#back_top_bottum
{
   width: 55px;
   height:111px;
   overflow:hidden;
   position:fixed;
   _position:absolute;
   z-index:9999;
   right:50px;
   _top:expression(eval(document.documentElement.scrollTop+document.documentElement.clientHeight-this.offsetHeight-(parseInt(this.currentStyle.marginTop,10)||200)-(parseInt(this.currentStyle.marginBottom,10)||0)));
   bottom:8%; 
   font-size:0;
   display:none;
}
.back_top
{
   width: 55px;
   height:55px;
   cursor:pointer;
   background:url('/images/back_top_bottom_gray.jpg') no-repeat;
}
.back_top:hover
{
   background:url('/images/back_top_bottom.jpg') no-repeat;
}
.back_dottom
{
   width: 55px;
   height:55px;
   cursor:pointer;
   margin-top:1px;
   background:url('/images/back_top_bottom_gray.jpg') 0 -56px no-repeat;
}
.back_dottom:hover
{
   background:url('/images/back_top_bottom.jpg') 0 -56px no-repeat;
}