THE PROBABLE AGE OF THE WORLD. 669 



is released from the sun's influence she revolves faster round the 

 earth. 



When it was seen how completely the difficulties in ancient obser- 

 vations were explained away by the calculations of Laplace, all doubt 

 was considered to be at an end, and astronomers supposed that the 

 whole truth was known. But, in 1853, it occurred to Prof. Adams to 

 recalculate Laplace's investigations, and the result was the detection 

 of a material error, which vitiated the whole series of observations. 

 The results of Prof. Adams's calculations were submitted to the 

 Royal Society ' in a paper, the explanatory part of which is very 

 short indeed, occupying but a couple of pages of the " Proceedings." 

 The brief statement is followed by a corroborative sea of high mathe- 

 matics, into which we have no intention of asking the reader to 

 plunge. The result, roughly stated, was to halve the amount of accel- 

 eration calculated by Laplace, and thus to leave half of the accelera- 

 tion of the moon necessary for his explanation of ancient eclipses to 

 be found in some other way. Astronomers were now in a condition 

 almost as bad as that from which they had been rescued by Laplace. 



Adams communicated his final result to M. Delaunay, one of the 

 great French mathematicians ; and it seems to have been during the 

 investigations which that astronomer undertook to verify the calcula- 

 tions of Adams that it occurred to him to inquire whether our meas- 

 ure of time itself remains unchanged ? in other words, whether the 

 earth itself may not be rotating more slowly, instead of the moon 

 more quickly, than in by-gone ages ? It is plain that the moon will 

 appear to be moving more quickly round the earth, if the earth itself 

 which is furnishing the standard by which the moon's revolution is 

 to be measured is rotating more and more slowly from age to age. 



Newton laid it down in his first law of motion that motion unresist- 

 ed remains uniform forever ; and he gave as an instance of constant mo- 

 tion, unaffected by any external causes, this very rotation of the earth 

 about its axis. But M. Delaunay remembered that Kant had pointed 

 out the resistance which the earth must incur from the tide-wave, and 

 had even approximately calculated its amount. The tidal wave is 

 lifted up toward the moon, and on the side of the earth opposite the 

 moon ; so that, as Prof. Tait puts it, the earth has always to revolve 

 within a friction-brake. Adams adopted this theory of tidal friction ; 

 and, in conjunction with Prof. Tait and Sir William Thomson, assigned 

 twenty-two seconds per century as the error by which the earth 

 would, in the course of a century, get behind a thoroughly-perfect 

 clock (if such a machine were possible). 



It may be asked, If the earth's movement be diminishing gradu- 

 ally in rapidity, will it eventually stop altogether ? No ; if ever the 

 earth shall so far yield to the action of the tidal wave as to rotate not 

 more rapidly than the moon, she will present to the moon always the 



1 June 16, 1853. 



