1858.] Geodetic Operations of the Ordnance Survey, 621 



Then, if the mountain be assumed to be of the same specific 

 gravity as the earth, the computed deflection at the 



South end ... = 4-'2 

 North end . . . = 3*8 



Or the whole disturbance =8*0 



but the observed sum of the deflections, that is, the excess of the 

 celestial arc above the geodetic arc, was found to be only 4*07, 

 or little more than ^ what it would have been had the earth 

 and mountain been of the same specific gravity ; and consequently 

 the earth must be of nearly double the specific gravity of the 

 mountain. 



The specific gravity of the mountain was ascertained to be 

 2-75, and therefore as 4*07 : 8*0 : : 2*75 = 5*45 the mean den- 

 sity of the earth ; by employing the full number of decimals, we 

 have 5*316 as the mean density of the earth. From similar obser- 

 vations at Schehallien Mountain, Hutton derived the mean density 

 =: 5 • 0. From experiments on the attraction of balls, 

 Cavendish obtained . . 5 * 44 

 Baily „ . .5-67 



Reich „ . . 5*44 



The Astronomer-Royal, from experiments with pendulums on 

 the surface of the earth, and at a great depth, obtained 6*55. , 



Col. James concluded his address by saying, " I have endea- 

 voured to give what may be called a mere outline sketch of the 

 geodetic operations of the survey. A full account of all these 

 operations, and of the very intricate and laborious computations 

 which have been made, has just been published. This account has 

 been drawn up by Capt. Alexander Clark, R.E., who is employed 

 with me on the survey, and I must refer all those who desire to 

 have more precise information on these subjects to it. 



" But I trust it will be understood, from what I have said, how 

 necessary and important these operations are for the execution of a 

 survey with that perfect accuracy which the nation has a right to 

 expect from the officers entrusted with its execution ; and that we 

 have, at the same time, contributed data for determining the exact 

 figure, dimensions, and specific gravity of the earth, which form the 

 only units of measure for estimating the distances, the size, and the 

 specific gravity of all the heavenly bodies which surround us." 



[H. J.] 



[The standard of length, and the compensation-bars used in 

 the measurement of the bases, were exhibited in the liecture-room 

 and described, and a series of diagrams were referred to in the 

 course of the lecture.] 



