For those of you who may not be aware of this, the University of Cambridge's website has freely available digitized copies of many original papers and print publications of the Board of Longitude who were central players in the development of chronometers in the 18th and early 19th centuries: http://cudl.lib.cam.ac.uk/collections/longitude Many of the primary sources cited in Gould's The Marine Chronometer are available on that website. Just one example: the published records of the Greenwich trials of Thomas Mudge's first chronometer. So we can for example graph the recorded rates of Mudge's first chronometer from June 21, 1776 to November 30, 1777: The rate acceleration that can be plainly seen is the reason Maskelyne and the Board judged Mudge's No. 1 unsatisfactory. The straight period right after the second half is the one that Gould on the other hand focuses on to argue that Mudge's chronometer was perhaps judged unfairly (see p. 79, and Appendices I and II).