4. An igloo or Eskimo hut is built in the form of a hemispherical shell with an inside diameter of 12 ft. If the igloo is constructed of snow blocks having a uniform thickness of 2ft and weighing 40 lb per cu. ft, find the weight of the igloo, neglecting entrance. Also, if fresh air contains 0.04 per cent carbon dioxide, find the amount this gas in the igloo when freshly ventilated.