In the eclipse of 1842, it was not only the vivacious Frenchman who was carried away in the impulse of the moment, and had afterwards to plead that "_he was no more than a man_" as an excuse for his unfulfilled part in the observations, but the same was the case with the grave Englishman and the more stolid German. In 1851, much the same failure in the observations occurred; and on some person asking a worthy American, who had come with his instruments from the other side of the world expressly to observe the eclipse, what he had succeeded in doing? he merely answered, with much quiet impressiveness, "_That if it was to be observed over again, he hoped he would be able to do something, but that, as it was, he had done nothing: it had been too much for him._" This is not quite so bad as the fashionable lady who had been invited to look at an eclipse of the sun through a grand telescope, but arriving too late, inquired whether "it could not be shown _over again_." With this brief glance at the science of astronomy, we once more return to the term "gravity," which will introduce to us some new and interesting facts, under the head of what is called "centre of gravity." 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. 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 [Page 33] 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 [Page 34] 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.