608 ON The figure, dimensions, and mean specific gravity of the earth. 
chnrcli towers and the loftiest mountains in the kingdom, from the Shetlands to the 
Scilly Islands, it is at this day in perfect working order, and probably one of the very 
best instruments that was ever made. 
The great dVigonometrical operations of the Survey have been carried on under so 
many officers, from the time of their commencement under General Roy down to the 
present time, that it wmuld be quite impossible, in this short notice, to mention more 
than tlie names of several Superintendents wlio have succeeded General Roy, viz. 
Colonel Williams, Major-General Mudge, Major-General Colby, and Colonel Hall; 
but in justice to the highly meritorious body of non-commissioned officers of the 
Corps of Royal Sappers and Miners, I should state, that whilst in the early part of 
the Survey the most important and delicate observations were entrusted solely to the 
commissioned officers, these duties have of late years been performed by the non- 
commissioned officers with the greatest skill and accuracy. 
In the Historical sketch of the Survey which I purpose publishing shortly, I hope 
to be able to do justice to the individual merits of all employed. 
The computations connected with the corrections of the observed angles, to make 
the whole triangulation as nearly as possible perfectly consistent, have been most 
voluminous, and have been made under the direction of Lieut.-Colonel Yollano, 
Captain Cameron, and Captain Alexander R. Clarke; but I gladly avail myself of 
this opportunity to acknowledge the great and important assistance and advice which, 
both as regards the insti-uments and the calculations, Mm have at ail times received 
from the Astronomer Royal. 
The triangulation by the methods which will be explained, is now made consistent 
in every part, so that any side of any triangle being taken as a base, the same distance 
will he reproduced when it is computed through any portion or the whole series of 
triangles ; and when the five measured bases on which Mm rely are incorporated in this 
triangulation, the greatest difference between their measured and coinjiuted lengths is 
not as much as 3 inches, and yet some of the bases are upwards of 400 miles apart. 
Several bases of from five to seven miles long have been measured, but those upon 
which the chief reliance has been placed are the Lough Foyle and Salisbury Plain 
bases, Mdiich were measured with General Colby’s compensation bars. The difference 
between the measured and computed length of the one base from the other through 
the triangulation is 0’4178 ft., or about 5 inches. 
This difference has been divided in proportion to the square root of the lengths of 
the measured bases, by wdiicli we have obtained the mean base which has been used 
in the triangulation; there is therefore a difference of -|- or — 0'2 ft., or 2^ inches 
between the measured and computed length of these bases from the mean base. 
The Hounsloiv Heath base was measured with Ramsden’s 100-ft. steel chains, and 
only differs 0*173 ft., or about 2 inches, from its computed length from the mean base. 
d'he Belhelvie base in Aberdeenshire, also measured with tiie steel chains, differs 
only 0*24 ft., or less than 3 inches, from the computed length. 
