回到顶端的jquery

现在的淘宝啊，京东啊，各种网站都有一个功能，有一个按钮，在页面最顶端的时候不会显示，当往下拉到一定的时候会出现。点击他会直接跳到页面的顶端。代码如下：

html代码：

<div id="top">
        <p id="back-to-top"><a href="#top"><span><img src="images/gotop.jpg"></span>返回顶部</a></p>
</div>

css代码：

p#back-to-top{position:fixed;display:none;bottom:100px;right:10px;overflow: hidden;}
p#back-to-top a{text-align:center;text-decoration:none;color:#d1d1d1;display:block;-moz-transition:color 1s;-webkit-transition:color 1s;-o-transition:color 1s;overflow: hidden;}
p#back-to-top a:hover{color:#979797;}
p#back-to-top a span{display:block;height:50px;50px;margin-bottom:5px;-moz-transition:background 1s;-webkit-transition:background 1s;-o-transition:background 1s;}
#back-to-top a:hover span{overflow: hidden;}
#back-to-top a span img{border:none; 50px;height: 50px;}

jquery代码：

/*top特效*/
        $.fn.smartFloat = function() {
            var position = function(element) {
                var top = element.position().top, pos = element.css("position");
                $(window).scroll(function() {
                    var scrolls = $(this).scrollTop();
                    if (scrolls > top) {
                        if (window.XMLHttpRequest) {
                            element.css({
                                position: "fixed",
                                top: 0
                            });
                        } else {
                            element.css({
                                top: scrolls
                            });
                        }
                    }else {
                        element.css({
                            position: "absolute",
                            top: top
                        });
                    }
                });
            };
            return $(this).each(function() {
                position($(this));
            });
        };
        //绑定
        $("#down").smartFloat();