CHAPTER IV.
CENTRE OF GRAVITY.
That point about which all the parts of a body do, in any situation, exactly balance each other.
The discovery of this fact is due to Archimedes, and it is a point in every solid body (whatever the form may be) in which the forces of gravity may be considered as united. In our globe, which is a sphere, or rather an oblate spheroid, the centre of gravity will be the centre. Thus, if a plummet be suspended on the surface of the earth, it points directly to the centre of gravity, and, consequently, two plummet-lines suspended side by side cannot, strictly speaking, be parallel to each other.
Fig. 40.
f. The centre. a b c d e. Plummet-lines, all pointing to the centre, and therefore diverging from each other.
If it were possible to bore or dig a gallery through the whole substance of the earth from pole to pole, and then to allow a stone or the fabled Mahomet's coffin to fall through it, the momentum—i.e., the force of the moving body, would carry it beyond the centre of gravity. This force, however, being exhausted, there would be a retrograde movement, and after many oscillations it would gradually come to rest, and then, unsupported by anything material, it would be suspended by the force of gravitation, and now enter into and take part in the general attracting force; and being equally attracted on every side, the stone or coffin must be totally without weight.
Momentum is prettily illustrated by a series of inclined planes cut in mahogany, with a grooved channel at the top, in imitation of the famous Russian ice mountains: and if a marble is allowed to run down the first incline, the momentum will carry it up the second, from which it will again descend and pass up and down the third and last miniature mountain.
