The letter concerns observations of the eclipses of Jupiter's satellites which were taken to establish the exact location of several rivers to determine the boundary between New Brunswick and the District of Maine (then part of Massachusetts).

The manuscript notes that the observations were made "in order to determine the latitude and longitude of the mouths of the rivers Scoodiac and Magaguadaweek, the two rivers in question known as the true St Croix". The observations were performed by Thomas Wright and Samuel Webber on behalf of the British and American governments and were based on the immersions and emersions of Jupiter's satellites, with observed lunar distances.