We should, however, receive a very inadequate impression of what tides are able to accomplish if we merely contemplated this change in the length of the day, striking and significant though it doubtless is. The student of natural philosophy is well aware that there is no action without a corresponding reaction, and it is instructive to examine in this case the form which the reaction assumes. Our reasoning has been founded on the supposition that it is the attraction of the moon on the waters of our globe that gives rise to the tides. It is, therefore, the influence of the moon which checks the speed of the earth’s rotation and adds to the length of the day. As the moon acts in this fashion on the earth, so, by the general law that I have mentioned, the earth reacts upon the moon. The form which this reaction assumes expresses itself in a tendency to allow the moon gradually to move further and further away from the earth than the earth’s attraction would permit if our globe were a solid mass void of all liquid capable of being distracted by tides. It is, therefore, certain that the distance of the moon, which is at present about two hundred and forty thousand miles, must be gradually increasing; but we need not look for any appreciable change in the moon’s distance arising from this cause when only an interval of a few centuries is considered. We need not expect to measure the difference due to tides between the size of the moon’s orbit this month and the size of the orbit last month. In fact, there are so many periodic causes of change in the dimensions of the moon’s orbit that it becomes impossible to detect the tidal influence even in the course of centuries. Here, again, we have to remember that in dealing with the history of our earth we are to consider not merely the thousands of years that include the human period, not merely the millions of years that are required by the necessities of geology, but also those unknown periods anterior to geological phenomena to which we have already referred.