INTRODUCTION. 55 



gain to science, which is worth the pains of the na- 

 vigator. First, therefore, let a chart, on Mercator's 

 projection, of the country surrounding a mountain, 

 be made by the above-mentioned method, accu- 

 rately determining the various stations in which 

 the angles of elevation are measured. The dis- 

 tance of these stations from the point where the 

 summit of the mountain lies, are measured on the 

 scale of degrees of latitude lying on the side, and 

 the minutes (or Italian miles) found, multiplied by 

 951', b, to reduce them to French toises. The mea- 

 sured angle of elevation, if it was taken on board 

 the ship, is to be corrected for the dip of the hori- 

 zon, and in all cases deduct the twelfth part of the 

 distance given in minutes of a degree, from the 

 angle of elevation, as the amount of the terrestrial 

 refraction. If, then, the distance is not consider- 

 able, and the angle of elevation pretty large, we 

 have h = D X tang, e'; t^; w^here h represents 

 the height of the mountain, D the distance mea- 

 sured, (both in toises,) and e' the angle of elevation, 

 rectified by index correction, the dip of the hori- 

 zon, and terrestrial refraction. But if the distance 

 of the mountain is considerable, regard must like- 

 wise be had to the convexity of the earth, and the 

 formula for the calculation of the elevation will 

 then be h' = D x si n, (e' + j- c) where c represents 



cos. (e' + s) 

 the measured distance of the mountain, in minutes 



E 4 



