152 DISCOVERY CH. 



and the improvements he effected at Greenwich during 

 his tenure of the office provided astronomers with a 

 mine of exact knowledge of the positions and movements 

 of stars. The lesson taught by his life is that perfection 

 should be aimed at, and the best use made of the instru- 

 mental means available, even though the significance of 

 the observations may not be understood. 



Many men of science have built better than they knew 

 for future generations because of their attention to this 

 principle of precision throughout their investigations. 

 About the middle of the eighteenth century L. Euler, 

 a Swiss mathematician, showed that, considering the 

 earth to be a rigid body, it should oscillate slightly, as a 

 top does when it spins ; that is to say, the direction of 

 the axis should vary in a period which was calculated 

 to be about three hundred days, and therefore the 

 positions of the north and south poles, which are the 

 extremities of the axis, and the latitudes of all places, 

 should be subject to a like periodic variation. 



Observations of sufficient precision to establish this 

 variation of latitude were not made until one hundred 

 and fifty years later, when S. C. Chandler, of Cambridge, 

 Mass., and F. Kiistner, of Berlin, proved that it 

 amounted to nearly six seconds of angular measurement 

 an amount which would cause each of the poles of 

 the earth to wander in a curve about sixty feet in 

 diameter around the mean position. More detailed 

 observations have since been made in various parts 

 of the world, and the results have enabled the details 

 of the paths and periods of this wandering of the 

 poles to be made out, as well as the conclusion to 

 be arrived at that the earth must be a little more rigid 

 than a globe of steel in order that the observed effects 

 may take place. Here, again, a result was arrived at 



