My life : a record of events and opinions - Wallace-online.org

6oMY LIFEchurch spires of Barton and Higham Gobion hadbeen thus used, and the distance between them accuratelygiven ;and as the line from one to the otherran diagonally across the middle of the parish wewere surveying, this was made our chief base-line,and the distance as measured found to agree veryclosely with that given in the survey. This volumewas eagerly read by me, as it gave an account of allthe instruments used, including the great theodolitethree feet in diameter for measuring the angles of thelarger triangles formed by distant mountain tops oftentwenty or thirty miles apart, and in a few cases morethan a hundred miles ; the accurate measurement ofthe base-lines by steel chains laid in wooden troughs,and carefully tightened by exactly the same weightpassing over a pulley, while the ends were adjustedby means of microscopes ; the exact temperaturebeing also taken by several thermometers in order toallow for contraction or expansion of the chains ;andby all these refinements several base-lines of sevenor eight miles in length were measured with extremeaccuracy in distant parts of the country. Thesebase-lines were tested by repeated measurements inopposite directions, which were found to differ onlyby about an inch, so that the mean of all the measurementswas probably correct to less than half thatamount.These bases were connected by the system of triangulationalready referred to, the angles at all thestations being taken with the best available instrumentsand often repeated by different observers,while allowance had also to be made for height abovethe sea-level, to which all the distances had to bereduced. In this way, starting from any one base,the lengths of the sides of all the triangles were