why the gl_ClipDistance[] doesn't work?

I just can't understand why the gl_ClipDistance doesn't work. The result is the same as I didn't set the gl_ClipDistance.

I had set "glEnable(GL_CLIP_DISTANCE0) ;glEnable(GL_CLIP_DISTANCE1) ;"by application.

the vertex shader is like this:

#version 410

uniform mat4    mvpMatrix;
//.......some other uniforms here
in vec2 vVertex;
uniform vec4 clip_plane=vec4(1.0,1.0,0.0,0.85);
uniform vec4 clip_sphere=vec4(0.0,0.0,0.0,10.0);

void main(void) 
    {     //........some other code,include vec2 worldPos    vec4 worldPosition=vec4(worldPos.x,height,worldPos.y,1.0);    gl_ClipDistance[0]=dot(worldPosition,clip_plane);    gl_ClipDistance[1]=length(worldPosition.xyz/worldPosition.w-clip_sphere.xyz)-clip_sphere.w;    gl_Position = mvpMatrix * position;    }

anyone can help me figure this out?