As the Topographical engineers moved westward into the uncharted regions of North America, the need for navigational instruments, such as those used by mariners, became immediately apparent.    Latitude was determined by measuring the angular distance between the horizon and the sun.  This was accomplished using a sextant and had to be done precisely at 12 o'clock noon.  Since daily calibration of watches was not possible on the frontier, a timepiece of superior construction was required.

Chronometer shown with case removed.

The first successful chronometer was produced in 1762 by John Harrison, an English carpenter turned clockmaker, and tested successfully in 1764.   The chronometer, being a timepiece of relatively extreme precision, was equipped with a special compensation arrangement to reduce the effect of changes in temperature upon its rate.  As the accuracy of the navigator's work is largely dependent upon the accuracy of the chronometer, it is treated with great care.  The chronometer is calibrated at the beginning the expedition and is kept running continuously with no change being made to the setting of the hands.