192 ANNUAL OF SCIENTIFIC DISCO VEKY. 



published of value on the subject. "We would also say, that the mechanical 

 arrangement referred to is not of recent origin, but has been known for many 

 years. Those interested hi the subject would do well to examine the articles 

 published in the Annual of Sci. Dis. for 1855, pp. 179-186. 



The following is an abstract of a paper recently read before the American 

 Academy, by Prof. Lovering : 



"Since the time of Foucault's celebrated experiment for illustrating the 

 rotation of the earth by the stability of the plane of oscillation, increased 

 attention has been given to the law of inertia as determining the stability of 

 planes of motion. The planes of rotation conform to this general rule of sta- 

 bility. Astronomy furnishes the only examples of perfect free rotating 

 bodies : and astronomy, here, as elsewhere, must be invoked, whenever it is 

 required to give an exact experimental illustration of the fundamental laws 

 of mechanics. Artificial experiments realize but imperfectly this perfect 

 freedom of the spinning earth and other planets. Besides the top and the 

 devil-on-tioo-sticks, in which ' philosophy in sport ' has been made ' science in 

 earnest,' there are Bohnenberger's less familiar apparatus, first described in 

 1817,* and Johnston's JRotascope.\ The necessity has recently been shown 

 of adding to the description of the former the new condition of placing the 

 axis of the apparatus parallel to the earth's axis to avoid the disturbance of 

 the earth's rotation, and the new application of the instrument, when other- 

 wise placed, to detecting this rotation.:}: 



" In 1853, Pliicker published an account of Fessel's apparatus for experi- 

 ments on the laws of rotation ; and, in 1854, Magnus presented to the public 

 an account of his Polytrop, also designed for similar illustrations. || 



"Pliicker preludes his description of the Fessel machine with some remarks 

 'on Poisson's mathematical investigations on the subject of rotations,^" 

 and alludes to Poinset's successful attempt to make the motions generally 

 hidden under the veil of mathematical analysis more sensible to the imagi- 

 nation and the eye.** Poinset thinks that, if many new truths are contained 

 in analysis, they are buried in it for all but a few gifted minds. ' Thus our 

 true method is but this happy mixture of analysis and synthesis, where cal- 

 culation is employed only as an instrument, a precious instrument, and neces- 

 sary without doubt, because it assures and facilitates our progress ; but which 

 has of itself no peculiar virtue ; which does not direct the mind, but which 

 the mind must direct like any other instrument.' 



" The origin of Fessel's machine was as follows : About 1851 this skilful 

 artist of Cologne, who a few years before had distinguished himself by his beau- 

 tiful Wave-machine, particularly adapted for illustrating the mechanical laws of 

 light, was examining the wheel of a model steam engine, and observed that, 

 while rolling it on his hand, the horizontal axis did not require to be supported 

 at both ends, while there was a tendency in the axis to move in a horizontal 

 plane. Fessel's practical skill, aided by the suggestions of the eminent physi- 



* Ann. Gilbert, LX. p. 65. t Silliman's Journal, XXI. p. 265. 



$ Ann. Pogg., XC., pp. 350, 351. Ann. Pogg., XC. p. 174 



1 Ann. Pogg., XCI. p. 298. ^ Journ. de Polytecbn. Ecole, XVI. p. 247. 



** Elemens dc Statiquc, 8th edition, 1842. 



