Animation directions

programming flash
1

I am using this turtural:

http://www.kirupa.com/developer/flashcs4/falling_snow_as3_cs4_pg4.htm

and will to using bubbles instead of snowflakes. I want the animation to go up, instead of down.

Can anyone help me to change the code in the .as-file, so my bubble will go up instead of falling down?

I have tried to change the following code:

xSpeed = .05 + Math.random()*.1;

to
xSpeed = .05 - Math.random()*.1;

This makes the bubbles go up, the the animations does not continue…