At present, 110 doubt, the effect of the tides in changing the length of the day is very small. A day now is nofc appreciably longer Iban a day a hundred years aga. Even in a thousand years"the chauge in the length of the day is only a fractiori of a second. But the importaace arises from the fact that the change, slow though it is, lies always in one direction. ïhe day is eontinually increasing. In uüllions of years the accumlated eiïect becomes not orly of appreciable, but esren of startling, magnitude. The change in the length of the day must involve a corresponding change in the motion of the moon. This is by 110 rneans obvious. It depends upon an elabórate mathematical theorem. I cannot attempt to provo this for yon, but I think I can state the result so tbat it can b& understcod without the proof, [f 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 its persecutor. At present the moon revolves round the earth at a distance oL 240,000 mile: . The reaction oí the earth teiMs to crease that dislance, and to toree the moon to involve in in orbit whieh is continually getting larger and larger. Here, then, we have two íemarkable consequences oí the tides which are inseparably connected. üeinember, also, that we are not enunciatihg any mere speculative doctrine. These results are the inevitable consequences of the tides. If theearthhadno seasor oceans,. no lakea or rivera; if it were an absolutely rigid solid throughout its entire mass, then these changes could not take place. The length of the day would never alter, and the distance of the moon would only (luctuate betweeu narrow limits. As thousands of years roll on the length oí the day increases second by second, and the distance of the moon increases mile by mile. These changes are never reversed. It is the old story of the perpetual dropping. As the perpetual dropping wears away the stone, so the perpetual action of the tides has sculptured out the earth and moon. Still the action of the tides continúes. To-day is longer than yesterday; ycsterday is longer than tüe day bcfore. 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, epocb, which was a crisis in the history of our system. Let me say at once that there is great uncertainty about the date of that crisis. It must have been at least 50,000,000 years ago. I may have been very mach earJer. This crisis was the interesting ocasión when the moon was born. , wish I could chronicle the event m ith perfect accuracy, but I eannot sure of anything exceüt that it wus more than 50,000,000 yeárs ago.- Prof. Ball in Nature.