C. B. WARRING. 181 



In any given case, therefore, we need only to deter- 

 mine whether the pull is an increasing or diminishing 

 one, to decide whether the tilting of the equator is to, or 

 from the ecliptic. 



If to this we add : The lateral or precessional move- 

 ment increases, or diminishes, as the applied force 

 varies, but does not change its direction, we have an 

 explanation of nutation in longitude also. 



Now we can see why the average obliquity of the 

 earth's axis is a constant quantity, notwithstanding the 

 fact that the pull of the sun, moon and planets is always 

 towards the ecliptic. When their pull decreases the obliq- 

 uity increases, and when the pull increases the obliquity 

 decreases, and as the time of the variations are equal, no 

 real change is produced. 



Now, too, we can see why it is that Frisi's law was 

 not long ago disproved by the accumulated effect of that 

 tilting whose existence it denies. For by this curious 

 law of nutation, although the tilting takes place, its 

 effects do not accumulate. Like Saturn's offspring, 

 they are destroyed as soon as they are born. 



Sir John Herschel, in his "Outlines of Astronomy,' ' 

 sect. 649, says : ' ' The solar influence alone would pro- 

 duce no effect upon the excursions of the earth's axis to 

 and from the pole of the ecliptic. It would increase the 

 length of the wave, but not its depth." 



But we have seen that nutation, in its depth as well as 

 its length, is the effect of an alternate increase and de- 

 crease of the pull of the attracting body. The pull of 

 the sun varies from solstice to equinox, and from equi- 

 nox to solstice. Hence, solar influence alone would pro- 

 duce an effect in the excursions of the earth's axis to 

 and from the pole of the ecliptic. 



As, however, the changes in the tilting force of the 

 sun are verv much slower and less in their extent than 



165 



