130 Royal Society : — 



mediate space, the maps of which are made of various sizes to ac- 

 commodate them to the convergence of the meridian. 



In 1799 the Royal Society gave further proof of the interest it 

 took in the progress of the Survey, by lending to the Ordnance its 

 great 3-foot theodolite, made by Ramsden, for the purpose of expe- 

 diting the work of the Survey ; and although this instrument has 

 been in almost constant use for the last sixty-seven years, during 

 which time it has been placed on the highest church 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 Trigonometrical operations of the Survey have been 

 carried on under so many officers, from the time of their commence- 

 ment under General Roy down to the present time, that it would be 

 quite impossible, in this short notice, to mention more than the names 

 of several Superintendents who ha\e 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, it 

 should be stated, that whilst in the early part of the Survey the most 

 important and delicate observations were entrusted solely to the com- 

 missioned officers, these duties have of late years been performed by 

 the non-commissioned officers with the greatest skill and accuracy. 



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 Yolland, Captain Cameron, and 

 Captain Alexander R. Clarke ; but Col. James gladly avails himself 

 of this opportunity to acknowledge the great and important assist- 

 ance and advice which, both as regards the instruments and the cal- 

 culations, have at all times been 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 be reproduced when it is 

 computed through any portion or the whole series of triangles ; and 

 when the five measured bases relied on are incorporated in this 

 triangulation, the greatest difference between their measured and 

 computed 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 which 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 which the mean base which 

 has been used in the triangulation has been obtained ; there is there- 

 fore a difference of + or — 0'2ft., or 2^ inches between the mea- 

 sured and computed length of these bases from the mean base, 



