318 TRIGONOMETRICAL SURVEV3. 



cesses, differing from each other, and directed to different 

 objects, must be performed and combined. The first is the 

 measurement of one or more bases, each from five to seven 

 miles long, and the longer the better. These should be on 

 a straight and level line ; but as this can hardly ever be 

 exactly obtained, it must be nearly such a line. It has 

 been usual to measure this line by placing straight rods, 

 sometimes of deal, sometimes of metal, and on one occasion 

 ofglass, all of the same length, end to end in succession, 

 and each supported horizontally, and those behind brought 

 forward and placed in advance along the whole line. Gene- 

 ral Roy, after having tried the deal, and also the glass rods, 

 at last had recourse to steel chains made like a watch chain. 

 He found that these, when laid in wooden troughs, and 

 stretched by weights that were always the same, were as 

 much to be trusted to as rods, and greatly more convenient. 



In the grand series of operations carried on by the French 

 mathematicians to determine the length of a quadrant of the 

 meridian, rods of platina were used ; and in the latest ope- 

 ration of this kind, viz. the base measured along the shores 

 of Loch Foyle for the trigonometrical survey of Ireland, two 

 parallel rods of different metals, united together in a par- 

 ticular way by cross-bars at their ends, have been employed. 

 Rods of this description we believe have been sent to India, 

 and will be employed there in future geodetical operations. 



The next part of the process is the selection of a number 

 of points, called stations, all over the country to be surveyed. 

 These are supposed to be joined by straight lines which 

 form a series of triangles. In each triangle the angles are 

 to be taken by a theodolite of large dimensions, and then, 

 when one side is known, the others may be found by trigo- 

 nometry. These being known, the whole series may be 

 delineated on paper, and the position of each point on the 

 survey found in respect to all others. This is sufficient for 

 determining every line, and every figure within a given ex- 

 tent ;" but something more is necessary to determine its po- 

 sition on the earth's surface, and its situation in respect to 

 the quarters of the heavens, and the parallels of latitude, 

 and the meridians on the surface of the globe. The first 

 of these objects is determined by observing the angles which 

 one or more sides of the triangles make with the meridians 

 passing through the stations which are the intersections of 



