54f INTRODUCTION, 



it is required to take soundings, or to determine 

 the extent of sand banks that He below water. 



An object that deserves the attention of the 

 navigator, is the measuring of remarkable, and 

 lofty mountains. The knowledge of their ele- 

 vation is not only important to physical geo- 

 graphy, but it may serve future navigators to 

 learn, from the measured angle of elevation of the 

 mountain, their distance from it. 



There are two principal methods of determining 

 the heights of mountains ; the one by the height of 

 the mercury in the barometer, the other by the 

 trigonometrical calculation of the right-angled tri- 

 angle, in which the horizontal distance of the ob- 

 server from the mountain, and the angle of ele- 

 vation under which it appears to him, are given. 

 The barometrical method, even if we leave out of 

 the account the defects inherent in the instrument 

 itself, and the variable elements of the calculation, 

 is not easily applicable in such voyages, because 

 in unknown, thinly peopled, and mostly savage 

 countries, a path is scarcely to be found upon the 

 plain ground, much less over rocks and forests, on 

 the top of a high mountain, never before visited. 

 Only the trigonometrical method, therefore, is left 

 us, and this in but an imperfect degree. Not only 

 is the horizontal distance but approximately deter- 

 mined, but the angles of elevation also cannot be 

 measured at sea with the utmost precision. Still 

 an approximation to an accurate determination is a 



