PHYSIOGRAPHY. 89 



The length of the quadrant passing through Paris is 10 001 472*5 m, 

 and that of the minimum quadrant (105 34' E.) is 10 000 024*5 m. 

 A geographical mile or knot, which is the distance on the 

 equator subtended by 1' of longitude, is 6087 feet, 1*153 statute 

 mile, or 1855 '3 metres. 



If the earth be considered as an oblate spheroid : 



Equatorial semi-diameter (a) 

 20 926 062 ft. 1 3963*3 miles. | 6 378 206 '4 m. 



Polar semi-diameter (c) 

 20 855 121 ft. | 394979 miles | 6 356 503*8 m. 



a-c - 13-51 miles. -~~ = ~- - 0*00339 



If the earth be considered as a sphere the radius is 3959 miles o;- 

 6 371 300 metres, and the number of miles subtending 1 of 

 longitude at any latitude is 69*09 X cosine latitude. 



Surface of the earth 197 000 000 ? square miles (about | is land 

 and f ocean). 



Volume of the earth 260 000 000 000 ? cubic miles. 

 Density of the earth 5*6 ? times that of water. 

 Mass of the earth 6 x 10 21 ? tons. 



(113) MEAN DENSITY OF THE EARTH. 



From experiments with the 



Plumb-line at Schiehallien (Maskelyne and Playfair)... 4 '713 



,, at Arthur's Seat (James) 5*316 



Pendulum at Mont Cenis (Carlini and Giulio) 4-95 



,, at Harton coal-pit (Airy) 6*565 



Torsion-balance (Cavendish 1798) 5-48 



(Reich 1838) '.'." 5*49 



(Batty 1843) 5*66 



,, (Cornu and Bailie 1872) 5*5 5'56 



Horizontal pendulum ("Wibring 1887) 5'6 



