Morning journal and courier., 1882-01-10
The Story of the Tides.
What They Tell of the Growing Length of the Day and Of the Birth of the Moon.
 From Naturo.l

From Naturo.l
 From scientific point of view the work done by the tides is of unspeakable impor tance Whence is this energy derived with which the tides do their work If the tides are caused by the moon, the energy they pos sess must also be derived from the moon. This looks plain enough, but unfortunately it is not true. Would it be true to assert that the finger of the rifeman which pulls the trigger supplies the energy with which the rifle bullet is animated? Of course 1t would not. The energy is derived from the explosion of the gunpowder, and the pulling of the trigger is merely the means by which thaY energy is liberated. In 8 somewhat similar manner the tidal wave produced by the moon is the means whereby part of the energy stored in the earth is compelled to expend itself in work. Let mo illustrate this by a comparison between the earth rotating on it's axis and the fy-wheel of an engine. The fy-wheel is sort of reservoir, into which the engine pours it's power at each stroke of the piston. The various machines in the mill merely draw off power from the store accumulated in the fy-wkeel The earth is like a gigantic fy-wheel detached from the engine, though still connected with the machines in the mill. In that mighty fy-wheel stupendous quantity of energy IS stored up, and stupendous quantity of ener- gy would be given out before that hy-wheel
 
 the reservoir from whence the tides draw the energy they require for doing work Hence it is that though the tides caused by the moon, yet whenever they require energy they draw on tho supply ready to hand in the TO- tation of the earth. The earth differs from the fy-wheel of the engine in very impor- tant point. As the energy is withdrawn from the fy-wheel by the machine in the mill, so it is restored thereto by the power of the steam-engine, and the fly runs uniformly. But the earth is merely the fy-wheel without the engine. When the work done by the tides withdraws energy from the earth, that energy is never restored. It therefore foI- lows that the earth's rotation must be de- creasing This leads to a consequence of the most wonderful importance. It tells us that the speed with which the earth rotates on it's axis is diminishing. We can state the result in manner which has the merits of simp1ici- ty and brevity. The tides are increasing the length of the day. At present no doubt the effect of the tides in changing the length of the day is very small. day now is not ap preciabIy longer than day hundred years ago. Even in 8 thousand years the change in the length of the day is only fraction of 8 second. But the importance arises from the fact that the change, slow though 1t is, lies always in one direction. The day is continu- ally increasing. In millions of years the ac- cumulated effect becomes not only apprecia- ble but even of startling magnitude.
 The change in the length of the day must involve corresponding changein the motion of the moon. If the moon acts on the earth and retards the rotation of the earth, so, conversely, does the earth react upon the moon The earth is tormented by the moon, so it strives to drive away it's prosecutor. At present the moon revolyes round the earth at distance of about 240, GOO miles. The reaction of the earth tends to increase that distance, and to force the moon to revolve in an orbit which is continually getting larger and larger As thousands of years roll on, the length of the day increases second by second, and the distance of the moon in- creases mile by mile. A million years ago the day probably contained some minutes less than our present day of twenty-four hours. Our retrospect does not halt here; we at once project our view back to an incredibly remote epoch which was crisis in the histo- ry of our system. It must have been at least 50, OO0,0O years ago. It may have been very much earlier. This crisis was the interesting occasion when the moon was born. Tie length of the day was only very few hours. If we call it three hours we shall not be far