260 Description of the Roiascope, 



ertia of the parts being separately determined and allowed for, the 

 iheoreiical laws of motion are seen to be perfectly confirmed by the 

 experiments. 



As to the manner in which the principles of rotation have gener- 

 ally been explained, it maybe briefly stated on the plan of what are 

 called rectangular coordinates. As by referring the effect of any 

 force applied opposite to the centre of gravity of a body at rest, to 

 three lines mutually crossing each other at right angles, the resulting 

 direction which the centre of gravity of that body will take in free 

 space, is inferred ; so, by a consideration of three perpendicular axes 

 of revolution within the body itself, we may determine the effect of 

 any number of forces tending to produce rotation. Combining these 

 two together, we have the resultant motions, both of rotation and 

 translation. One of the most important propositions pertaining to 

 the physical character of the subject, is that discovered and demon- 

 strated by Frisi ; — that " when <i body revolves on an axis, and a 

 force is impressed, tending to make it revolve on another, it will re- 

 volve on neither, but on a line in the same plane with them, dividing 

 the angle which they contain so that the sines of the parts are in the 

 inverse ratio of the angular velocities with which the body would 

 have revolved about the said axes separately." 



The following elementary experiments and observations were the 

 leading facts, which suggested the construction of the instrument 

 hereafter described. 



1. When we take up by its hrnzen meridian a common artificial 

 globe, and, having given it a rapid motion about its axis, attempt to 

 move the poles from their position in space, we shall find our efforts 

 resisted and the globe impelled in various directions, in a manner 

 which will generally surprise those to whom the experiment is new. 

 If the globe be held by the meridian at points over the equatorial 

 circle of the sphere and the axis be placed nearly vertical, and if in 

 this state of things the revolving globe be carried alternately from 

 right to left, and the reverse by the extended arms around in a small 

 orbit or portion of an orbit, the tendency of the sphere to change the 

 position of its axis will be felt in one or both directions of the move- 

 ment. 



2. Whenever we suspend, in the manner of a pendulum, by a rod 

 a few feet in length, an artificial globe, by its brazen meridian, at a 

 point at or near its equator, the globe if not caused to revolve on its 

 axis, may be made to oscillate in any direction, without tending to 



