1858.] on Rotatory Stability/, 481 



these results excited so much wonder when exhibited with a refined 

 apparatus, and in a scientific form, it was forgotten how perfectly 

 similar, and equally paradoxical in its nature, is the common and 

 familiar result of a top sustained by the mere act of spinning in a 

 position from which it directly falls when the rotation ceases. 



Some results of this kind, which had then become known, were 

 on a former occasion brought under the notice of the members of 

 the Royal Institution (March 3, 1854*). On that occasion, there- 

 fore, the discussion was confined to the principle of " composition 

 of rotations,'* and those applications of it which had been found in 

 certain rotatory phenomena of projectiles, illustrated by the gyro- 

 scope in its several earlier forms as successively modified by 

 Bonenberger, Atkinson, Fessel, and Wheatstone, showing the 

 identity of these results on a small scale with the grand cosmical 

 phenomenon of the precession of equinoxes. Since the date of that 

 lecture, the striking results produced by merely carrying out the 

 same principles, and applying the gyroscope to demonstrate directly 

 the fact of the earth's rotation, as well as under other conditions to 

 point to the poles, by M. Foucault, have become familiarly known. 

 It is, however, an act of justice to mention that the former result 

 (the proof of the earth's rotation) was eighteen years before 

 fully pointed out by Mr. Sang,-|- of Edinburgh, and only not prac- 

 tically accomplished from the expense of the necessary apparatus. 

 In recurring to the subject on the present occasion, the object is to 

 explain another application of the same principles, like the former 

 very obvious when once discloSi$d; but which nevertheless remained 

 unknown and unthought of until it was pointed out and actually 

 effected by the inventions of Prof. C. P. Smyth : the use of rota- 

 tory apparatus for giving an invariable plane or platform for 

 astronomical instruments used at sea. To render the subject intel- 

 ligible it is necessary to recur to two simple first principles in dyna- 

 mics, which, when distinctly apprehended, give the clue to the 

 whole of the applications. 



The first of these is the tendency of a body in rotation to retain 

 that rotation in the same plane, when perfectly balanced, irrespee- 

 tive of the motion of external objects, which is termed " the fixity 

 of the plane of rotation." 



The second is " the composition of rotatory motion :" or that 

 when a force is impressed on a body in rotation, it does not show 

 itself directly, but is compounded with the first motion ; so that the 

 rotation takes an intermediate direction, or the axis shifts its posi- 

 tion in space. This being the cause of the motion of the earth's 

 axis, giving rise to the precession of equinoxes, it is called gene- 

 rally a " precessional motion." 



* Proceedings of the Royal Institution, Vol. i., p. 393. 

 t See Edinburgh New Phil. Jounial,Oct. 183G and 1837 ; and Proceedings 

 of the Royal Scottish Society of Arts, 1856. 



Vol. II. 2 l 



