36 FROM NEBULA TO NEBULA 



his absolute south pole doing the turning, while the 

 north pole remains fixed so that on the northern sky 

 his axis extended will describe an oval figure. For ex- 

 ample: Take a pear and pass its stem * vertically up- 

 ward through a small aperture in glass, and so that 

 you can grasp the tip of the stem. Now if with the 

 other hand you twiddle the pear around you can get my 

 idea exactly. The stem is the medial line of the Prime 

 Resultant; where it joins the fruit, will be the point of 

 the north pole ; and the eye of the pear will be the true 

 south pole. From this explanation it will be seen that 

 the sun may greatly shift his mass without markedly 

 changing his apparent position, and it is this nutation 

 of his which seems to me to afford the key to Mercury's 

 remarkable perihelion advance. If the existence of a 

 small planet between the sun and Mercury could serve 

 to explain the phenomenon, as the mathematicians de- 

 clare, this oscillatory movement of the sun's center of 

 weight ought equally to suffice, since any strategic ad- 

 vantage in the imaginary planet's position with refer- 

 ence to the pivot would be offset by the sun's excess 

 mass. 



Incidentally it may here be remarked, in order 

 to be referred to later, that owing to the general 

 gaseous condition of the sun, his center of weight 

 would naturally be composed of his heavier sub- 

 stances, which, by the same token, are the most refrac- 

 tory, forming, as it were, a sort of knot in his other- 

 wise comparatively homogeneous structure. 



To the reader who has been vividly picturing to 

 himself this stupendous whorl of countless orbs de- 

 scending spirally through space, there must have oc- 

 curred its likeness to a giant top. And just as the axis 

 of a top, when going at low speed, possesses a gentle 



