400 Note on some Eccperiments on Rotatory Motion. 



gained. If the second motion was quickened by an extraneous 

 force, tlien the ring with the disc appeared to become lighter, 

 since it rose up ; on the contrary, by a retardation of the hori- 

 zontal movement, it seemed to become heavier, or sank. With 

 the same rotatory velocity of the disc, the horizontal rotation 

 increased or diminished according as the force of gravitation was 

 increased or diminished; as, for example, if the rotating disc be 

 made of soft ix'on, it rises or sinks if we precede or follow it with 

 a magnet, thereby accelerating or retarding the horizontal revo- 

 lution." 



He also points out the analogy of this apparatus to that of 

 Bonenbcrger. In the Annalen, No. 10 (Oct. 1853), the editor 

 adds " one more word on the Fessel rotatory machine," in which 

 he confirms the account of its inventor by his own experience of 

 its use, and adds an explanation of its principle, as he observes, 

 for the sake of those who may wish to understand it without 

 going into calculation. 



The genei-al nature of this explanation may be stated thus : 

 the disc being supposed to rotate in a vertical plane, or the axis 

 B horizontal ; then at any point in its circumference the motion 

 in the direction of the tangent may be conceived resolved into a 

 horizontal and a vertical component, the direction of motion in 

 each being of course opposed at opposite parts of the circle. 



Now if the ring C be free to be acted on by gravitation, or 

 the axis B tend to become inclined, or the rotation to take place 

 in an oblique plane, the horizontal components will not change 

 their direction, but the vertical components will ; and will no 

 longer have their direction in the plane of the disc : they will 

 consequently cause it to change its plane, or exert a lateral force, 

 and thus give the whole a motion of revolution round the vertical 

 axis E. But while this takes place, the tendency to inclination 

 of the axis B is counteracted, and the opposing latei'al effects of 

 the horizontal components result in the rotation of the disc 

 round its horizontal axis in a vertical plane. 



M. Fessel has added a small slider below D, which, when 

 pushed out, supports the ring C against gravitation ; and in this 

 case there is no revolution round the vertical. M. Poggendorff 

 also observes, that if the i-otatory part of the instrument be 

 balanced about a point near D, either on a pivot, or suspended 

 by a string with a counterpoise, the same results ai'e more satis- 

 factorily exhibited. 



He observes that the same principles apply to the explanation 

 of Prof. Magnus's remarkable experiment with two discs rotating 

 at opposite ends of a suspended axis. (See his paper translated 

 in Taylor's Foreign Scientific Memoirs, Part III. May 1853, 

 p. 229.) 



