Originally used primarily in the livestock industry for regulating valves in stock watering troughs, they are now widely used as buoyancy mediums in liquid storage reservoirs, gauging stations along streams, water cooling towers, etc. Many are weighted to offset counterweights on automatic gauging devices. Has tube through the center. 36100010: 10” x 3.75” Buoyancy: 96-97. 36100020: 12” x 4-3/4” Buoyancy: 159-161.