I had this gifted to me from an estate of good friends. Its called a sailor's weather gauge. As a storm or rain approaches, the barometric pressure ahead the weather rises. By watching the fluid in the tube you can tell when rain is eminent because the high pressure pushes the water up the tube. The higher up in the tube the water goes, the closer the rain is and/or the more intense the storm is about to be. This little thing has never failed, unless the water level is allowed to get too low. With the colored water [for instant visibility] that little cup is imperative as the paper in the bottom shows.
We got this about 5 or 6 years ago and since I have looked in may antique shops for any others and have seen only one and that one, while much older, was in real tough shape...so no sale. The last couple storms we've seen here have had the water bouncing up and down like a yoyo. I had to add water this morning, but prior to doing so the water was about 2/3 of the way up the tube....not crazy weather for here but not that far away. This unit works better than the tv weather people. lol