Mr Buchanan's Description and Uses of Ftotracting Table, 148 



other, and then the error can at once be adjusted by a standard 

 index on the frame by which the ruler is brought back to its 

 original position. Should the wheel start upon the rack only 

 one tooth, it would produce an error of only 5 minutes on the 

 angle. The weight of the ruler and wheels is balanced by two 

 counter-weights. 



Tlie great advantage of this instrument consists in the fa- 

 cility and accuracy with which the numerous angles that occur 

 in extensive surveys can be laid down. In many cases, I have 

 seen 50 or 60 angles taken from one station in a system of 

 triangulation, all laid down in the course of a few minutes, 

 and the angles from different points meeting and intersecting 

 one another with an exactness truly surprising, and almost 

 rivalling the accuracy of a trigonometrical calculation. This 

 instrument, therefore, must be of great use in laying down 

 surveys generally ; and more particularly, all those surveys 

 now so general which are carried on by a system of triangida- 

 tion. In all extensive land surveys, I have long since intro- 

 duced this system, and with great advantage, both as to ex* 

 pedition and accuracy. In county surveys it is indispensable; 

 and also in those surveys for marine engineering and hydro- 

 graphical purposes, of which a very interesting account has 

 just been given by my esteemed friend Mr D. Stevenson, in his 

 recent work on marine surveying. But above all, I should 

 think the use of this instrument would be invaluable in those 

 extensive trigonometrical surveys of Britain which are now go- 

 ing on under the Board of Ordnance. The great triangles in 

 these surveys extending often to 50, 60, and even 100 miles be- 

 tween the different points, must necessarily be calculated by tri- 

 gonometry, and still require all the resources and refinements 

 of that perfect science ; but for all the subordinate stations, 

 comprehending those whose distances do not exceed 6, 8, or 10 

 miles, I am satisfied that an instrument of this kind would 

 prove of essential service in filling up the details and expedit- 

 ing the laborious operations of that great work. 



Geobge Buchanan. 



E&iMBURQH, IXii February 1842. 



