May 26, 1856.] - ORDNANCE SURVEY. 135 



Dimensions, and Mean Density of the Earth, as derived from the 

 Ordnance Survey.' In this communication, he states that now 

 that the observed angles have been corrected by the most refined 

 methods of mathematical science, the triangulation is rendered 

 perfectly symmetrical and consistent in itself, so that, any side 

 being taken as a base, the same length will be reproduced when 

 it is calculated through the whole or any part of the triangulation. 

 This, as regards the angular measurements, leaves nothing to be 

 desired ; and when the five measured bases are incoi-porated in the 

 triangulation, although some of them are 400 miles apart, and from 

 5 to 7 miles in length, the greatest difference between the measured 

 and computed lengths of the bases does not amount to 3 inches ; and 

 it may be safely affirmed that such a degree of accuracy was never 

 before attained in so extensive a triangulation. 



Astronomical observations have been taken at numerous trigono- ^ 

 metrical stations for the purposes of determining their latitudes, and ^v^ 

 by comparing the amplitudes of the astronomical with the geodetical 

 arcs, the figure and dimensions of the Earth have been determined. J^ j<y 

 In determining the most probable spheroid from all the observed *^ 

 amplitudes, continues this talented officer, it was evident that the 

 plumb-line was deflected from the true direction of the zenith at '^^ 

 several points, and that this was the case at the Eoyal Observatory of 

 Edinburgh and Arthur's Seat near it, to the extent of 27" to the south, iz fki\ 

 The configuration of the ground — the great valley of the Frith of "'" ■^f-> 

 Forth being on the north, and the range of the Pentland Hills on the 

 south— presented a tangible cause for the deflection ; but as the con- 

 toured plans of this district were published, and Colonel James was 

 himself personally acquainted with the geological structure of the 

 country, he had observations made on the summit and on the north 

 and south flanks of Arthur's Seat, with the view of determining the 

 amount of the attraction of its mass, and from thence deducing the ^ ''-'":?" 

 mean specific gravity of the Earth. The computed deflection of 

 the plumb-line due to the configuration of the ground, accounted 

 in great measure for the observed anomaly in the amplitudes of the 

 arcs of the meridian. The longest arc of meridian which has been 

 computed in Great Britain, extends from Dunnose in the Isle of 

 Wight, to Saxaford in the Shetland Islands, and is 10° in length.>^^; '^^ 

 The Polar diameter of the Earth, as determined by the Ord-' ^^ 

 nance Survey = 7,900 miles; the Equatorial = 7,926 ; the com- 

 pression = a 9T-3Tr ; ^^ mean density = 5*316. 



These great geodetical operations have now been brought to a 



'j^r 



