How a temperature change moved the glass spheres
The device turned an invisible change in warmth into a visible change in buoyancy. Buoyancy is the upward push a liquid exerts on an object placed in it. Each glass sphere had a slightly different weight relative to its size, so some were just able to float while others tended to sink.
As the liquid warmed, it expanded. Its mass stayed the same, but that mass occupied more space, so the liquid became less dense. The upward push on each sphere weakened; spheres that had been barely floating could sink. When the liquid cooled and contracted, it became denser and could lift spheres that had been lower in the tube.
A thermometer without a modern temperature scale
The spheres acted like a set of physical thresholds. Their positions showed whether the surrounding liquid was warm enough or cool enough to support them, turning temperature change into a simple rise-and-fall display. A sealed tube kept the liquid contained while leaving the spheres free to move within it.
This was a useful way to compare conditions, but it was not the same as reading a precise number from a calibrated thermometer. The device’s message was positional: as the liquid’s density shifted, a sphere crossed the narrow boundary between floating and sinking. A small change in warmth could therefore make the reading visible without a dial or electronic sensor.
