_Stability of the Solar System._--The well-known demonstrations of the stability of the solar system, given by Lagrange and Laplace, are not to be accepted in an unlimited sense. They make no provision against the destructive agency of a resisting medium, or the entrance of matter into the solar domain from the interstellar spaces. In short, the conservative influence ascribed to these celebrated theorems extends only to the major planets; and even in their case it is to be understood as applying only to their mutual perturbations. The phenomena of shooting-stars and aerolites have demonstrated the existence of considerable quantities of matter moving in _unstable_ orbits. The amount of such matter within the solar system cannot now be determined; but the term probably includes the zodiacal light, many, if not all, of the meteoric rings, and a large number of comets. These unstable parts of the system are being gradually incorporated with the sun, the earth, and doubtless also with the other large planets. It is highly probable that at former epochs the quantity of such matter was much greater than at present, and that, unless new supplies be received _ab extra_, it must, by slow degrees, disappear from the system. The fact, now well established, of the extensive diffusion of meteoric matter through the interplanetary spaces has an obvious bearing on Encke's theory of a resisting medium. If we grant the existence of such an ether, it would seem unphilosophical to ascribe to it one of the properties of a material fluid--the power of resisting the motion of all bodies moving through it--and to deny it such properties in other respects. Its condensation, therefore, about the sun and other large bodies must be a necessary consequence. This condensation existed in the primitive solar spheroid, before the formation of the planets: the rotation of the spheroid would be communicated to the coexisting ether; and hence, _during the entire history of the planetary system, the ether has revolved around the sun in the same direction with the planets_. This condensed ether, it is also obvious, must participate in the progressive motion of the solar system. But again; even if we reject the doctrine of the development of the planetary bodies from a rotating nebula, we must still regard the density of the ether as increasing to the center of the system. The sun's rotation, therefore, would communicate motion to the first and denser portions; this motion would be transmitted outward through successive strata, with a constantly diminishing angular velocity. The motion of the planets themselves through the medium in nearly circular orbits would concur in imparting to it a revolution in the same direction. Whether, therefore, we receive or reject the nebular hypothesis, the resistance of the ethereal medium to bodies moving in orbits of small eccentricity and in the direction of the sun's rotation, becomes an infinitesimal quantity.