22 



[No. 1. 



Consequently the eccentricity acts, here, in an opposite di- 

 rection, therefore in such a way that the force diminishes as the 

 eccentricity increases; but as the factor q by which ^fee* is mul- 

 tiplied, is only whilst the quantity outside the brackets 



Jr§ = 5 /a ■ ^§r ( as tne l unar f° rce b ears to tne s °l ar force, the 

 ratio as 5:2 nearly); the effect on thej tidal-wave is only 

 An • I = t& of the former." 



We see then that the tidal-force rises and falls with the 

 eccentricity of the Earht's orbit. It becomes altered to the 

 extent of T £ ¥ of its value from the greatest to the least eccen- 

 tricity. This force is the most important in its effect on the 

 day and causes it to become longer. The most important force 

 in shortening the day is, according to Thomson, the cooling of 

 the Earth, but he has estimated its effect at only P art °* 

 that of the tidal-force (and, here, attention is only paid to the 

 oceanic tidal-wave). When, therefore, the tidal-force diminishes 

 and increases with r ^ of its value, this periodical change is 

 not balanced by forces acting in opposite direction, and we 

 must therefore conclude that the sidereal day steadily becomes 

 longer, but that the increase is periodically greater and lesser. 

 It increases in length, quicker "and quicker, as long as the 

 eccentricity | of the Earth's orbit increases, and slower and 

 slower, as long as the eccentricity diminishes; in other words, 

 the centrifugal force diminishes, and the equatorial regions in- 

 crease in weight, quicker and quicker, during increasing, and 

 slower and slower during decreasing eccentricity. 



There prevails, even among physicists, as previously stated, 

 a difference of opinion as to whether the Earth will change its 

 form, in the event of change in the centrifugal force. Thomson 

 is most disposed to believe that it will not, Darwin thinks that 

 it will. And among other physicists, with whom I have con- 

 sulted, there prevails a similar difference of opinion in relation 

 to this subject. One thinks that even several hours lengthening 

 of the day would be insufficient to alter the form of the solid 

 Earth; another thinks that the solid Earth would perhaps alter 



