on Rotator}/ Motion. 



399 



as they will form an appropriate introduction to the beautiful 

 researches on the subject which Professor Wheatstone has very 

 recently brought before the Royal Society. 



In Poggendorff's Annalen, Sept. 1853, No. 9, Prof. Plucker 

 has given some account of the rotatory machine of M. Fessel. 

 Notwithstanding the luminous theoretical researches on the sub- 

 ject in the papers of MM. Poinsot and Poisson, he observes that 

 experimental illustration of it is still desirable, and mentions 

 that M. Fessel, "formerly teacher in the provincial school of 

 industiy, now by the kindness or unkiudness of circumstances 

 led to exercise his art as a mechanician," has constructed an ap- 

 paratus, which, from the way in which it presents the phseno- 

 mena to the eye, excites surprise even in those who are acquainted 

 with the principle. 



The instrument will be understood at once from the sketch 

 annexed. 



The disc A, loaded 

 round its circumfer- 

 ence, can be spun 

 rapidly on the axis B, 

 whose ends rest in the 

 ring C, itself joined by 

 a hinge at I) to the 

 axis bent at a right 

 angle at E, the ver- 

 tical pai't of which 

 turns freely in the 

 tube F, supported on 

 the stand G, 



The obsei-ved phsenomenon is this. When the disc is set in 

 rapid rotation about B, the whole begins also to revolve bodily 

 round the vertical axis E, and this is easily seen to be due to 

 the action of gravitation tending to bring down the ring C 

 about D. 



M. Fessel adds, in a note, an account of the way in which he 

 accidentally discovered the principle, by rotating a heavy wheel 

 on one end of an axis. And M. Pliicker proposed to him to 

 modify the construction by attaching the ring C to one end of a 

 bar carrying a sliding counterpoise, and capal)le of revolving 

 about a jtivot ; and this, he says, answered adniiraljly. 



"The apparatus thus improved," Prof. Pliicker continues, 

 " gave the jjrevious phicnomena, but at the same time others 

 immediately appeared. 



" If the disc rotated in a vertical plane, then the axis moved 

 its(;If round towards the opposite direction in the horizontal 

 plane. If the first rotatory motion gradually lost, the second 



