GEODESY. 



Ixv 



When the positions of the points A, B^ C are given on a map, the position of 

 P on the same map may be found graphically by drawing hnes making angles 

 with each other equal to the given angles a and /:? from a point on a piece of 

 tracing paper, and then placing this tracing on the map so as to meet the required 

 conditions. This ready method of solving the problem is often sufficient. 



17. Salient Facts of Physical Geodesy. 



a. Area of earth's surface, areas of continents, area of oceans.* 



Square miles. 



Total area of earth's surface 196940000 



Area continent of Europe 3820000 



" " Asia 17230000 



" " Africa 11 480 000 



" " Australia 3 406 000 



" " America 15950000 



Total area of continents 51886000 



Total area of oceans 145054000 



b. Average heights of continents and depths of oceans.f 



Feet. Metres. 

 Average height of continent of Europe . 



Asia . . 



Africa . 



Australia 



" " " America . 



Average height of all 



980 

 1640 

 1640 



820 



1340 

 1440 



300 

 500 

 500 

 250 

 410 

 440 



Feet. 



Average depth of Atlantic Ocean 12 100 



" " Pacific Ocean 12 700 



" " Indian Ocean 11 000 



Average depth of all 11 300 



Metres. 

 3680 

 3890 

 3340 

 3440 



c. Volume, surface density, mean density, and mass of earth. 



Volume of earth = 259 880 000 000 cubic miles. 



= I 083 200 000 000 cubic kilometres. 



= 260 X 10° cubic miles (about). 



= 108 X lo^*^ cubic kilometres (about). 



Surface density of earth ^=^ 2.56 ± 0.16 t 

 Mean density of earth = 5.576 ± 0.016. 



* Derived from relative areas given in Helmert's Geoddsie, Band II. p. 313. 



t Helmert's Geoddsie, Band II. p. 313. 



I These densities are given by Professor Wm. Harkness in his memoir on The Solar Parallax 

 and Related Constants. The surface density applies to that portion of the earth's crust which lies 

 above and within a shell ten miles thick, the lower surface of this shell being ten miles below sea 

 level. 



