Earth; but we may approach very near to the centre of their action, and this joined to the precision of modern observations, ought to render their effects perceptible.

The mountains of Peru, the highest in the world, seemed the most proper for this object. Bouguer did not neglect so important an observation in the journey which he undertook, for the measure of the meridional degrees at the equator.

But these great bodies being volcanic and hollow in their interior, the effect of their attraction was found to be much less than might be expected from their size. However it was perceptible; the diminution of the force of gravity at the summit of Pichincha, would have been 0.00149, without the attraction of the mountain, and it was observed to be 0.00118. The effect of the deviation of the plumb-line, from the action of another mountain, surpassed 20". Dr. Maskelyne has since

* 6" 4.

measured, with great care, a similar effect, produced by the action of a mountain in Scotland: the result was, that the mean density of the Earth, is double that of the mountain, and four or five times greater than that of the common water. This curious observation deserves to be repeated several times on different mountains, whose interior construction is well known.

Let us apply the preceding theory to Jupiter.

The centrifugal force due to the motion of rotation of this planet, is nearly

of the force of gravity at its equator; at least, if the distance of the fourth satellite from its centre, as given in the second Book, be adopted.

If Jupiter was homogeneous, the diameter of its equator might be obtained, by adding five-fourths of the preceding fraction to its shorter axis; taken as unity, these two axes would, therefore, be in the proportion of 41 to 36. According to observation, their proportion is that of 14 to 13. Jupiter, therefore, is not homogene

Supposing it to consist of strata, if the densities diminish from the centre to the surface, its ellipticity should be included between and, the observed ellipticity being within these limits, proves the heterogenity of its strata, and by analogy that of the strata of the terrestrial spheroid, already rendered very probable in itself, and from the observations of the pendulum.

CHAP. VIII.

On the Figure of Saturn's Ring.

THE ring of Saturn, as has been shewn in the first Book, is formed of two concentric rings of very small thickness. By what mechanism do these rings sustain themselves round the planet? It is not probable that this should take place from the simple adhesion of their particles. Since, were this the case, the parts nearest to Saturn, sollicited by the constantly renewed action of gravity, would be at length detached from the rings, which would, by an insensible diminution, finally disappear, like all those works of nature which have not had sufficient force to resist the action of external causes. These rings support themselves then without effort, and only by the lines of equilibrium. But for this

it is requisite to suppose them possessed of a rotary motion round an axis perpendicular to their plane, and passing through the centre of Saturn, so that their gravitation towards the planet, may be balanced by the centrifugal force due to this motion.

Let us imagine a homogeneous fluid spread round Saturn in the form of a ring, and let us see what ought to be its figure, for it to remain in equilibrium, in consequence of the mutual attraction of its particles, of their gravitation towards Saturn, and their centrifugal force. If, through the centre of the planet, a plane is imagined to pass, perpendicular to that of the ring, the section of the ring by this plane, is what I shall call the generating curve. Analysis proves that if the magnitude of the ring is small in proportion to its distance from the centre of Saturn, the equilibrium of the fluid is possible when the generating curve is an ellipse of which the greater axis is directed towards the centre of the planet. The duration of the rota