90 Mile Beach Lantern

Origin
When early settlers began colonizing New Zealand almost all travel was done by horse. Knowing they could travel 30 miles in a day on horseback, the beach along the northernmost point of the country was marked as being 90 miles long, having taken three days from point to point. Not taking into account the slower movement speed across sand, the beach is really around 55 miles. By an interesting coincidence, however, the beach is almost exactly 90 kilometers.

This lamp was blown from glass taken from the beach by an early settler, possibly the same who made the error in measurement.

Effects
Light shone through the lamp causes those who see it to confuse units of measurements. There seems to be no real pattern to the changes, only that miles to kilometers seems to be a constant. The effect wears off after 90 miles have been traveled from the source of light.