Page:Popular Astronomy - Airy - 1881.djvu/296

282 when it is dropped ; otherwise, (referring to Figure 66,) it would at once be carried out of the plane BAN, and the previous reasoning would no longer hold. In practice, this object is attained by fastening the sphere to a wall or other fixed body by means of a second string, so as to keep it in such a position as AB ; and then setting the second string on fire at an instant when the sphere is observed to be conpletely at rest.

In general, a body which is set rotating about a line will not continue to revolve about it; on the contrary, the axis of rotation will usually change its position relatively to the body. Whatever be the shape of the body, however, it is always possible to find a line fixed relatively to the body, such, that the body having once begun to rotate about it will continue to do so, provided no external force act: such a line is called a permanent axis of rotation. If the body be perfectly symmetrical about any line within it, that line is a permanent axis; thus the diameter of a sphere, the diagonal of a cube, and the axis of revolution of a spheroid, (page 60,) are permanent axes of the sphere, cube, and spheroid. If, further, when a body has once been made to rotate