Trigonometrical Survey, 585 



divisions; consequently 17^ — 13,=4| divisions, was the wear of 

 B, in measuring 203 chains. Therefore, the whole wear is 

 found by this proportion, viz. 203 : 4^ : : 273 : 5,223 divisions, 

 = t§o °f an inch; which very inconsiderable quantity, like the 

 wear on Salisbury Plain, no doubt, arose from the pivots and 

 pivot holes of the joints being polished by continual use. 

 This supposition seems just; as the wear of the chain, after 

 the measurement on Hounslow Heath, was found to be much 

 greater. 



The length of the chain A, as well as that of the standard B, 

 was accurately ascertained by Mr. Ramsden, in the year 1793, 

 as particularly shewn in the Philosophical Transactions for 1795. 

 In the temperature of 54 , A was found to exceed 100 feet, 

 iVoooo °f an mc ^ ' tnere f° re > adding the wear which took 

 place on Salisbury Plain, viz. ~^ part of an inch, we get the 

 length of A at the commencement of the measurement on 

 Sedgemoor = 100,01009 feet. 



From repeated trials, as before observed, the standard B was 

 found to exceed the length of twice that of the new fifty-feet 

 chain, 14 divisions of the micrometer head; and, after the mea- 

 surement, the same chain fell short of A, ih-% of those divisions : 

 hence, A exceeds twice the length of the 50-feet chain, 31^ divi- 

 sions. Therefore the length of the short chain, in the temperature 

 of 54 , may be taken at 50,00075 feet. 



