352 SCHLESINGER— VARIATIONS OF LATITUDE. [April 24, 



tion to our knowledge, yet on the theoretical side, at least, we find 

 the beginning more than a century and a half ago. In 1755 Euler 

 considered " the rotation of solid and rigid bodies " in a memoir that 

 is now recognized as the foundation stone for our edifice. He 

 showed that if such a body is projected into space it will exhibit two 

 kinds of rotation; the first of these is the famihar one that corre- 

 sponds to the day in the case of the earth; the other is more subtle 

 and corresponds to the variation of latitude. By reason of this the 

 axis of the diurnal rotation is continually changing within the body, 

 progressing in a regular way and coming back after a time to its 

 earlier positions. An ordinary top gives us a simple example of this 

 kind of rotation. The spinner imparts to the top a motion of trans- 

 lation as well as a rotation, and if we wish to study the rotation we 

 must arrest the translation in some way. This we can do by letting 

 the top fall upon a hard surface in which the iron peg soon wears a 

 minute hole for itself, and the effect is to stop the translation of the 

 top without modifying seriously the rotation. Then we can see that 

 while the top is turning very rapidly around an axis, this axis is 

 itself rotating in a comparatively leisurely way. Just the same thing 

 is occurring with the earth: the point (or pole) at which the axis of 

 the daily rotation pierces the surface of the earth is continually in 

 motion. If we could take to the neighborhood of the pole a modern 

 instrument, and if we could observe there at leisure and in comfort, 

 we should have no particular difficulty in finding the position of the 

 pole within a meter. But if we should repeat these observations a 

 few months later we should find that the pole had wandered away 

 to some distance. To be sure, this distance would not be great and 

 all the wanderings of the pole that have thus far been observed could 

 be plotted to true scale on the floor of a room not much larger than 

 the one we are in. Of course if the pole is moving, so too is the 

 earth's equator ; and thus the latitudes of all points on the earth are 

 varying. Such wanderings as these need not disturb the peace of 

 mind of those gentlemen who like to discover the arctic or the ant- 

 arctic pole. Under the circumstances that the polar explorer must 

 work and with the meager instruments he can transport, he is glad to 

 determine his latitude within half a mile of the truth. 



