ASTRONOMY. 41 



difference of the latitudes, will give the length of 

 a degree. 



ERATOSTHENES was the first who applied this method 

 to the estimation of the earth's circumference. By 

 measuring, or rather estimating, the difference of 

 latitude between Alexandria and Syene, and also 

 their distance, he concluded the circumference of ( 

 the earth to be 250000 stadia. MONTUCLA, His- 

 toire des Math. torn. I. p. 242. 2d edit. 



The length of a degree, or of any arch of the meri- 

 dian, is determined by taking two points nearly at 

 the distance of the arch required, and nearly north 

 and south of one another. A series of triangles is 

 then to be carried from the one point to the other 

 by means of stations taken on the tops of hills or 

 other elevated grounds. The angles of these tri- 

 angles are to be measured, as also the azimuths 

 of the sides, at the points where the series of 

 triangles begins and ends. Thus, there is a series 

 of triangles all given in species which connects the 

 two extreme points, and the bearings of the sides 

 of these triangles, in respect of the meridian of the 

 first station, are also given. 



The lengths of the sides of the triangles in known 

 measures, toises or fathoms, is next found by mea- 

 suring a base on a level ground, and connecting it 

 by angles with the sides of one of the triangles. 

 Jn all this we proceed as if the triangles were plane, 

 whereas they are in fact spherical, and the three 



angles 



