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<script>
  $('#fade-nav').NavFadeIn({
	'position': 350
	'speed'	  : 250
});
</script>

(function($){
	$.fn.NavFadeIn = function(option){
		var x  = this;

		var settings = $.extend({
			'position': 80,
			'speed'   : 900
		}, option);


		// settings
		var position = settings['position'];
		var speed    = settings['speed'];

		$(document).scroll(function(){
			var scrollPosition = $(window).scrollTop();

			if (scrollPosition >= position) {
				x.fadeIn();
			}else{
				x.fadeOut();
			}
		});
	}

})(jQuery);