292 Royal Institution. 



is also explained in some German elementary works. Attention has 

 been more recently drawn to the subject by a highly interesting 

 paper of Professor Magnus of Berlin {Verhandlungen der Kiinigl, 

 Preuss. Akad. 1852, translated in Taylor's Foreign Scientific Me- 

 moirs, N. S. part 3, p. 210), in which some remarkable applications 

 of this apparatus are given ; he also describes it (with a figure), and 

 observes that the execution of it requires great delicacy and correct- 

 ness of workmanship. Copies of this instrument have indeed been 

 made in this country (one of which was exhibited through the kind- 

 ness of Professor Wheatstone) ; but of these the author believes no 

 description has ever appeared in English works, and they are cer- 

 tainly very little known, notwithstanding their manifest value to 

 every lecturer : the essential parts are a sphere capable of rotating 

 about an axis whose extremities rest in opposite points of a hoop 

 which can turn on pivots horizontaUy, within another hoop turning 

 on pivots about a vertical line. 



In fact, the author of the present communication has long felt the 

 want of such an apparatus for lecture illustration ; and before he 

 was aware of the existence of any of those just alluded to, had con- 

 structed one in a diff'erent form, and which is found to answer fully 

 the purposes of illustration for which it is designed, without any nice 

 workmanship or complex machinery, (See Astronomical Society's 

 Notices, vol. xiii. pp. 221-248.) 



Its object, like that of the instrument last mentioned, is to exhibit 

 experimentally the actual composition of rotations about two different 

 axes impressed at once on the same body. 



The essential parts are merely a bar capable of rotating freely 

 about one end of an axis (and loaded at its extremities to keep up 

 the rotation), while the axis itself can turn about a point in its length 

 near the end carrying the bar, upon a horizontal axis, capable of 

 moving freely round a vertical pillar. At the lower end of the first 

 axis is a weight which more than counterpoises the upper part. 



If, then, there be no rotation in the bar about the first axis, the 

 effect of the weight is to produce a rotation about the second alone, 

 bringing down the first axis into a vertical position. 



If now the first axis be held horizontally or obliquely, and a rota- 

 tory motion be given to the bar about it, on letting the axis go we 

 compou7id both rotations ; and the resulting effect is, that the weight 

 will no longer bring the axis down, or alter its inclination at all, but 

 will cause it to take a new position, or make the whole to turn 

 round the vertical, in a direction opposite to that of the rotation. 



Thus, although confessedly not new in principle, to make public 

 an experimental illustration in so simple a form may not be without 

 its use for a gi-eat majority of students. 



Even the theoretical principle is capable of being stated in a way 

 quite intelligible to those acquainted only with the very first rudi- 

 ments of theoretical mechanics, presenting itself in close analogy to 

 that well-known first principle, the composition of rectilinear motion. 



As in this last case, if a body be in motion in one direction, and 

 any cause tend to make it move in another, it will move in neither, 

 but in an intermediate direction,— so we have the strictly analogous 



