It has long been recognized that the earth in its daily rotation is ever swinging against the tidal waves, pushing them aside with its lands much as a ship breaks the wind-made waves. The result is necessarily somewhat to slow the turning movement of the sphere. The action is like that of a brake on a fly-wheel which continually diminishes the power that keeps it in motion. There is no means by which this energy of turning can effectively be replaced. It is a part of the original movement impressed on the earth at the time when the nebulous mass became separated from the other parts of the solar system: any subtraction, however small, necessarily slows and presently prevents the movement. When a man climbs a westward-sloping hill, he applies an infinitesimal amount of energy to accelerating the earth's movement; as he descends the eastern slope he does like immeasurably small work in speeding the machine. The tides are giants in this treadmill, and computation shows that in the course of a few thousand years they should mark their action by the shortening of the day by a second or two. But now come the astronomers, with fair proof drawn from evidence as to the time of occurrence of ancient eclipses, showing that the day cannot have shortened by as much as a second for all that tidal friction should have brought a vastly greater result about. The only discernible way out of this tangle is through the following considerations:

We thus see that if

When a sphere is whirling with a certain fixed momentum, as in the case of the earth, as we lessen its diameter, we increase the speed of the rotation. A familiar and fairly good instance of this may be had by swinging a weight attached by a string so that the cord winds around the finger; as the line shortens the turns are made in less and less time. Effectively the same principle is applied to the steam governor. the oceanic tides tend to diminish the rotation of the earth, as they surely do, then there is reason to believe that this action is neutralized by the shrinking of the sphere. This action of the oceanic tides is only a small part of the tidal work which has profoundly affected the celestial spheres and is continually acting, so long as they are not rigid to the gravitative pulls of other bodies. The wide-ranging effect of this action has recently been made known to us by George Darwin, and has, as yet, not

entered into the field of popular science. It may, therefore, be worth while briefly to set it forth.

The effect of the tidal action of two spheres, while they are in the fluid or plastic state in which the tides can by their attraction cause the shapes of their masses to alter, is to send them farther apart. Thus when the moon was set off from the earth, both spheres were, doubtless, much nearer to each other than they are at present. They may have been almost in contact, but at that time both of them had such a mobility of their particles that each produced great tides in the other. The effect of the interaction of these tidal protuberances was to push the bodies apart. The way in which the process is effected cannot be set forth, save in rather recondite mathematical form, or by complicated diagrams. Hence it may better go as a bald statement, with the assurance that the result is unquestionable.

So long as the earth and moon remained sufficiently fluid to allow their whole spheres to be tidalized, the constant, slight, but efficient strain, due to the action, pulled them away from one another. When they became so far solidified that the tides ceased to deform their spheres, they ceased to work apart. The relatively slight uplifts of the oceans still have effect in this way, but it is so small that we cannot expect to trace it. In the case of nebulous masses which are passing into the state of solar systems where there are for a time fluid spheres, this sundering action of the tides has much to do with their shaping. This is particularly the case with those most puzzling wonders of the spaces, the double stars of the type when two neighboring suns revolve about their common center of gravity. Because of the heat which their shining indicates, we have to believe that they are fluid enough to have vast tides which, in the manner above suggested, are driving them apart until they become separated, it may be, farther than the most remote planet from the sun.

Coming back to the matter of the continuance of the earth in something like its present condition, we see that all the discernible facts point to the conclusion that, so far as the conditions of the ancient relations between the heat of the sun and of the earth's interior, the important elements in the mechanism, are concerned, there is no reason why a hundred million fair years of life may not be before this planet. As for the tidal effect, the earth