266 Description of the Rotascope. 
ertia of the parts being separately determined and allowed for, the 
theoretical 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 may be briefly stated on the plan of what are 
called rectangular coérdinates. 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 a 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 brazen meridian a common artificial 
globe, and, having given it a rapid motion about its axis, attempt f° 
move the poles from their position in space, we shall find our efforts 
resisted and the globe impelled in various directions,’ in 4 mente 
which will generally surprise those to whom the experiment 18 new: 
If the globe be held by the meridian at points over the equator! 
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 meridians wt 
point at or near its equator, the globe if not caused to revolve oP s 
axis, may be made to oscillate in any direction, without ten 
