Relating to the FIGURE of the EARTH. 25 



In this way may many ufeful conclufions be derived con- 

 cerning the degree of credit due to meafurements already made,, 

 as well as with refpect to the felection of the places where they 

 are to be made hereafter. On thefe I mall enter no further at 

 prefent, and mail only add, that, befides the advantages or dis- 

 advantages which the method of comparing together degrees of 

 the meridian and perpendicular in the fame latitude has, and 

 which are fubjedls of calculation, it has another advantage, which 

 in the cafe of the Britifh furvey is undoubtedly very great, viz. 

 that all the data are furnifhed from one fyftem of trigonometrical 

 operations ; executed according to the fame plan, with the fame 

 inftruments, and by the fame obfervers. 



30. One other application of geometrical meafurements to dif- 

 cover the figure of the earth yet remains to be confidered. This is; 

 the comparifon of an arch of the meridian with an arch of a pa- 

 rallel of latitude which crofTes it. The meafure of a parallel of lati- 

 tude can be executed readily, and is not confined to a fmall arch 

 as in the cafe of a perpendicular to the meridian. The plumb- 

 line, while it is carried along the circumference of a parallel to 

 the equator, tends continually to the fame point in the earth's 

 axis, fo that there is no difficulty in afcertaining the amplitude 

 of the arch meafured, providing there be no unufual difturbance 

 of the direction of gravity. As an arch of a parallel to the equa- 

 tor, however, is not the fhorteft line between two points on the 

 furface of the fpheroid, the meafurement along that furface will 

 not give the length of the arch truly. To obviate this difficulty, 

 it is only neceflary to follow the method fo properly introduced 

 into the Trigonometrical Survey, of reducing the meafures, both 

 of lines and angles, to the chords and to the planes of the recti- 

 lineal triangles contained by them. In this way, the chord of 

 an arch of a parallel of latitude may be determined, however 

 great the arch ; and it is worthy of being remarked, that, what- 

 ever be the deflections of the plumb-line at the intermediate fta- 



Part I. D tions > 



