opened for the explanation of some of the most difficult problems with which the mind has been obliged to contend.

The diminution of the periodic time of Encke's comet has led some astronomers to adopt the idea of the existence of a resisting medium. But in case the sun possesses the power of repelling the matter of comets in their perihelion passage, a part of the matter thus repelled may be driven entirely beyond the attractive influence of the nucleus, and be irrecoverably lost. In this case, a diminution of mass would inevitably involve a like diminution of periodic time, a contraction of the orbit, and all the phenomena presented by this mysterious object. Herschel even thinks it possible, on this theory, to account for the separation of Biela's comet into two distinct objects, and it appears to me that it presents the most reasonable explanation of the luminous appearance seen at certain seasons of the year, called the zodiacal light. This phenomenon appears to be a ring of nebulous matter surrounding the sun, and some of whose particles are sustained at a much greater distance than could be accounted for by gravitation. Admitting the repulsive power already adverted to, there is no difficulty in understanding how this nebulous ring may be sustained at a vast distance from the sun.

Here we freely admit that we enter the confines of the unknown. We have left the solid ground of truth and certainty, and are pushing our investigations into the dim twilight of the invisible and uncertain. But as antiquity predicted that the time would come when the comets would be traced in their career, their periods revealed, and their orbits ascertained, so we may confidently hope that, at no very distant day, all the mysteries which hang around these chaotic worlds will be fully revealed, and a knowledge of their physical condition shall reward the long study and deep research of the human mind.

LECTURE IX.

THE SCALE ON WHICH THE UNIVERSE IS BUILT.

HUS far our attention has been directed to an examination of the achievements of the human mind within the limits of our own peculiar system. We have swept outward from the sun through the planetary worlds, until we have reached the frontier limits of this mighty family. Standing upon the latest found of all the planets, at a distance of more than 3,000,000,000 of miles from the sun, we are able to look backwards, and examine the worlds and systems which are all embraced within the vast circumference of Neptune's orbit. An occasional comet, overleaping this mighty boundary, and flying swiftly past us, buries itself in the great abyss of space, to return after its "long journey of a thousand years," and report to the inhabitants of earth the influences which have swayed its movements in the invisible regions whither it speeds it flight.

The magnificence and complexity of the great system of planets, and satellites, and comets, which constitute the sun's retinue, the immense magnitude of some of these globes, their periods of revolution, and reciprocal action, would seem to furnish a sufficient exercise, not only for the highest intellectual efforts, but for the entire energy which the human mind can exert. But the whole of this stupendous scheme, as we shall

soon see, is but an infinitesimal portion of the universe of God, one unit among the unnumbered millions which fill the crowded regions of space. Standing at the verge of the planetary system, we find ourselves surrounded by a multitude of shining orbs, some radiant with splendour, others faintly gleaming with beauty. The smallest telescopic aid suffices to increase their number in an incredible degree, while with the full power of the grand instruments no in use, the scenes presented in the starry heavens become actually so magnificent as to stun the imagination and overwhen the reason. Worlds and systems, and schemes and clusters, and universes, rise in sublime perspective, fading away in the unfathomable regions of space, until even thought itself fails in its efforts to plunge across the gulf by which we are separated from these wonderful objects.

In our measurements within the limits of the solar system, the radius of the earth's orbit has sufficed for a unit with which to exhibit the distances of the planets and comets. Great as is this unit, measuring no less than 95,000,000 miles, we shall soon find it far too minute and insignificant to serve in our researches with reference to the grand scale of the visible universe. To obtain comprehensible ideas with reference to the interstellar spaces, we shall be obliged to call to our aid a unit, not exactly of distance, but of velocity; and before entering on the full exhibition of the main object of this lecture, permit me to direct your attention to a remarkable discovery, by which the important fact has been revealed, that light does not pass instantly from a luminous body to any remote object on which it may fall, but with a progressive motion, whose actual velocity has been ascertained. The important bearing of this discovery will become apparent as we advance in our examination of the sidereal heavens.

After the motions of the four moons of Jupiter had been sufficiently observed to construct tables of their movements, with a view to predict their eclipses, some unaccountable phenomena presented themselves, which, for a long time, baffled all efforts to explain them. It should be remembered, that the orbit of Jupiter encloses that of the earth, and when the two planets happen to be on the same side of the sun, and in a straight line passing through that orb, they are then at their least distance from each other, and are said to be in conjunction. Now suppose Jupiter to remain stationary, at the end of half a year the earth will