a new chain of 100 links, each one foot in length. He also asked Ramsden to prepare three precisely-calibrated wooden rods 20 feet long. These proved to be a failure because of fluctuations in length due to varying humidity; instead three calibrated glass tubes were used. The chain proved to be as accurate as the glass tubes, and it was in use for baseline surveys over the next thirty years. Later, even more precise measurements showed that the accuracy of Ramsden's 100-foot (30.48 m) chain was within about 3 inches in 5 miles. In actual use the chain was supported throughout its length by wooden trestles, and tensioned with a known constant weight. Its coefficient of thermal expansion was carefully measured so that temperature fluctuations could be taken into account. Full details (with plates) are given in Roy's account of the measurement of the Hounslow Heath baseline. American surveyors sometimes also used a chain of 100 feet, also with 100 links, known as the engineer's chain. The term chain in both cases usually refers to the measuring instrument rather than a unit of length, and distances measured are normally given in feet and decimal fractions of a foot (not inches). Despite Ramsden's chain originating in the UK, Gunter's predominated there: "When a chain is spoken of without qualification, Gunter's chain is meant", noted Macquorn Rankine's mid-Victorian A Manual of Civil Engineering. The zenith telescope The zenith telescope constructed by Jesse Ramsden in 1802 was used to determine the latitude of many stations of the Principal Triangulation of Great Britain. This portable instrument was designed to bring observatory precision to fieldwork. The outer frame stood about tall and the telescope mounted on an inner frame was long. The telescope was restricted to observations within a few degrees of the zenith in order to prevent errors due to refraction. A complete description of the instrument is given by Pearson. See also Repeating circle References Bibliography External links Photograph of a section of Ramsden's chain, at Science & Society Picture Library Surveying instruments Historical scientific instruments