In the course of such vast ages the reaction of the earth on the moon’s orbit has not only become perceptible, it has become conspicuous; it has not only become conspicuous, but it has become the chief determining agent in making the moon’s orbit as we find it at the present day. We have seen that as we look into the past the length of the day seems ever shorter and shorter; and concurrent with this decline in the day is the diminution in the moon’s distance from the earth. There was a time when the moon, instead of revolving at a distance of two hundred and forty thousand miles, as it does at present, revolved at a distance of only two hundred thousand miles. As we think of epochs still earlier we discern the moon ever closer and closer to the earth, until at last, at that critical time in the history of the earth-moon system, when the earth was quickly revolving in a period of a few hours, our satellite seems to have been quite close to the earth; in fact, the two bodies were almost in contact The study of the tides has therefore conducted us to the knowledge of a remarkable configuration exhibited in the primitive earth-moon system. The earth was then spinning round rapidly in a day which was only a few hours long, while close to the earth, or almost in contact with it, the moon coursed around our globe, the period of its revolution being shorter to such an extent that the satellite completed its circuit in the same time as the earth required for one turn round its axis. We must remember that the materials destined to form the pair of allied planets did not then form two solid bodies as they do at present; they were both, in all probability, incandescent masses glowing with fervor, and soft, if not actually molten, or incoherent, or even gaseous. These aggregations were close together, and one of them was whirling around the other in a period of a few hours, the duration of that period being equal to the time in which the larger mass revolved on its axis. In fact, the two objects, even though distinct, seem to have revolved the one around the other as if they had been bound together by rigid bonds. The rapid rotation with which they were animated suggests a cause for this state of things. It is well known that a fly-wheel, when driven at an unduly high speed, is liable to break asunder in consequence of its rapid motion. If a grindstone be urged around with excessive velocity the force tending to rend the stone into fragments may overcome its cohesion, and it will fly into pieces, often projected with such violence that fearful accidents have been the consequence.