Surveyor's Plain Compass

This surveyor’s compass was made in about 1775 by Benjamin Rittenhouse, younger brother of the famed instrument and clockmaker David Rittenhouse of Philadelphia. Benjamin became especially well known for the quality of his surveyor’s compasses; in 1798 Rufus Putnam, Surveyor General of the United States, referred to Benjamin Rittenhouse’s compasses as “the best I have seen.” In addition to their functional qualities, Benjamin Rittenhouse’s compasses are distinguished by their boldly designed and beautifully executed compass roses—the engraving of their faces indicating the cardinal and intermediate directions.

On the face of the compass, east and west are reversed from their ordinary configuration. This is explained by the fact that this compass was used not for finding directions, but for determining the deviation of a bearing from the north-south axis. Imagine that the compass needle and the indicator for north are aligned, but that the bearing to be measured is to the east. As the compass turns clockwise to align with the desired bearing, the needle remains stationary and will indicate the bearing in degrees to the east. After taking bearings and distance measurements in the field, the surveyor would then use the notes to draw a plan of the land parcel. Such work was much in demand in colonial America as settlement expanded and land changed hands.