116 



terials yet exist *for so important a work. 

 Without the aid of geodesical instruments, 

 by which we measure very small angles, it 

 is almost impossible to determine the outlines 

 with sufficient exactness. While I was em- 

 ployed in these measurements in the southern 

 hemisphere, on the ridge of the Cordilleras 

 of the Andes, Mr. Osterwald, with the assis- 

 tance of a celebrated astronomer, Mr. Tralles, 

 was sketching, by a similar method, the chain 

 of the Alps of Switzerland, as it appears when 

 viewed from the banks of the lake of Neuf- 

 chatel. This view, lately published, is so very 

 exact, that, the distance of each summit being 

 known, their relative height is found by em- 

 ploying in the calculation only the simple mea- 

 sure of the outlines of the drawing. Mr. Tralles 

 made use of a repeating circle. The angles by 

 which I determined the size of the different 

 parts of the mountain were taken with a sextant 

 by Ramsden, the limb of which marked with 

 certainty six or eight seconds. By the repetition 

 of this operation at. the interval of distant pe- 

 riods, the accidental changes, which take place 

 on the surface of the Globe, may some day be 

 verified. In a country exposed to earthqukes, 

 and overwhelmed by volcanoes, it is very diffi- 

 cult to determine whether the mountains dimi- 

 nish, or whether by the ejection of ashes and 

 scoriae they insensibly augment. The simple 



