Apply multiple transparencies to image

Hi. I'm using Photoshop CS3 and trying to figure out how to apply multiple transparencies to a single image. I've created a mask layer and am using the Gradient tool. But after I apply the first gradient transparency and try to apply the second, it removes the first one and only applies the second!! What am I doing wrong?

Nevermind! I found out through trial-and-error: I needed to right-click the mask layer and do an Apply. That fixed it.

OR, instead of doing something destructive you could have used the solid to transparent Gradient instead of Black to White.