Trigonometrical Survey. 587 



Art. x. Reduction of the Base. 



The overplus of the 273d chain was measured Feet. 



by Mr. Ramsden, and found to be 23,517 feet; 

 wherefore, the apparent length of the base was - 27676,4,830 



From the measurement in the Riding-house of 

 his Grace the Duke of Marlborough, the chain 

 A was found to exceed 100 feet, in the temperature 

 of 54 , 0,11425 parts of an inch ; to which, add- 

 ing the wear by the measurement on Salisbury 

 Plain, viz. -—, and also half the wear by the 

 measurement of this base, viz. —^ part of an inch, 



we get — — — for the excess of the chain's length 



above 100 feet ; therefore, ' 1191 x 272,8 = 2,7075 



feet; which add --_."'- -1-2,7075 



The sum of all the degrees shewn by the ther- 

 mometer was 985 11 ; wherefore, 9 -^ — 54°* 272,8 



x 0,0 ° 7? = 3,1069 feet ; which also add - +3* 1 °^9 



Again, from the comparison of the 50-feet chain 

 with the standard B, it appeared that the excess 

 above 50 feet, in the temperature of 54°, was 0,09075 



parts of an inch ; therefore, Q, ° 9 ° 75 x 8 = 0,0605 



parts of a foot. This likewise add - -j- 0,0605 



The sum of all the degrees shewn by the ther- 

 mometers placed by the sides of the 50-feet chain, 



was 1372; therefore-^— 54, x 4 x-~ =0,0365 



parts of a foot : and this add - - -f 0,0365 



27682,3944 



