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I bought this little glass bottle about 15 or 20 years ago here in France. The seller told me that it was a special bottle for watering caged birds. Well it has moved about with me since then but I never thought to question it until today. It is, in fact, a fiendishly clever device. If you fill the tower bit with water and turn it right way up, then the little bowl in the bottom fills with water but doesn’t spill over, it only fills by the amount extracted, as in sipped out by the bird.
Ingenious !
Now my question is how in the name of all that’s good and holy does it manage to do this? I have no idea but the vaguest notion that it was something I did know long long ago when I studied science in school. Answers please.
Comments
Terry Cunningham
on May 27, 2014Hi Martin. An interesting problem that takes us back to school science. You will remember the long tube full of mercury that was up-ended in a bowl of mercury and acted as a primitive barometer to measure air pressure. It was the column of air 50 miles high pressing down on the surface of the mercury in the bowl with a pressure of 15 pounds per square inch that was able to support a column of mercury 30 inches high in the tube. 50 miles of air weighs the same as 30 inches of mercury. If the tube were longer than 30 inches, the space at the top of the tube would contain a vacuum. You can do the same with a tube of water and 50 miles of air would support about 20 feet of water. In your tower there must be air above the water, not a vacuum (the tower would have to be more than 20 feet high for that). Initially, the tower is full of water but a bird will sip water from the bowl until the level in the bowl drops below the roof of the hole connecting the tower to the bowl. That allows air to enter through the hole and rise to the top of the tower. At the same time, water travels through the hole in the opposite direction into the bowl to raise the level in the bowl until it is just higher than the roof of the hole. At that point no more water can leave through the hole as no more air can get in. All you see is, now and again, a few bubbles rising up inside the tower and the water level in the tower gradually falling. Birds are clever creatures aren’t they?
Regards, Terry.
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