.. it’s how long it would take to travel from one side to another if you could dig a hole between any two points on earth and ride a vehicle powered only by gravity! Who knew Douglas knew so much?
A Day in the Life of a Presson…
Co-discover Interesting things :)
A Day in the Life of a Presson... 
I have known about the *84 minute period of this oscillation from my early assignments at McDonnell Douglas, related to computer navigation systems: “As first explained by Max Schuler in his classic 1923 paper, a pendulum whose period exactly equals the orbital period of a hypothetical satellite orbiting just above the surface of the earth (about 84 minutes) will tend to remain pointing at the center of the earth when its support is suddenly displaced. This is the basic principle of Schuler tuning that must be included in the design of any inertial guidance system that will be operated near the earth, such as in ships and aircraft.” The “elevator” through the earth is just one version of a Schuler Pendulum.
*Actually, 84.4 minutes, but who’s counting. (grin).
🙂