The tendency of the tides on a tide-disturbed globe is to adjust the movements of that globe in such a way that the tides shall no longer ebb or flow, but that permanent high tide shall be established in some places and permanent low tide in others. If the rotation of the body be not fast enough the tide will pull the body round in order to effect this object. If the rotation of the body be too rapid, then the influence of the tide will tend to check the movement and bring down the speed of rotation until the desired adjustment is obtained. At present the earth is spinning too fast to permit the high tides to remain at permanent localities, and consequently tides are applied with the effect of checking the rotation. The earth is, however, so vast, and the tides generated by so small a body as the moon are relatively so impotent, that their effects in reducing the speed of the earth’s rotation are insignificant. Nevertheless, small though they are, they unquestionably exist, and there can not be a doubt that to some extent the earth is affected by the unremitting action of the tides; the consequence is that the rapidity with which the earth rotates upon its axis is gradually declining. One result of this can be stated in a very simple manner. The length of the day must be increasing. It is true that this gradual stretching of the day is very slow; it is indeed quite inappreciable in so far as our ordinary use of the day as a measure of time is concerned. The alteration almost eludes any means of measurement at our disposal. Even in a thousand years the change is so small that the increase in the length of the day is only a fraction of a second. We can doubtless afford to disregard so trifling a variation in our standard of time so far as the period contemplated in mere human affairs is concerned. In fact the change is absolutely devoid of significance within such periods as are contemplated since the erection of the Pyramids, or indeed since any other human monument has been reared. We must not, however, conclude that the change in the length of the day has no significance in earth history. The significance of the gradual elongation of the day by the tides arises from the circumstance that the change always takes place in one direction. In this form of effect the tide differs from other more familiar astronomical phenomena which sometimes advance in one direction and then after the lapse of suitable periods return in the opposite direction, and thus restore again the initial state of things. But the alteration of the length of the day is not of this character, it is not periodic, its motion is never reversed, is never even arrested. Only one condition is therefore necessary to enable it to obtain tremendous dimensions, and that is sufficient time in which it can operate.