Attempt to determine the mean height of Continents. 327 



lity of such a determination of limits, and, from the comparison 

 which depends on it, I have deduced the extent of the surface 

 of the plains, and the horizontal and flat portions of moun- 

 tains, in my geognostical researches on South America ; a por- 

 tion of the globe in regard to which the length of the im- 

 mense wall which forms the Cordillera of the Andes, and of 

 the elevated masses of Parima and Brazil, was so incorrectly 

 limited and circiunscribed on all maps. In fact, there is a 

 general tendency in all graphic representations to give the 

 mountains a greater degree of breadth than they really pos- 

 sess, and even in the flat portions to confound plateaux of va- 

 rious kinds with each other." 



M. de Humboldt published, in 1825, two memoirs inserted 

 in the Memoires de I'Academie des Sciences of Paris, on the 

 mean height of continents, and an estimate of the volume 

 of the elevated ridges of mountains, compared with the 

 extent of the surface of the lower regions. An assertion of 

 Laplace in the Mecanique Celeste (vol, v., book xi. chap. i. 

 page 13), gave rise to these researches. This great geo- 

 meter had established in principle, that the agreement ob- 

 served between the results of experiments made with the pen- 

 dulum and the compression of the earth, deduced as well 

 from the trigonometrical measurement of the degrees of the 

 meridian as from the inequality of the moon, furnished a 

 proof " that the surface of the terrestrial spheroid would 

 be nearly that of equilibrium, if that surface became fluid. 

 Hence, and from the consideration that the sea leaves vast 

 continents uncovered, we conclude that it cannot be of great 

 depth, and that its mean depth is of the same order as the 

 mean height of the continents and islands above its level, 

 a height which does not exceed 1000 metres" (or 3073 

 Parisian feet, that is to say, only 463 feet less than the sum- 

 mit of the Brocken, according to M. Gauss, or a little more 

 than the most elevated mountains of Thuringia). Laplace 

 further adds, " This height is, then, a small fraction of the 

 excess of the radius of the equator over that of the pole, an 

 excess which exceeds 20,000 metres. Just as high moun- 

 tains cover some pai-ts of continents, so there may be great 

 cavities in the bed of the sea ; but it is natural to suppose 



