You can try something like this, adding a height to your #bot div:
<div id="bot" style="overflow:auto;z-index:3;height:200px;">
EDIT
In case you want this captions container to be fluid, remember that in order to set the height of a container in percentage, its parent container needs to have an explicit height (i.e. in px for instance).
So you need you can set the height of the main container in this case in px:
<div id="box" style="height:600px">
And then you can set the height of your box in percentage:
<div id="bot" style="overflow:auto;z-index:3;height:20%;">
You can test it here: http://jsfiddle.net/H7uhM/11/
Ps. Another thing, I suggest you to get rid of all these inline styles for CSS.
HTML for structure
CSS for presentation
JS for behaviour
Keep CSS code separated from html, your code will be easy to modify and reusable.
I suggest you to read something about the concept of CSS specificity and after that you will never use inline styles again.
http://www.smashingmagazine.com/2007/07/27/css-specificity-things-you-should-know/