I have a strange piece of equipment left by an ex meteorologist friend who has recently died. I am wondering if you can help me identify it and explain what it is used for.

It consist of two glass tubes, joined together, side by side. The lager tube is about 60mm diametr and 145mm tall & is sealed at both ends. The small tube is 25mm dia & 70 mm long, open at one end. The other end is linked into the side of the larger tube.

The only clue that I have is that on the box containing this item it says 'Constant Level Water Bottle'. I have searched on the internet and can find nothing like it.
Can you help.

Thank you, Bernard Tuck

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