Attempt to determine the mean height of Continents. 329 



The comparison which Laplace has instituted in the pas- 

 sage quoted from the Mecanique Celeste between the depth of 

 the sea and the height of continents, recalls a passage of Plu- 

 tarch, in the 15th chapter of his Life of jEmilius Paulas (cd. 

 Reiskii, vol. ii. page 276), — a passage the more remarkable, as 

 it makes us acquainted with an opinion which generally pre- 

 vailed among the philosophers of the Alexandrian school. 

 After quoting an inscription found on Mount Olympus, and 

 giving the result of the measurement of its height by Xenago- 

 ras, Plutarch adds, " But geometricians (probably those of 

 Alexandria) believe that there is no mountain higher, and no 

 sea deeper, than ten stadia^ We can entertain no doubt about 

 the exactness of the measurement made by Xenagoras ; but it 

 is striking to observe, that the philosophers of this school esta- 

 blished in the structure of the earth a perfect equality be- 

 tween the heights or positive and negative ordinates. Here 

 the maximum of the heights and depths is alone taken into 

 account, and not the mean height, — a consideration which 

 rarely presented itself to the mind of the ancient philosophers, 

 and which, for variable magnitudes, was applied in a useful 

 manner to astronomy by the Arabs. Even in the Metereologius 

 of Cleomedes (i. 10), we meet with an assertion similar to that 

 of Plutarch ; while in the Meteorolcgicis of the philosopher of 

 Stagira (Arist. Met- ii. 2), the only point considered is the in- 

 fluence of the inclination of the bottom of the sea, from east 

 to west, on its currents. 



When we try to determine the mean height of the elevation 

 of continents above the present level of the seas, it means 

 that the object is to find the centre of gravity of the volume 

 of these continents above that level, — an investigation very dif- 

 ferent from that which consists in searching for the centre of 

 gravity of the volume of the continental mass, or the centre of 

 gravity of the masses, seeing that the portion which rises above 

 the sea, in the crust of the globe, is by no means of the same 

 density, as has been demonstrated both by geognosy and ex- 

 periments with the pendulum. The mode of simple calculation 

 is as follows : — Each chain of mountains is considered as a tri- 

 angular prism placed horizontally. The mean height of the 

 defiles or passes, which determine the mean height of the crest 



