I found what looks like a "crystal ball" in a meteorological station in the Vlădeasa mountains (Romania). What is its purpose?
It's a Campbell-Stokes sunshine recorder, used to record the times at which the sun is shining. It acts as a lens, focusing the sunlight onto a piece of card. If the sun is shining, the focused beam will burn a hole through the card at that point.
Here's a closer view of a sunshine recorder in Wendelstein, Bavaria: Source: Wikimedia Commons
And here's a used recording card with holes burned through it: Source: Wikimedia Commons