ANNIVEESARY ADDRESS OF THE PRESIDENT. xHx 



limitation of the period during which living beings have inhabited 

 this planet to one, two, or three hundred million years requires a 

 complete revolution in geological speculation, the onus prohandi rests 

 on the maker of the assertion, who brings forward not a shadow of 

 evidence in its support. 



Thus, if we accept the limitation of time placed before us by 

 Sir W. Thomson, it is not obvious, on the face of the matter, that 

 we shall have to alter, or reform, our ways in any appreciable 

 degree ; and we may therefore proceed with much calmness, and, 

 indeed, much indifference to the result, to inquire whether that 

 limitation is justified by the arguments employed in its support. 



These arguments are three in number : — 



I. The first is based upon the undoubted fact that the tides 

 tend to retard the rate of the earth's rotation upon its axis. That 

 this must be so is obvious, if one considers roughly that the tides 

 result from the pull which the sun and the moon exert upon the 

 sea, causing it to act as a sort of break upon the rotating solid 

 earth. 



Kant, who was by no means a mere " abstract philosopher," but 

 a good mathematician and well versed in the physical science of 

 his time, not only proved this in an essay of exquisite clearness 

 and intelligibility, now more than a century old *, but deduced from 

 it some of its more important consequences, such as the constant 

 turning of one face of the moon towards the earth. 



But there is a long step from the demonstration of a tendency to 

 the estimation of the practical value of that tendency, which is all 

 with which we are at present concerned. The facts bearing on this 

 point appear to stand as follows : — 



It is a matter of observation that the moon's mean motion is (and 

 has for the last 3000 years been) undergoing an acceleration rela- 

 tively to the rotation of the earth. Of course this may result from 

 one of two causes : the moon may really have been moving more 

 swiftly in its orbit; or the earth may have been rotating more slowly 

 on its axis. 



Laplace believed he had accounted for this phenomenon by the 

 fact that the eccentricity of the earth's orbit has been diminishing 

 throughout these 3000 years. This would produce a diminution of 

 the mean attraction of the sun on the moon, or, in other words, an 

 increase in the attraction of the earth on the moon, and, conse- 

 quently, an increase in the rapidity of the orbital motion of the 

 latter body. Laplace, therefore, laid the responsibility of the ac- 

 celeration upon the moon ; and if his views were correct, the tidal 

 retardation must either be insignificant in amount, or be counteracted 

 by some other agency. 



Our great astronomer Adams, however, appears to have found a 

 flaw in Laplace's calculation, and to have shown that only half the 



* " Untersucliuiig der Frage ob die Erde in ihrer Umdrehmig um die Achse, 

 wodurch sie die Abwechselung des Tages und der Nacht hervorbringt, einige 

 Veranderang seit den ersten Zeiten ihres Ursprunges erlitten habe, &c. — Ka)it's 

 ' Sammtliche Werke,' Bd. i. p. 178. 



