200 



MEASURING APPARATUS OF THE 



2nd. Of giving an equal degree of tension. 



3rd. Of preserving the joints from the effects of oxydation and fric- 

 tion. Other reasons there are, but they might perhaps be remedied — ■ 

 those now detailed are insuperable. 



It is not necessary to enter into the details of the various measure- 

 ments made in France, or of that made by Maire and Boscovich, com- 

 mencing near the tomb of Cecilia Metella, and the high way to Alba 

 Longa, or the two operations of Yarouqui and Tarqui in Peru. 



/ 



Suffice it to say that in all such operations, the coincidence of a mea- 

 sured base of verification with one computed has never been free from the 

 imputation of arising from a happy compensation of errors, whilst if there 

 were a difference, no surprise was excited by it. 



The apparatus before us is the invention of Colonel Colby , of the 

 Royal Engineers, and contains the principle of self-correction for increase 

 of temperature. 



* It consists of a series of bars arranged in pairs — each pair consisting 

 of one bar of brass and one of iron. The pair thus formed is firmly 

 clamped together in the middle by two cylindrical pieces of iron, so that 

 no motion can take place towards the centre. 



The length of each bar is 10 feet 1-4 inch, the breadth is 55 of an 

 inch, and the depth 1*5 inch ; at each of the extremities an aperture is 

 worked out 0-9 of an inch high, and 11 inch longitudinally, extending 



* The reader is referred to the accompanying plates containing plans, sections and elevations of 

 the apparatus and its principal parts, for an explanation of this description. 



