lubber line

Talking to engineers at Dallas-based Deejay Industries, Ritchie's engineers decided to try the screw-machine specialists' suggestion of knurling the end of each lubber line. Pressed into the hole in the, plastic capsule, the knurl would upset the plastic sufficiently to ensure that the brass rods would stay put during ultrasonic welding.

Vibrations during ultrasonic welding of capsule halves were shaking loose the tiny brass rods, called lubber lines, along which mariners make their sightings.

You would then fly so as to put the selected numbers on the lubber line on that instrument.

The lubber line also went straight across the attitude indicator between the airspeed and the altitude and--this is the important part--when everything was centered, all you had to do was stare at the attitude indicator.