function backToTop(ymin) {
	var vitesse = 0;
	var gravite = 0.5;
	var ymin = (ymin!==null) ? ymin: 0;    	
    var ymax;
    var fps = 25;
	
	if(document.documentElement.scrollTop) {
		ymax = document.documentElement.scrollTop;
	} else {
		ymax = document.body.scrollTop;
	}
	
	if(ymax>ymin) {		
		var y = ymin;
		var positions = new Array();
		while(y<ymax)
		{
			vitesse += gravite;
			y += vitesse;
			positions.push(y);
		}
		positions.reverse();
		for(var i=0;i<positions.length;i++)
		{
			setTimeout("window.scrollTo(0,"+positions[i]+");", i*fps);
		}
	} else if (ymax<ymin) {
		var y = ymax;
		var positions = new Array();
		while(y<ymin)
		{
			vitesse += gravite;
			y += vitesse;
			positions.push(y);
		}
		for(var i=0;i<positions.length;i++)
		{
			setTimeout("window.scrollTo(0,"+positions[i]+");", i*fps);
		}		
	}
	
}