584 The Account of a 



o 



Art. vhi. Particulars relati?ig to the Base on King's Sedgemoor, 

 and the Reduction of that Base. Piate XXVIII. 



Comparisons of the Chains. 



As the chains, after the measurement on Salisbury Plain, 

 were oiled, and laid up in the Tower, no apprehensions were 

 entertained that either of them was elongated by the rusting of 

 the joints. It was, however, our wish to have compared them 

 with each other, previous to the commencement of this operation, 

 and attempts were made, but rendered unsatisfactory, from the 

 want of sufficient firmness in the soil. It was not till we arrived 

 at the 70th chain, that a good opportunity presented itself: 

 the measuring chain A, was then compared with the standard 

 B, and found to be thirteen divisions of the micrometer head, 

 attached to the brass scale, in excess. In these trials, the tem- 

 perature remained constant; the mercury in Fahrenheit's 

 thermometer being at 66^°. 



The 50-feet chain, spoken of in a former article, came from 

 the hands of Mr. Ramsden without being very accurately mea- 

 sured ; therefore it now became proper to ascertain its length, 

 by means of the standard chain. This was accordingly done at 

 the present time ; when B was found to exceed twice the length 

 of the 50-feet chain, by 14 divisions of the micrometer screw ; 

 the thermometer, at the time of trial, standing at 6g±°. 



At the conclusion of the measurement, the chains were again 

 compared, when the working chain A, was found to exceed the 

 standard, 17^ divisions on the micrometer head : this was after 

 273 chains were measured. Now, when 70 chains only had 

 been measured, the difference between A and B was 13 of those 



