C. B. WARRING. 169 



part would always be counterbalanced by that upon 

 another symmetrically placed. 



But the earth is not a sphere. Encircling its equator 

 is an immense protuberant ring inclined 23^° to the 

 ecliptic. The part of this xorotuberance towards the 

 moon, or the sun, being four thousand miles nearer to 

 them than is the centre of the earth, is attracted by them 

 more strongly than the spherical part, which may be 

 regarded as if its mass were a point at the centre. The 

 opposite part of the ring being four thousand miles more 

 distant is attracted less strongly. 



The effect of this difference is easily seen in fig. 41. 

 If S (the sun or the moon) pulls A more strongly than it 

 pulls c, A will tend to tilt in the direction of the small 

 arrow, m. 1 And if c is pulled more strongly than B, it 

 will tend to go towards S faster than B, or what is the 

 same thing, B will pull back, and, as a consequence, 

 take the direction of the other small arrow, n. Notice 

 that m and n act together, and tend to lessen the inclina- 

 tion of the equator to the ecliptic. In six months the 

 earth will have moved to the opjDosite side of the sun, and 

 a similar tilting tendency will be produced, and here, as 

 before, the effect is to lessen the obliquity of the equator. 

 In either case, and consequently, in all cases, the tenden- 

 cy of the solar and lunar attraction is to make the 

 earth's axis more and more nearly perpendicular to the 

 plane of its orbit. 



This tilting force varies greatly, reaching its maxi- 

 mum when the ring experiences the combined ef- 

 fect of the sun and moon, when at time of conjunction 

 or opposition, both happen to be in perigee, and the moon 

 and the equator to be on opposite sides of the ecliptic 

 and the sun at one of the solstitial points. For then 

 they are nearest to the earth, and have, if I may so 



1 The small arrows spoken of above have been omitted by the engraver. They are seen. 

 however, although not lettered, on the right hand part of the diagram. 



153 



