850 



Professor Arthur Schuster 



[May 18, 



it as far as possible to the depth below our feet and the height above 

 our heads. 



In passing from the arbitrary units to which we refer our terrestrial 

 measurements of length, to the scale on which we measure the dimen- 

 sions of the solar system, and from them to stellar distances, the mag- 

 nitude of the earth's radius or circumference forms an all important 

 immediate quantity. One of the first acts of the French Academy of 

 Sciences, founded in 1666, consisted in organising the work of 

 accurately measuring the dimensions of the earth, and this at once 

 enabled Newton to confirm his celebrated theory of universal gravita- 

 tion. As improvements in the methods of measuring kept pace 

 with the work actually accomplished, our knowledge steadily increased, 

 but we are still improving on it. New problems have arisen requiring 

 more minute study, and the measurement of the shape and size of the 

 earth still remain a question of the first importance. The actual 

 surveys and triangulation required for the purpose are of necessity left 

 to the initiative of individual states or to the combination of the states 

 primarily concerned, but the general discussion of results, as far as they 

 apply to the earth as a whole, is entrusted to an International Geodetic 

 Association, which at present consists of twenty-one states. These, 

 together with their annual contributions to the general fund, are 

 entered in the followino- table : — 



I 



The Central Bureau of this Association is attached to the Royal 

 Geodetic Institute of Potsdam, which is under the distinguished 

 direction of Professor Helmert, who acts as secretary to the Associa- 

 tion. 



The question of measuring the size of the earth depends to a great 

 extent on the measurement of arcs of meridian. As long as we were 

 confined to Europe for the measurements of these arcs they remained 

 necessarily short, but larger portions of our globe have become ac- 

 cessible to the theodolite, and there is especially one arc which is 

 distinguished by the fact that it is the longest possible which can be 

 traced along the land covering the earth's surface. It runs about 30° 

 east of Greenwich, and a large portion of it passes through Africa. 

 Owing to the great energy and enterjDrise of Sir David Gill, the work 



