THE INTERIOR OF THE EARTH. 251 



very much in a practical study of the subject. There are two reasons 

 for this: first, it is only for European stations (and for the most part 

 only for Greenwich) that we have any knowledge of these earlier 

 variations; the other component of the wanderings of the pole, 

 namely that in the meridian at right angles to the meridian of 

 Greenwich, did not begin to be known until very recently. Again, 

 these ancient observations were undertaken for certain definite pur- 

 poses that they served as well as could be expected for their time; 

 but they were not intended and are not well suited for precise deter- 

 minations of the latitude. Close acquaintance with the subject has 

 taught us that exceedingly delicate observations are necessary to 

 define the variations with adequate accuracy. If I held in my hands 

 two plumb lines half a meter apart, they would not be quite parallel 

 to each other, though both are exactly vertical; if they were pro- 

 longed they would meet somewhere near the center of the earth, 

 4,000 miles below. The angle between them is a little less than 

 0".02 and represents approximately the accuracy that is demanded 

 and that has recently been attained in latitude observations. This 

 success is due chiefly to the International Geodetic Association which 

 has organized an " international latitude service " of high efficiency, 

 and to whose efforts and experience are due the improvements in 

 instruments and methods that have made possible this extraordinary 

 degree of precision. Since 1899, the association has maintained six 

 observing stations for this sole purpose, two of these being in our 

 own country. One of the minor effects of the war that is now raging 

 in Europe will be the discontinuance of some of these stations. 

 One of the American stations has already been abandoned, and the 

 same fate will overtake the other in June, 1916, unless some inde- 

 pendent means of maintaining it, at least temporarily, presents itself 

 soon.* An interruption of these observations would be a great pity, 

 for this is one of the cases where a continuous record is highly 

 desirable. 



To return to Chandler and his work on these variations, perhaps, 

 the most important of his achievements was to show that the prin- 

 cipal term in the variations, instead of having a period of 10 months 

 in accordance with Euler's theory, has in reality a period of 14 

 months. This difference explains the failure of Bessel and all the 

 others, who preceded Kiistner, to find a latitude variation in their 

 observations, for, relying upon Euler's results, they had all tested 

 their observations for the 10-month variation and had sought for 

 no other variation. For the same reason. Chandler's announcement 

 of the longer period was received with incredulity in some quarters, 



» since this sentence was spoken the United States Coast and Geodetic Survey has 

 secured legislation that guarantees the continuation of this station. 



