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//
   	$(window).scroll(function(){          		totalheight =parseFloat($(window).scrollTop())+ parseFloat($(window).height());  	  if(($(document).height()/2) <= totalheight ) {  //有滚动条 且滚动到触发点(一半)	     next();	  }  	});  	$(window).on('mousewheel', function(event) { //手机不支持		totalheight =parseFloat($(window).scrollTop())+ parseFloat($(window).height());		if(totalheight==$(document).height()) //无滚动条		{		if(event.originalEvent.wheelDelta<0) //且 向下滚动		   		next();		}	});