48 DIMENSIONS OF THE EARTH. SECT. VI. 



rotation, and all the degrees measured between the pole and the 

 equator would give the same compression when combined two 

 and two. That, however, is far from being the case. Scarcely 

 any of the measurements give exactly the same results, chiefly 

 on account of local attractions, which cause the plumb-line to 

 deviate from the vertical. The vicinity of mountains produces 

 that effect. One of the most remarkable anomalies of this kind 

 has been observed in certain localities of northern Italy, where 

 the action of some dense subterraneous matter causes the plumb- 

 line to deviate seven or eight times more than it did from 

 the attraction of Chimborazo, in the observations of Bouguer, 

 while measuring a degree of the meridian at the equator. In 

 consequence of this local attraction, the degrees of the meridian 

 in that part of Italy seem to increase towards the equator through 

 a small space, instead of decreasing, as if the earth was drawn 

 out at the poles, instead of being flattened. 



Many other discrepancies occur, but from the mean of the five 

 principal measurements of arcs in Peru, India, France, England, 

 and Lapland, Mr. Ivory has deduced that the figure which most 

 nearly follows this law is an ellipsoid of revolution whose equa- 

 torial radius is 3962-824 miles, and the polar radius 3949-585 

 miles. The difference, or 13'239 miles, divided by the equatorial 

 radius, is 555 nearly * (N. 128). This fraction is called the com- 

 pression of the earth, and does not differ much from that given 

 by the lunar inequalities. Since the preceding quantities were 

 determined, arcs of the meridian have been measured in various 

 parts of the globe, of which the most extensive are the Eussian 

 arc of 25 20" between the Glacial Sea and the Danube, conducted 

 under the superintendence of M. Struve, and the Indian arc ex- 

 tended to 21 21', by Colonel Everest. The compression deduced 

 by Bessel from the sum of ten arcs is 298, the equatorial radius 

 3962-802, and the polar 3949'554 miles, whilst Mr. Airy arrives 

 at an almost identical result (3962-824, 3949*585, and 298^) 

 from a consideration of all the arcs, measured up to 1831, in- 

 cluding the great Indian and Russian ones. If we assume the 

 earth to be a sphere, the length of a degree of the meridian is 

 69^ English miles. Therefore 360 degrees, or the whole equa- 



* Sir John Herschel remarks that there are just as many thousands of 

 feet in a degree of the meridian in our latitude as there are days in the 

 year, viz. 365,000. 



The Greenwich Observatory is in N. lat. 51 28' 40". 



