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, assuming the earth 

 to be a perfectly 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, at Cambridge, 

 Mass., and F. Kiistner, at Bonn, proved that the variation 

 amounted to a few tenths of a second of angular measure- 

 ment. On account of this, the north pole of the earth 

 is not a fixed point, and may sometimes be nearly thirty 

 feet distant from the average position. Euler assumed 

 the earth to possess no elasticity ; and in that case the 

 period of variation of latitude would be about 305 days. 

 The results of prolonged observations give, however, 

 unmistakeable evidence of a period of 428 days, which 

 is what would be expected if the earth possesses a certain 

 amount of elastic yielding. ' The conclusion of the whole 

 matter is that the observed effects can be accounted for 



