Pennsylvania Statutes Title 71 P.S. State Government § 1860. Proviso

Provided, That wherever upon said line the locations of any of the monuments, or posts, erected by the said commissioners in 1786 and 1787 have been lost and cannot otherwise be definitely fixed, then and in that case, and in every case where it is required to establish intervening points in said line, a straight line drawn between the nearest adjacent monuments whose localities are ascertained shall be understood to be, and shall be, the true boundary line.