"In the late 1890s, strange airships started being spotted sailing across the country's skies. While lighter-than-air airships had been around since the 1850s, their capabilities were far more limited, and they were not nearly as big as those being observed. Witnesses described the ship, or ships, as being 150 to 200 feet long and moving at speeds of roughly 135 to 150 mph. Airships at the time were generally smaller and only moved at speeds of around 5 to 7 mph."
https://www.planeandpilotmag.com/article/ghost-airships-of-the-1800s/
"Attached to the keyboard were many tuned resonation plates and
vibratory mechanisms. Mr. Keely said that when the plates were
polarized with “negative attraction” the airship would rise and
float above the ground.
Keely could make the airship accelerate to any desired speed by
damping out certain notes on the keyboard."
https://www.unexplainable.net/info-theories/history-of-the-mystery-airship-sightings-in-the-late-1800s.php
"Heating the lifting gas causes expansion of the gas in order to further lower the density of the lifting gas, which results in greater lift."
https://en.m.wikipedia.org/wiki/Thermal_airship
"the concept of a vacuum airship. This concept is similar to a standard balloon, whereas a balloon uses helium or hydrogen to displace air and provide lift, a vacuum airship uses a rigid structure to maintain a vacuum to displace air and provide lift."
https://www.nasa.gov/general/evacuated-airship-for-mars-missions/
