sary for including all the minuter variations detected by Bradley, 

 and subsequent observers, showing the nutation of the axis, and the 

 inequalities of precession due to the varying configurations of the 

 attracting luminaries. 



