Joseph Saxton (1799-1873) worked for 30 years --from 1843 until his death-- as chief mechanic in the Office of Weights and Measures (OWM) of the U.S. Coast Survey. Saxton was born in rural Pennsylvania where he worked in his father's nail factory and then apprenticed with a clockmaker. He moved to Philadelphia in 1818 to receive further training in horology and engraving. While working for the machinist Isaiah Lukens in Philadelphia, Saxton built a pyrometer with an optical level for measuring the thermal expansion of metal, a principle he would apply in several other designs, including the reflecting length comparator that he designed and built while at OWM.