Photostats of a manuscript written by John Harrison in 1730 concerning the construction of his first marine chronometer. The document describes the problem of finding longitude at sea, the solution of using a marine timekeeper, the accuracy of contemporary clocks, errors in pendulum regulators and their correction, and the effects of temperature on clocks. It continues with notes on the construction of Harrison's chronometer, including the theory employed and compensations introduced. There are two diagrams at the end.