296 Royal Institution. 



These higher mathematical views, though of course the most 

 complete and systematic, are not the most direct or easy mode of 

 explaining the subject to the student. Greater simplicity certainly 

 characterizes the method adopted by Mr. Airy (in the tract before 

 cited) of applying directly the theorem of the composition of rotatory 

 motion, as doubtless Newton would have done had it been known to 

 him. But here, as in so many other instances, the first explanation 

 presented itself mixed up with more complex considerations ; and 

 as has been well observed, " simplicity is not always a fruit of the 

 first growth." 



To those not versed in the mathematical theory of all points in 

 physical astronomy, the " modus operandi" of the precession, per- 

 haps, usually seems the most paradoxical ; and the explanations 

 given in some of the best popular treatises are seldom found satis- 

 factory, following as they do the letter of Newton's illustration, and 

 omitting the direct introduction of the principle of composition, 

 W'hich, if only from what has been here off^ered, is at once seen to be 

 easily capable of the most elementary explanation. Indeed it was 

 from this consideration forcing itself on the mind of the author, in 

 several courses of popular lectures on astronomy, that he was led to 

 seek the means of experimental illustration above described ; and 

 which would more palpably imitate the pheenomena to the eye, if, 

 instead of the rotating bar a terrestrial globe be substituted (as in 

 Bohnenberger's instrument) — for better illustration made protuberant 

 at the equator — where the weiglit at the south pole acts the part of 

 the sun's and moon's attraction, to pulldown the protuberant matter 

 of the spheroid at the equator if at rest, but when combined with 

 the earth's rotation results in a transference of the position of its 

 axis, or slow revolution of its pole round the pole of the ecliptic in 

 a direction opposite to its rotation, carrying with it the equinoctial 

 points, and causing the signs of the zodiac to shift backwards from 

 their respective constellations. 



It always affords a sort of intellectual surprise to perceive for the 

 first time the application of some simple and familiar mechanical 

 principle to the grand phsenomena of astronomy ; to see that it is 

 but one and the same set of laws which governs the motions of 

 matter on the earth and in the most distant regions of the heavens ; 

 to find the revolution of the apsides in a pendulum vibrating in 

 ellipses, or the conservation of areas in a ball whirled round by a 

 string suddenly shortened ; or (as in the present case) to perceive a 

 celestial phsenomenon, vast in its relations both to time and space, 

 and complex in its conditions, identified, as to its mechanical cause, 

 with the rotatory movement of a little apparatus on the table before 

 us, or to discover tlie jirecession of equinoxes in the deviation of a 

 rifle or a boomerang. And the simple experimental elucidation of 

 such phaenomena and their laws will not be useless, as it tends to 

 confirm in the mind of the student the great characteristic of the 

 modern ])hysical philosophy first asserted by Galileo, the identity of the 

 causes of the celestial and terrestrial motions, and to aid and elevate 

 our conception of those grand and simple principles according to 

 which the whole machinery of the universe is so profoundly adjusted. 



