The resonance apparatus (shown below) is used to calculate the speed of sound using constructive and destructive interference of sound waves. 

While conducting the experiment, we continuously vary the length of water column until we achieve resonance. 

**We alter the length of water in the water column in the following way-** 

* We raise the water reservoir higher than it's original height. The water level in both the tubes (column and reservoir) is seen to increase.
* We lower the water reservoir below it's original height. The water level in both the tubes (column and reservoir) is seen to decrease.

**This doesn't seem intuitive at all**.  

 - If the water increases in both tubes then where does this extra water come from?  

 - If the water decreases in both tubes then where does some of the water go?  

 - Shouldn't liquid stay at the same level? so that the same pressure is maintained? 

**Further Clarification**:  

If the length of the height in the water column (BC) be $h$. Then I expect to see the height of the water reservoir also $h$ so that they are at the same level.  

Now as we raise the water reservoir, the water level should remain exactly the same (equal to $h$) level. But what happens is the height of the water column and the level of water in the reservoir both increase and become more than $h$.  

The opposite happens when we decrease the height of the water reservoir. 

All help is appreciated.
[![Resonance Tube Apparatus](https://i.stack.imgur.com/bGFcr.gif)](https://i.stack.imgur.com/bGFcr.gif)