var speed_k = 800; //pixels per second for 1000 pixels height
var steps = 20; //number of steps for jump

var c = 0;
var height = 1000;  
var step_height;  
var step_time;  
function MoveUp()  {  
	c = 0;  
	height = document.body.scrollTop+300;    
	speed = speed_k*(height/1000);     
	step_height = height/steps;     
	step_time = (height/speed)/steps;     
	_MoveUp();   
}   
function _MoveUp(){ 
	c++; 
	if(c<steps) { 
		window.scrollBy(0,-step_height); 
		setTimeout(
				function () { 
					_MoveUp()
				}
				, step_time);  
	}   
}