Under the steam engine in the lower engine room–the camera is mounted right over the beginning of the cam shaft.
The white mark allowed for a manual RPM check on this big steel flywheel on the ground floor. Note how dark the bottom level of the mills is—that’s because all of the equipment is blocking out the light.
A wide view of the steam pump room, complete with pistons (taken apart for their brass), flywheels (covered in graffiti and rust) and pressure gauges (smashed apart for fun). I guess what I’m trying to say is, I was not disappointed.