piter, gave rise to the discovery of the successive motion of light, which the phenomenon of aberration has ascertained with still greater precision. It appeared to me that the theory of the motion of this satellite being now better known, and the observation of its eclipses become more numerous, their discussion should give the quantity of aberration more exactly than by direct observation. Delambre undertook this investigation at my request, and found the entire quantity of aberration * 625, which is exactly that which Dr. Bradley derived from his observations. It is very curious to observe such a perfect agreement, in results which have been obtained by such very different methods.

It follows from this agreement, that the velocity of light is uniform through the whole space comprehended by the terrestrial orbit. In fact, the velocity of light given by the aberration, is that which sub

* 20" 2.

sists at the circumference of the terrestrial orbit, and which, being combined with the motion of the Earth, produces this phenomenon. The velocity of light, as given by the eclipses of the satellites of Jupiter, is determined by the time which light employs to traverse the terrestrial orbit; these two velocities being the same, the velocity is uniform through the whole length of the diameter of the terrestrial orbit. It results also from these eclipses, that the velocity of light is uniform through the whole diameter of the orbit of Jupiter; for, from the excentricity of this orbit, the effect of the variations in the radii vectores, is very sensible in the eclipses of the satellites; and these exactly correspond to the hypothesis of a uniform velocity, in the motion of light.

If light is an emanation from luminous bodies, the uniformity of its velocity requires that it should be projected from each of them with the same force, and that its motion should not be sensibly retarded by their attraction. If we suppose

light to consist in the vibrations of an elastic fluid, we must then, to explain the uniformity of their velocity, suppose the density of the fluid throughout the whole extent of the planetary system, proportionate to its elasticity. But the simplicity with which the aberration of the stars, and the phenomena of the refraction of light, in passing from one medium to another, are explained by considering light as an emanation from a luminous body, renders this hypothesis extremely probable.

CHAP. VII.

Of the Figure of the Earth and Planets, and of the Law of Gravity at their Surface.

It has been shewn in the First Book, what we have learnt from observations on the figure of the Earth, and of the planets: let us compare these results with those of universal gravitation.

The force of gravity towards the planets, is composed of the attractions of all their particles. If their mass was fluid, and without motion, their strata would be spherical, those nearer the centre being more dense. The force of gravity at their exterior surface, and at any distance whatever, without the sphere, would be exactly the same, as if the whole mass of the planet was compressed into the centre of gravity. It is in consequence of this remarkable property, that the Sun, the

planets, comets, and satellites, act upon each other, very nearly as if they were so many material points. At very great distances. the attraction of the particles of a body of any figure, which are the most remote, and those which are nearest the particle attracted, compensate each other in nearly the same manner as if they were united in the centre of gravity; and if the ratio of the dimensions of the body be considered as a very small quantity of the first order, this result will be exact to a quantity of the second order. But in a sphere, it is rigorously true, and in a spheroid differing but little from a sphere, it is of the same order as the product of its excentricity, by the square of the ratio of its radius, to the distance of the point attracted. This property of the sphere, of attracting as if its mass was concentrated in its centre, contributes greatly to the simplicity of the motions of the heavenly bodies. It does not belong exclusively to the law of nature, it equally appertains to the law of the attraction