of that density. Gravitation and attraction are generally used as synonymous terms, more especially gravitation--somewhat after the manner of the likeness between the two negroes, Cæsar and Pompey, the latter being most especial in the likeness--but there is a very appreciable distinction between them, if we want to use each of them in its proper and strict sense. Gravitation implies the conception of a weight of some kind falling to a fixed centre, while attraction gives the idea of two weights, or masses, drawing each other to a common centre, which when properly looked at is a different thing; because the centre may be anywhere between the two, depending entirely on the difference, if any, in the weights of the masses. The confounding of the two, or rather the almost universal adoption of the less correct term, name, expression--whichever it may be called--has been the cause of wrong conceptions being formed of the construction of almost all--probably all--celestial bodies, and of that most absurd expression, _attraction of gravitation_, used by all our most eminent physicists. The _gravitation_ of _attraction_ might be excused, but putting cause for effect is hardly scientific. A name is nothing as long as what is meant by it is understood and taken into consideration, but that is not always the case, as we shall proceed to show. The term gravitation may be applied with almost, but not absolutely, perfect strictness to the attraction between the sun and the planets, because the common centres of their attractions and the centre of the sun are so near each other that they may be looked upon as one and the same thing, or point; but it is not so with the attractions of the planets for each other where there is no common fixed centre, or if there is something approaching to it in a far off way, it is constantly varying, so that the term gravitation cannot be strictly applied to them, nor even to the sun, to speak truly. Planets sometimes _gravitate away_ from each other and from the sun, otherwise Adams and Leverrier could not have discovered Neptune from the perturbations of Uranus. Neither can it be properly applied to the different masses of matter in the sun or in the earth--although it was no doubt notions connected with the earth that gave rise to the term, from all ponderable matter falling upon it--because _per se_ they could have no tendency to fall to the centre, for _at the centre_ there is no sufficient attractive force to draw them towards it. Gravitation was a known term long before the days of Newton, who had the glory of enlightening the world by showing that attraction was the cause of it; and, perhaps unfortunately, the name was continued to represent what it in reality does not.