The gunpowder mill was probably powered by Powder Mill Run.
The following survey (Book C77 Page 187) relates to the 31-1/4-acre property of Jacob Comp "Situate on the waters of Powdermill run in Southampton Township Somerset County & Surveyed the 11th day of January 1838 in pursuance of a warrant dated the 10th day of July A.D. 1837." The Book C-63 Page 111 survey and the Book C77 Page 187 survey were performed on the same day by the same surveyor.
The following image is an excerpt from the northeast corner of the WPA survey map of Southampton Township, Somerset County, Pennsylvania. I prepared it as an index to the original property surveys in the vicinity of Powder Mill Run. North is generally to the right-hand side of the image.
Yellow highlighting identifies some of the properties associated with families that produced muzzleloader gunsmiths. Peter Troutman's son Benjamin Troutman (1780-1856) was a gunsmith who worked in the area shown by the map excerpt. I don't know if "Adam Laply's" property was owned by Adam Lepley I or the gunsmith Adam Lepley II (1776-1853). I don't know if the Christian Sturtz properties belonged to Christian Sturtz Junior or Senior. Christian Sturtz, Junior's son Jacob Sturtz (1787-1834) was a gunsmith who made gunpowder in Ohio.
Although I do not know if it is relevant to the mill that Powder Mill Run is named for, John Albright's December 24, 1807 survey (Book C-5, Page 75) mentions a mill dam and improvements (also see Book C-5 Page 76). Powder Mill Run and Gladdens Run both run through John Albright's property, which is located just above the mouth of Powder Mill Run.
The following table identifies surveys in the vicinity of Powder Mill Run:
Saltpeter:
An article in the July 13, 1898 issue of the "Somerset Herald" newspaper states, "Ordinary gunpowder, the kind one buys in a store, is a mechanical mixture of 75 parts of saltpetre, 15 parts of charcoal and 10 parts of sulphur." If gunpowder was manufactured near Comps Crossroads, then an obvious question would be where did they get the key ingredient of saltpeter? The answer seems to be a cave in Bedford County, in what is now the Sweet Root Natural Area, about 15.3 miles to the eastward of Comps Crossroads in a straight line. An article about the cave is published in the January 26, 1967 issue of the "Bedford County Press and Everett Press" newspaper. The following excerpt about the cave is from the Southampton Township, Bedford County portion of the 1884 book "History of Bedford, Somerset and Fulton Counties, Pennsylvania":
In lieu of mining, saltpeter could also be manufactured using a process that involved urine. For a brief summary of how black powder was made, see pages 128 and 129 of Kauffman's 1960 book "The Pennsylvania-Kentucky Rifle". For an overview of how Saltpeter was made, see the July, 1968 and November, 1990 issues of the "Muzzle Blasts" magazine. For more detailed information, see the 1776 pamphlet "Essays Upon the Making of Salt-Petre and Gun-Powder" that was published by the New York Committee of Safety.
Go to the Gunsmith Project Index for info about other Somerset & Bedford co. powder mills and gunsmiths.