THE WANDERINGS OF THE NORTH POLE. 81 



which likens tlie earth to an orange. Let ns thrust a knitting needle 

 through the orange along its shortest diameter to represent the axis 

 about which the eartli rotates. Not only does the earth perform one 

 revolution about this axis in the space of each sidereal day, but the 

 axis itself lias a movement. If the earth's axis always remained fixed, 

 or never had any motion except in a direction parallel to itself, then the 

 point to whicli it was directed on the sky would never change. We 

 have however seen that the pole in the sky is incessantly altering its 

 position; we are therefore taught that the direction of the earth's axis 

 of rotation is constantly changing. To simulate the movement by the 

 orange and knitting needle, we nnist imagine the cn-ange to rotate 

 around its axis once in that period of twenty-three lionrs and fifty sis 

 minutes which is well known as the length of the sidereal day; while at 

 the same time the knitting needle, itself bearing, of course, the orange 

 with it, performs a conical movement with such extreme slowness that 

 not less than twenty- live thousand years is occupied in making the cir- 

 cuit. The movement, as has often been pointed out, is like that of a 

 peg top which rotates rapidly on its axis while at the same time the 

 axis itself has a slow revolving motion. Thus the phenomena which 

 are presented in the rotation of the eartli demonstrate that the axis 

 about which the earth rotates occupies what is, at all events, approx- 

 imately a fixed position in the earth, though not a fixed position in 

 space. We can hardly be surprised at this result; it merely implies 

 that the earth acts like a rigid body on the whole, and does not permit 

 the axis about which it is turning to change its position. 



It will now be easily understood how it comes to pass that the posi- 

 tion of the North Pole upon the earth has not appreciably changed in 

 the course of thousands of years. The axis around which the earth 

 rotates has retained a permanent position relative to the earth itself; 

 it has however continuously changed, it is at this moment changing, 

 and it will continue to change with regard to its direction in space. So 

 far our knowledge extended up to within the last few years, but in these 

 modern days a closer inquiry has been made into this, as into as many 

 other physical subjects, and the result has been to disclose the impor- 

 tant fact that, though the ])heiiomena as just described are very nearly 

 true, they must receive a certain minute qualification. Complete exam- 

 ination of this subject is desirable, not only on account of its natural 

 importance, but also l)ecause it illustrates the refinements of which 

 modern astronomical processes are susceptible. I have stated the 

 broad general fact that the position of the terrestrial pole undergoes 

 no large oi- considerable fiuctuation. But while we admit that nolarge 

 fluctuation is possible, it is yet very proper to consider whether there 

 may not be a small fluctuation. It is certain that t\\e position of the 

 pole as it would bo marked by a post driven into the earth today can 

 not difler by a mile from tlie position in which the same point would be 

 marked last year or next year. But does it difler at all? Is it abso- 

 SM 93 6 



