One of my first projects out of college involved the construction of a seven story office building on Galveston Island in Texas for an insurance company. Part of my job was engineering controls; establishing grids (plan lines) and grades (elevations).

One of my favorite things to do was to set up the Dumpy Level that I was using for "shooting" grades (elevations) on one of the upper floors of the building and gaze through the optics at the horizon. There are verniers in the scope; an upper and lower that are horizontal and then two that cross in the middle. The middle horizontal vernier represents the HI (height of instrument) above mean sea level.

There is a level tube that is below the scope which has about a 30 foot radius ground in the glass of the tube where the leveling bubble resides. The instrument is leveled by adjusting leveling screws on two opposite axes of the instrument to bring the bubble in the center of the tube.

If the earth were flat, then no matter how high the instrument is, the horizon line would match the sea level line when the instrument is level. But of course, this is not the case.

I would set up on the sixth or seventh floor and watch the ships approach Galveston bay. On a clear day, the ships could be seen beyond the sea level horizon line, and this line would bisect the ship above the draft level until the ship got close enough to my line of site that was above sea level. The HI (Height of Instrument) line may be above the ship, depending on the ships height above sea level. But as it approached, the line would get lower and lower on the image of the ship, until it resolved with my relative height above sea level when it was close proximity to my location.

Clearly this was an indication of the surface of the earth curvature.