TRIGONOMETRICAL SURVEY OF INDIA. 



213 



The base of Romney Marsh was measured with the steel chain, sup- 

 ported on coffers, it was begun on the 15th October, 1787, and ended 4th 

 December, and was 28535.677 feet, and here we find introduced two of 

 those cases of perplexity which I have alluded to before : viz. 



Correction for contraction of the chain, Sub. 1.0667 feet. 



Correction for wear of the chain, Add. 0.2735 



I do not mean to say these are incorrect — all I mean is, that they are 

 hypothetical, and as such are not as satisfactory to the common sense 

 and good understanding of mankind as a measurement which is depend- 

 ent on fact alone. 



This base when deduced from that on Hounslow Heath by the triangu- 

 lation, was 28533.3 feet, so that there is a difference between computation 

 and actual measurement of 2.277 feet, which is at the rate of 5.06 inches 

 in a mile. 



In the Summer of 1791, the base on Hounslow Heath was re-measured 

 with the steel chain in coffers, supported on pickets. The operation occu- 

 pied 43 days, and was found after all reductions, to be 27404.3155 feet, or 

 about .3018 of a foot greater than the former one — this is about .607 of an 

 inch per mile, and the daily progress was 638 feet. 



It is not necessary to go into a detail of all that has been executed with 

 the steel chain in Great Britain, but it is some criterion for us to form a 

 judgment respecting that apparatus, when a gentleman of Colonel Coluv 's 

 reputation and talent, who has been so active a participator in all connected 

 with its application, comes forward us an active K^foruicr, and by introduc- 

 ing the apparatus of which you now sec the f;u:-sinul(\ virtually pronoun- 

 ces his opinion as to the fallibility ol' ili;it \n liit h liv ^cclv> to supercede. 



