plunge the eastern portion of the wheel below the surface-The axis now is tilted towards the east.— The experiment is simple, and shows that, in case the successive portions of the wheel be submerged, the axis will always be tilted towards the point which goes under first. To reverse the experiment: in case we take hold of the axle and turn it east, it sinks the eastern half of the wheel below the surface of the water, while the western half is raised out of it, and then in case we make the upper extremity of the axis follow round the circumference of a circle whose surface is parallel to that of the water, and whose centre is exactly above the centre of the wheel, it will be seen that, as the axle moves round, successive portions of the wheel are submerged, until finally the water line will have divided the wheel into all its successive halves, and will have successively coincided with every possible diameter of the wheel.

Now for the application. The level surface of the water is the level plane of the earth's orbit, the wheel is the earth's equator, and the axle is the earth's axis of rotation. One-half of the equator is constantly submerged below the plane of the ecliptic -the other half rises above it. But the water line, or the intersection of these two planes, the equinoctial line, cannot remain fixed in the same line. A power does seize the equator and plunge successive halves of it beneath the plane of the ecliptic, changing perpetually the water line, until finally each half in succession, into which all its diameters can be divided, are sunk below the surface, or plane of the ecliptic, thus causing the earth's axis to tilt over towards the portions successively submerged, until it

finally sweeps entirely round and comes to resume its first position.

But do you now demand what power seizes the earth's protuberant equator, and tilts it successively towards every point of the compass? I answer, that the power is lodged in the sun and moon, and it is their combined action which works out these wonderful results. In case the sun and moon were so situated as always to be in the plane of the earth's equator, then they would have no power to change the position of the equator. But we know that they are not in this plane, except when passing through it, and are found sometimes on the north and sometimes on the south side of it. Wherever either of them may be, the nearest half of the redundant matter about the earth's equator will be more forcibly attracted than the remote half, and the equator will be tilted towords the attracting body, and the axis of the earth will follow the movement of the equator to which it is firmly fixed.

Thus does the earth's whole solid mass sway to the motion of the ring of matter heaped up around the equator, delicately and beautifully sensitive to all the changes in the relative places of the sun and moon. Neither the earth nor its axis are ever, for one moment, released from the action of these remote bodies. However slight the effects, however varied in action, oscillating to every point within certain prescribed limits, the stability is preserved, and the final effect is a small retrograde motion of the equinox at the end of every year, and a slight change in the place of the pole of the heavens.

But there is no isolated matter in the universe.-.

Every particle of matter attracts every other particle of matter, and it is impossible for the sun and moon to exert any influence on the equatorial ring of matter which belongs to our globe, without feeling, in their turn, the reaction of this ring on themselves.— The remote and ponderous sun may, in consequence of its vast size and distance, escape from any effect capable of being detected by observation. But this is not the case with the moon. Her proximity to the earth and diminutive mass, render her peculiarly sensitive to the influence of the redundant matter at the earth's equator, and as her attraction tilts the plane of the earth's equator, so does the equatorial ring tilt the plane of the moon's orbit. These effects have been accurately observed and measured, and strange to relate, their exact values have exhibited the figure which belongs to the earth with far greater precision than.can be obtained from measures on its surface. We may even go farther, for such is the intimate relationship between the earth and its attend ant satellite, that there is scarcely a question can be asked with reference to the one, that is not answered by the other.

If we demand the weight of the earth when compared with the sun, the moon answers. If the excess of the equatorial diameter of the earth over the polar be required, the moon answers. If the homogenity of the interior of the solid earth be required, the moon replies. If the thickness of the earth's crust is sought for, question the moon, and the answer comes. If you would know the sun's distance from the earth, ask the moon. If the permanency of the axis of rotation be required, ask the moon, and she alone yields

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a satisfactory reply. Finally, if curiosity leads us to inquire whether the length of the day and night, the revolution of the earth on its axis, be uniform, or whether it may not have changed by a single second in a thousand years, we go to the moon for an answer, and in each and every instance her replies to all these profound and mysterious questions are clear and satisfactory. How wonderful the structure of the universe! How gigantic the power of the human intellect! If all the stars of heaven were struck from existence; if every planet and satellite which the eye and the telescope descry, inside and beyond the earth's orbit, were swept away forever, and the sun, earth and moon alone remained for the study of man, and as evidences of the being and wisdom of God, in the exquisite adjustments of this system, in the reciprocal influences of its three bodies, in their vast cycles of configuration, in their relative masses, magnitudes, distances, motions, and perturbations, there would remain themes sufficient for the exercise of the most exalted genius, and proof of the being of God, so clear and positive, that no sane mind could com. prehend it and disbelieve.

LECTURE VI.

THE STABILITY OF THE PLANETARY SYSTEM.

WHEN, by the application of a single great law, the mind had succeeded in resolving the difficult problems presented by the motions of the earth and its satellite, the moon, it rose to the examination of the higher and more complicated questions of the stability of the entire system of planets, satellites, and comets, which are found to pursue their courses round the sun. The number of bodies involved in this investigation, their magnitudes and vast periods of revolution, their great distances from the observer, and the exceeding delicacy of the required observations, combined with the high interest which attaches itself to the final result, have united to render this investigation the most wonderful which has ever employed the energies of the human mind.

To comprehend the dignity and importance of this great subject, let us rapidly survey the system, and moving outward to its known boundaries, mark the number and variety of worlds involved in the investigation. Beginning, then, at the great centre, the grand controlling orb, the sun, we find its magnitude such as greatly to exceed the combined masses of all its attendant planets. Indeed, if these could all be arranged in a straight line on the same side of the sun, so that their joint effect might be exerted on that body, the centre of gravity of the entire system thus