var iTimer;
var iHeight = 401;
var iStopHeight = 900;
var oSmoke;

function doSmoke() {
	oSmoke = document.getElementById('Smoke');
	if (!iTimer) { iHeight = 401; }
	if (oSmoke) {		
		iTimer = setInterval('move_smoke()', 30)
	}
}

function move_smoke() {
	if (oSmoke) {
		iHeight += 10;
		(iHeight <= iStopHeight) ? ( oSmoke.style.height = iHeight + 'px' ) : ( clearInterval(iTimer) )
	}
}