583 



The Account of a 



And, for the reduction of the base to the tempe- 

 rature of 62 , viz. for 8° on the brass scale, we have 



- — =2,2497 feet ; which subtract 



12 



27682,3944, 



— 2,2497 



Therefore, the length of the base is - - - feet 27680,1447 

 which, neglecting decimals, may be taken at 27680 feet. 



As to the probable error of the above conclusion, I know not 

 how to form a just opinion. On ground sufficiently hard, and 

 otherwise favourable, I think a base of 5 miles might be mea- 

 sured so accurately, as to afford a result not differing from the 

 truth more than three inches : but, on this occasion, I should 

 not suppose the error can be less than six, nor more than nine 

 inches. Motives for adopting this supposition, have been related 

 in a foregoing article. 



art. xi. Calculation of the Sides of certain principal Triangles in 

 Cornwall and Devonshire. Plate XXVII. 



Distance from Hensbarrow to St. Agnes Beacon, 97084,8 Feet. Phil. Trans. 1797. p. 461. 



No. of 

 triangles 



Names of stations. 



St. Agnes Beacon 

 Hensbarrow - 

 Trevose Head - 



Observed 

 angles. 



47 i° °>75 

 67 6 13,25 



65 43 47 



180 



Diff. 



— 0,15 

 -0,58 



-0,57 



Spheri- 

 cal 



»>3» 



Error. 



—0,31 



Angles corrected 

 for calculation. 



47 1° 3> 2 S 



67 6 13 



6 5 43 43>75 



t> xj a c f St. Agnes Beacon 



Trevose Head from J Hens f arrow _ 



Distances. 



Feet. 



98108,1 



78099,9 



