Ordnance Survey Astronomical Observations. 351 



with the linear result, and with similar operations in other 

 parts of the world, certain coefficients, relativeto the size 

 and shape of the earth, and necessary to be employed in all 

 the calculations of the base lines and triangles of the terres- 

 trial measure. 



Of these coefficients, the most important is the quantity of 

 compression of the earth, which is derived with the greatest 

 accuracy by comparing together measured lengths of the meri- 

 dian in different latitudes ; and the practical operation consists 

 in each country in determining astronomically the differences 

 of latitude of two stations, and then from their measured 

 distance asunder in feet, and their bearing the one from 

 the other, ascertaining the angular space between them on 

 the meridian, as viewed from near* the centre of the earth, 

 and so deducing as it is popularly termed, the length of a 

 degree of the meridian. 



Of the two proceedings, the terrestrial admits of being 

 performed with far greater accuracy than the astronomical. 

 Thus the length of 60 miles, for example, as calculated from 

 the carefully measured base lines and angles of the Ord- 

 nance Survey, may be determined with certainty to consider- 

 ably under one foot ; and this will be equivalent to one-hun- 



the public, the omission has been supplied by the English officers in the recent 

 maps on the six-inch scale ; though to insert the points to such exactness that 

 they may be depended on to the full extent of the terrestrial accuracy of the 

 paper will be shewn presently to be a far more difficult matter than the ob- 

 jectors were probably aware of. 



* From near to, not at, the centre of the earth, by reason of the spheroidal 

 character of its figure ; which causes the direction of gravity in various parts 

 of the surface to point, not to one and the same internal spot, but to the locus 

 of a curve depending on the amount of compression ; and thus it is that as we 

 travel from the equator toward the Poles, the radius of curvature and the length 

 of a degree on the surface increase, while the distance from the centre decreases. 

 Hence may arise two different modes of reckoning Latitude, one by a practical 

 astronomical operation on the surface; and the other by a theoretic reference to 

 the centre of the world, and is deduced by calculation from the former, combined 

 with a knowledge of the ellipticity. This theoretic, or as it is called Geocen- 

 tric Latitude, is proper to be employed in certain astronomical calculations ; 

 but the former is that which is more generally used, and known as "Latitude," 

 and will be alone considered in the course of this notice. 



